Fauna: invertebrates - Udine Cultura

Fauna: invertebrates 

MARIA CRISTINA GAMBI · CARLA MORRI 

■ Invertebrate vagile fauna 

Vagile fauna indicates all mobile 

animals, i.e, those free to move 

autonomously or, if sedentary, capable 

of restricted movements in time and 

space. In this section, we examine 

vagile invertebrates, which include 

many of the most typical species 

faithfully associated with seagrasses in 

general and Posidonia meadows in 

particular, on whose leaves they 

usually develop. Vagile fauna includes Antedon mediterranea on Neptune grass 

a large group of organisms belonging 

to different phyla and with very different morphological and dimensional 

characteristics, ecological requirements and functional roles. However, many 

of these fauna living on seagrasses may also be found in other habitats, like 

photophilic macro-algae and the coralligenous, and are therefore highly 

cryptic organisms of herbivorous and herbivorous-detrivorous habits. 

The first research on vagile fauna associated with Neptune grass and other 

seagrasses, which we call “small” (seahorse grass, common eelgrass, dwarf 

eelgrass, halophila seagrass) was carried out by the French school in the mid- 

1960s. This research is now flanked by more studies along the Italian and 

Spanish coasts. Although the fauna related to these systems in the eastern 

Mediterranean is still little known, some new data are now available for the 

coasts of Greece, Turkey and Tunisia. From the methodological viewpoint, 

sampling of vagile fauna is very complex, at times poorly selective, and is 

carried out in various ways by researchers. As regards leaf layers, the most 

commonly used method by scuba divers is hitting the base of rhizomes hard 

with a net which then grazes the leaf surface. This is a semi-quantitative 

method which, once standardised, enables researchers to carry out reliable 

comparative analyses. 

Purple sea urchin (Sphaerechinus granularis) and sciophilous algae on rhizomes of Posidonia oceanica 

53

Larger, or extremely vagile fauna (e.g., 

large decapods, mysidiaceans) is 

captured with beam trawls (called 

chalut by the French and gangamo by 

the Italians) and epibenthic dredgenets. 

Underwater aspirators are the 

most commonly used and least 

destructive means of collecting fauna 

associated with rhizomes and the 

seabed. In addition, when this method 

is used on fixed surfaces, larger 

numbers of samples can be collected. 

Snake star (Ophidiaster ophidianus) 

Other techniques, like core-boring, the 

use of grabs and removing matte 

sections, with saws, cutting blades or shovels, are clearly very destructive. 

Generally speaking, a single method of sampling vagile fauna does not exist 

and therefore, according to the type of organisms analysed, their mobility, 

cryptic characteristics, behaviour and ecology, a sampling method should be 

devised especially for them. 

From the taxonomical viewpoint, the main groups of vagile fauna associated 

with seagrass systems are molluscs, crustaceans and polychaete annelids 

(segmented marine worms) and, to a lesser extent, nematodes (roundworms), 

platyhelminthes (flatworms) and echinoderms (like starfish). Flatworms and 

roundworms are generally small, and roundworms are actually part of what is 

called meiofauna (animals not longer than 0.5 mm) and are highly cryptic in 

seagrass systems, although very diversified. Segmented worms are highly 

diversified and well represented in fauna associated with Neptune grass, 

especially at rhizome level and in mattes, where they are the dominating 

invertebrate group. They have a metameric body structure, i.e., made up of a 

series of equal segments along the longitudinal axis - a structure they share with 

arthropods and one also frequently observed in the evolution of vertebrates. 

Molluscs, which are a very large phylum, including terrestrial and freshwater 

species, generally have a calcareous exterior shell to protect them. The shell 

may be in single spiral, like that of gastropods, or divided into two or more 

pieces (as in bivalves and chitons). Among gastropods, nudibranchs are 

shell-less, just like cephalopods (cuttlefish, octopus and squid), many of 

which have an interior supporing structure (the well-known cuttlebone). 

Seagrass systems host particularly diversified gastropods, including several 

nudibranchs and some cephalopods. 

Crustaceans (arthropods) are among the most diversified group of vagile 

fauna and, from many viewpoints, play a similar role to that of insects, their 

close relatives, in freshwater and terrestrial systems. 

Crustaceans associated with seagrasses are very small peracarids 

(amphipods, isopods, tanaidaceans, cumaceans or hooded shrimps, and 

mysidaceans) with very particular mouthparts. All these species develop 

directly, their eggs being contained in brood pouches, from which the young 

eventually emerge. 

Decapod crustaceans are also very abundant and widespread. They are larger 

than crustaceans and have 5 pairs of legs (hence the name decapod = tenlegged) 

with swimming forms (prawns and shrimps) and creeping forms like 

crabs and the common hermit crabs. 

Harpacticoid copepod crustaceans are also part of vagile fauna. They belong 

to meiofauna and are the only ones that lead benthic lives, as opposed to the 

majority of copepods, which are typically planktonic. 

Echinoderms are frequently found in seagrass systems, with a few, very 

characteristic and well-known species, including starfish, sea urchins and 

brittle stars (ophiuroids), sea cucumbers (holothuroids) and feather stars 

(crinoids). Generally, echinoderms exhibit fivefold radial symmetry, with 

particular external structures - the outer shell of urchins - which become very 

small plates or sclerites in starfish and sea cucumbers. 

54 55 

The gastropod Gibbula ardens, typically grazing Posidonia leaves

56 

Vagile invertebrate fauna associated 

with Posidonia oceanica canopies. 

The mobile fauna associated with 

Neptune grass canopies is generally 

very small, mostly herbivorous or 

herbivorous-detrivorous, and finds 

shelter and nutrition by exploiting the 

varied, complex epiphytic felt covering 

the leaf surfaces. Organisms that have 

this particular life-style are called 

mesoherbivores, typical forms of many 

other vegetated coastal systems. It is 

Starfish (Astropecten spinulosus) 

precisely in seagrass canopies that we 

find the most characteristic species 

associated with these plants, because leaf covers have unique micro-climatic 

conditions that select living forms which are specially adapted to them. Most 

of the vagile fauna associated with seagrass canopies (crustaceans, molluscs, 

echinoderms) carry out daily migrations along the vertical axis of meadows, 

indicating that species and numbers vary considerably between day and 

night, and it is also very difficult to determine whether some species rightly 

belong to the canopy or to rhizomes. 

Vagile fauna, which is variously associated with the dynamics of seagrass 

canopies and their animal and plant epiphytes, has very different temporal and 

spatial composition and population structures, due to the highly dynamic life 

habitat. The limited research carried out in the Mediterranean on the 

relationship between meadow structure and vagile fauna suggests a positive 

relationship between leaf bundle density, and therefore leaf cover, and the 

diversity and/or abundance of fauna, at least during the seasonal development 

of the canopies. However, many of the main groups of vagile fauna develop in 

patches, and the spatial variability scale is still unclear, as are the abiotic and 

biotic environmental factors causing it. 

From the functional viewpoint, vagile fauna plays an important although often 

underestimated role in Posidonia meadows, in which greater attention has 

been given to the debris chain or route, which seems to prevail in energy and 

biomass terms. The group of organisms that make up vagile fauna are the 

‘grazing chain’, which exploits plant epiphytes of leaves to transfer matter and 

energy to higher trophic levels, secondary consumers and large predators 

(large decapods, octopuses, fish). Although in biomass terms vagile fauna has 

clearly lower values than the large detrivorous invertebrates of Posidonia 

systems (urchins, sea cucumbers, many large decapods) and rhizome 

epibiotic filterers (sponges, sea squirts, bryozoans, sabellid polychaetes), the 

higher turnover of mesoherbivores - due to their small size and short lifecycles 

- compensates their net production. In this way, the two main routes of 

energy transportation - debris and grazing - probably converge. 

Posidonia canopies host few polychaetes (even at night), both as number of 

species and especially as individuals. Although this group is highly diversified 

in rhizomes and mattes (250 species counted in some meadows of the 

Tyrrhenian and Spanish coasts), only 5% of the species are estimated to live in 

the canopies. Among the most frequently found are the nereidid Platynereis 

dumerilii, a herbivorous species feeding on epiphytic macro-algae, the ophelid 

Polyophthalmus pictus, and several species of small, interstitial syllids like 

Sphaerosyllis spp. and Exogone spp., which feed on the felts of epiphytic 

diatoms, and macro-benthic syllids like Syllis, most of which are carnivores 

and live on animal epiphytes, especially hydrozoan colonies. Although there 

are few polychaetes in canopies, they become more frequent and diversified in 

the deeper sections of the meadows. 

Molluscs, especially prosobranch gastropods, make up one of the prevailing 

groups on canopies. Many of them are grazing mesoherbivores whose 

mouthparts (radulae) are specialised in scraping epiphytic layers. Although 

almost all of them are herbivores, each species is specialised for a certain 

A hermit crab and hydrozoans on the tip of a Posidonia leaf 

57

group of plant epiphytes, in order to 

restrict competition and better exploit 

food availability within the system. The 

most typical and frequent are the 

globular rissoids (minute sea snails) 

with species like Rissoa variabilis, R. 

ventricosa and R. violacea. Other 

common rissoids belong to the genus 

Alvania (A. discors, A. lineata) and to 

the genus Pusillina. 

Other gastropods typical of surface 

meadows are Gibbula ardens and G. 

umbilicalis, the trochid (top snail) 

Jujubinus striatus and J. exasperatus 

and the turbinids (turban snails) Tricolia 

pulla, T. speciosa and T. tenuis. More 

ubiquitous species, which migrate 

vertically between leaves and 

rhizomes, are also noteworthy, like 

Bittium reticulatum, B. latreilli and Columbella rustica. 

There are also opisthobranchs (gastropods with small shells) and nudibranchs 

(shell-less) occasionally found between leaves. These are quite specialised 

carnivorous organisms that feed on sessile epiphytic animals. Among them 

are the genera Doto, Eubranchus and Cuthona, which prey on hydrozoans, the 

genera Polycera and Janolus, which feed on epiphytic bryozoans, and 

Goniodoris and Berthella, preferring colonial tunicates (sea squirts). Other 

organisms like Chauvetia mamillata and Favorinus branchialis are specialised 

in preying on the eggs of other invertebrates. Cephalopod molluscs like 

cuttlefish (Sepia officinalis) and bobtail squids (Sepiola sp.) sometimes swim 

between the leaves of seagrass meadows looking for food and shelter from 

other predators. 

Molluscs are the group that best shows population zoning along meadow 

height, generally accompanied by particular morpho-functional adaptations 

of the species (shell and foot shape, locomotion, size, type of reproduction, 

nutrition) associated with the overall environmental gradient. Generally 

speaking, the upper sections of meadows (0-5 m) are less populated, 

although the species are larger and more characteristic (Gibbula, Jujubinus). 

Middle sections (10-15 m) host larger numbers of more diversified species, 

and deep sections (15-20 m) contain more ubiquitous species as well as 

those coming from nearby environments (soft seabeds, coastal debris, the 

coralligenous, etc.). 

These organisms exhibit great biodiversity and considerable variability in 

population composition and structure, according to geographic area, season, 

depth, soil characteristics, and the circadian rhythm that some of them have. 

However, the genera and some of the species listed above make up a 

constant nucleus often found in many environments and geographical areas of 

the Mediterranean, with some substitutions. Most mollusc species usually 

have short life-cycles (1-2 years) and reproduce directly by laying small 

clusters of eggs. Larval stages occur inside the eggs, which hatch to reveal 

already formed juveniles. This type of reproduction is associated with the 

small size of these animals in seagrass meadows. 

Crustaceans colonise canopies: most of them are peracarids, especially 

amphipods whose large numbers make them the favourite prey of many 

cephalopods and fish. They are therefore an essential link in the food-chain of 

Posidonia meadows, linking primary producers (plant-vegetal epiphytes) to 

higher trophic levels. Posidonia meadows host 80 amphipod species, which 

are those that carry out the most evident and greater daily migration: their 

numbers in canopies are large by day and even larger by night. 

Although there are no amphipod communities with species and structure 

exclusive to Posidonia meadows and constantly found in meadows, the species 

58 59 

Turban snail (Tricolia tenuis) grazing on a 

Posidonia leaf 

Cuttlefish (Sepia officinalis)

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The decapod Hippolyte inermis camouflaged 

on a Posidonia leaf 

most frequently found in canopies are 

Dexamine spinosa, Apherusa 

chiereghinii, Aora spinicornis, Ampithoe 

helleri, Caprella acanthifera, Hyale 

schmidtii, Phtisica marina, Eusiroides 

dellavallei, Ampelisca pseudospinimana 

and Maera inaequipes. 

Most of them are herbivorous or 

herbivorous-detrivorous and can feed 

on several species of plant epiphytes, 

from diatoms to filamentous macroalgae, 

which they remove and brush 

with their antennae equipped with 

thin filaments acting as combs. 

Amphipods, like all peracarids, 

develop directly, and adult females 

incubate their eggs. 

The great diversity of this crustacean 

group is thought to be favoured by this 

type of reproduction, which limits spatial dispersion and increases the 

possibility of reproductive isolation and adaptations to particular local 

conditions. 

Although isopods are another group of peracarid crustaceans less diversified 

than amphipods, their retinue species are more markedly adapted to 

Posidonia meadows and their canopies in particular. This is the case of Idotea 

hectica, one of the few directly herbivorous species, i.e., capable of feeding on 

the living tissues of Neptune grass leaves. Other typical species are Astacilla 

mediterranea and a few species of the genera Gnathia, Cymodocea and 

Cleantis. Large numbers of isopods also migrate daily, especially at night. 

Among peracarids floating near Posidonia leaves, there is a large group of 

mysidaceans - micro-shrimps - which form dense, fast-moving swarms, the 

favourite food of several fish. The species associated with seagrasses are 

Siriella clausii and Mysidopsis gibbosa, some species of the genus 

Leptomysis, with L. posidoniae and L. buergii, and a species recently 

described in Italian meadows, Heteromysis riedli. This was named in memory 

of one of the most important biologists of the Mediterranean, the Viennese 

Rupert Riedl, a pioneer in marine biology research and examination of sea 

caves by scuba-diving, as well as the inventor of an original benthos zoning 

chart based on the dynamics of shore water movement. 

Other, smaller groups of peracarid 

crustaceans are tanaidaceans and 

cumaceans, with a retinue of quite 

ubiquitous species, like Leptochelia 

savignyi, a species linked with plant 

debris generally found in many 

vegetated systems along the coast. 

Decapod crustaceans are well 

represented both by forms floating 

near leaves and by creeping ones. The 

most abundant and diversified family of 

floating animals are hippolytid shrimps 

of the genus Hippolyte. In particular, H. 

inermis is brilliant at camouflage: its 

original bright-green livery can change 

colour very rapidly, and a pinkish shade 

is adopted when imitating the colour of 

Creeping hermit crab (Calcinus tubularis) 

epiphytes, especially those of 

encrusting corallinaceous algae. In situ 

and laboratory studies have also revealed the particular life-cycle and 

reproductive biology of this species. The diatom-based diet of post-larvae 

living in leaves favours the precocious sexual transformation of males (all 

hippolytid shrimps are born males) into females, thus balancing the gender 

ratio in the population. 

Other decapod species typically living on leaves are Thoralus cranchii, 

Palaemon xiphias and species of the genus Processa. These last species are 

carnivores that migrate to the leaves at night to feed on other small 

invertebrates. Other, very numerous species, particularly at night, are the 

creeping hermit crabs Cestopagurus timidus and Calcinus tubularis, and the 

galatheids (squat lobsters) Galathea bolivari and G. squamifera. 

Among echinoderms, the only species truly typical of Posidonia canopies and 

very similar to the more common larger, pinkish starfish Asterina gibbosa, is the 

asteroid Asterina pancerii, whose greenish colour gives it effective camouflage 

in its habitat. A. pancerii is a typical Mediterranean endemic, strictly nocturnal 

and a carnivore, feeding, like many other starfish, on small molluscs. This 

species incubates eggs, a rather unusual characteristic among Mediterranean 

echinoderms, which may represent an adaptation to life in seagrass systems. 

Among other leaf-loving echinoderms is the common edible sea urchin, 

Paracentrotus lividus, whose numerous populations live in Posidonia meadows 

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62 

where they find protection between rhizomes by day. At night, especially their 

young migrate to the canopy to graze on epiphytic felts. The marks of their 

grazing are typically seen on the tips of the oldest leaves. Crinoids, or featherstars, 

are frequently seen on the leaves, like Antedon mediterranea, a species 

exhibiting different colours and which incubates its eggs. 

The vagile fauna associated with the canopies of meadows formed of small 

seagrasses (common eelgrass, seahorse grass, dwarf eelgrass) has been the 

least well examined in the Mediterranean. More information is available on 

seahorse grass, which is the most widely distributed species after Neptune 

grass, and may even be found at considerable depth (30 m). 

The mobile fauna on seahorse grass canopies is equal to that living on 

Neptune grass, although with fewer species for each characteristic group 

(amphipods, isopods, molluscs, polychaetes), i.e., an impoverished fauna 

compared with that living on Posidonia. Echinoderms, for instance, are almost 

absent on small seagrasses, except for the minute green sea urchin 

Psammechinus microturbeculatus which, precisely because of its tiny size, 

can graze on the leaves of seahorse grass and is not found on Neptune grass. 

Although their total biodiversity is reduced, some species may actually be more 

numerous in these habitats. For example, some interstitial syllid polychaetes of 

the genera Sphaerosyllis spp. and Exogone spp., the molluscs Bittium 

reticulatum and Jujubinus gravinae, which appear to replace J. exasperatus and 

J. striatus ecologically, are typically found on Neptune grass, and some 

peracarids (like the tanaidacean Leptochelia savignyi, and amphipods 

Synchelidium haplocheles and Pariambus typicus) also appear. Generally, the 

limited complexity of these canopies, due to the small size of leaves and faster 

temporal dynamics, favour small species with interstitial habits. 

The numbers of mobile animals on seahorse grass undergo greater seasonal 

variations than those living on Posidonia, due to the greater variations of leaf 

bundle density in meadows and the morphology of canopies over the year. 

The extent of faunal colonisation therefore depends on meadow density and 

general environmental conditions. When exposed to strong wave action, for 

example, Cymodocea meadows disappear in winter because tufts are 

uprooted by waves: this explains the extreme dynamics of these systems and 

the consequences for their associated communities. 

One of the few manipulative experimental studies available for the 

Mediterranean shows how reduction and gradual, complete removal of 

Cymodocea canopies have dramatic effects on the structure of fauna, with 

plummeting numbers of some groups, which may disappear completely. 

When wave action is very strong or leaf bundles are not very dense, as in 

meadows at 15-20 m, or in winter, Cymodocea systems are very similar to 

bare, soft seabeds. This explains why the benthic bionomy of the French 

school defines these systems as “epiflora facies” of fine sand. 

Asterina pancerii, the typical asteroid echinoderm of Posidonia meadows Green sea urchin (Psammechinus microturbeculatus) 

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64 

Vagile invertebrates associated with rhizomes and the seabed. The animals 

associated with Posidonia rhizomes and the seabed include species from 

several phyla, many of which have been mentioned above as living on 

Posidonia canopies. Vagile animals associated with rhizomes are generally 

larger, less specialised and more ubiquitous, and may be found in other 

vegetated habitats, even in softbed biotopes, because they are associated with 

the type of sediment in which Neptune grass grows. In addition to groups 

typical of canopies which, as mentioned before, carry out conspicuous daily 

migrations from rhizomes to leaves (amphipods, isopods, tanaidaceans, 

molluscs), the most frequently found forms in the rhizome-seabed area are 

polychaetes, decapod crustaceans, molluscs and echinoderms. Very important 

and numerous are also smaller groups of platyhelminthes, and meiofauna like 

nematodes and harpacticoid copepods. Similarly to sessile fauna, also for 

mobile fauna living on rhizomes and the seabed, there are fewer species 

associated with Posidonia systems than those living on this plant’s leaves or 

migrating to it at night. Near rhizomes and on the sea bottom, wave action and 

light are restricted and decrease with depth, and the accumulation of 

suspended particles increases as they are caught in seagrass canopies. 

Zoning of fauna associated with rhizomes is less clear, since it sometimes 

overlaps or may even be completely absent, both at different depths and in 

different meadows. In addition, the type of substrate on which meadows grow 

Hairy crab (Pilumnus hirtellus) 

(rock, debris, coarse sand, silty sand) 

and the different rates of local 

sedimentation cause the associated 

fauna to become richer in species 

related to hard substrates or typical of 

very different types of sediments. The 

numbers of animals living in these 

areas of meadows are also influenced 

by the density of bundles, i.e., by 

substrate availability for colonisation 

and meadow type in general. 

Continuous or patchy distribution of 

meadows, and the presence of 

clearings, channels and other 

interruptions to meadow continuity 

give rise to mosaic patterns that favour 

colonisation by other species, thus 

An isopod on a leaf of Neptune grass 

increasing overall biodiversity. 

Polychaetes are highly diversified precisely at rhizome and seabed levels, 

although no community is precisely typical of Neptune grass meadows. 

Polychaete populations associated with seagrasses are composed of a 

mixture of species with different ecology coming from vegetated 

environments, habitats with soft or hard beds, and none of them are exclusive 

to Posidonia systems. Exceptions are some species that appear to be more 

closely related to Neptune grass, like Pontogenia chrysocoma, Pholoe minuta, 

Kefersteinia cirrata and the sedentary species Polyophthalmus pictus. 

Generally, about one-third of polychaete species living in this area of 

meadows belong to the syllid family, with both macrobenthic organisms like 

Syllis garciai, S. columbretensis and S. gerlachi, and interstitial ones such as 

Sphaerosyllis spp., Exogone spp. and Salvatoria spp. 

Other very diversified families living at rhizome level are phyllodocids (paddle 

worms), polynoids (scale worms), nereidids (ragworms), hesionids, and some 

large species like the aphroditid Laetmonice hystrix (fireworm), a predator that 

migrates to meadows from nearby silty beds in search of food. Noteworthy are 

some species of eunicid worms (bobbit worms) living in Posidonia fascicles 

(the bases of fallen leaves that remain attached to rhizomes to form muffshaped 

covers), in which they burrow characteristic winding tunnels. These 

animals belong to the special category of borers, and are among the few 

which can chew into the horny fascicles of Neptune grass, use and move this 

65

type of Posidonia debris, which is 

usually considered inedible and is only 

attacked by fungi and bacteria. 

Examples of borers are Lysidice 

ninetta, L. collaris, Nematonereis 

unicornis and Marphysa fallax. Boring 

polychaetes were first described in 

meadows along the Italian coastline, 

and were later found in other 

Mediterranean areas (Spain, France, 

Croatia, Turkey and Greece). They 

colonise Posidonia rhizomes along all 

the distribution area of the plant, 

particularly in intermediate and deep 

meadows, and are usually found in 

fascicles between 2 and 4 years old 

(lepidochronological years). These 

species live in the fascicles throughout 

the year, become sexually mature in 

summer, and produce pelagic larvae, a 

fact which favours their large-scale 

dispersion. 

66 Among cephalopods, there is the common octopus (Octopus vulgaris) and the 

Lysidice ninetta, a polychaete boring into 

Posidonia fascicles 

Nematonereis unicornis, a polychaete which 

also bores into Posidonia fascicles 

Among molluscs associated with rhizomes, many species also live on leaves, 

like the genera Alvania, Gibberula, Jujubinus, Pusillina and Bittium which, as 

mentioned above, move between leaves and rhizomes in the daytime. There 

are also larger species, such as cerithids - Cerithiopsis tubercularis, C. 

minima, and Cerithium vulgatum - muricids like Hexaplex trunculus (banded 

dye-murex) and Bolinus brandaris (purple dye-murex), and members of other 

families like Conus mediterraneaus and Calliostoma laugeri. 

In meadows growing on rock, such as most of those along the Sicilian 

coasts, rhizomes are colonised by several species living on hard substrates, 

like green ormer (Haliotis tuberculata) and the cypraeids Mediterranean 

cowry (Erosaria spurca) and Luria lurida, whose empty shells are often found 

in clearings and channels near mattes. Attached to rhizomes and to the 

pebbles scattered on the bottom are sedentary chitons (sea cradles, like 

Lepidopleurus cajetanus). Clearings in between mattes often host gaudy 

species like the opistobranch (sea slug) Umbraculum mediterraneum and 

the knobbed triton Charonia lampas, which is a threatened species listed in 

the Habitats Directive. 

white-spotted octopus (Octopus macropus), whose typical lairs are found at 

the margins of meadows, and in clearings and channels between mattes. 

These cephalopods are the most active predators living in meadows where, 

especially at night, they feed on decapod crustaceans and other molluscs, like 

green ormer and many bivalves. 

Among peracarid crustaceans which, as previously described, are those 

carrying out the greatest vertical migrations from rhizomes to leaves, some 

isopods are noteworthy because they are associated with the rhizome layer, 

like Cleantis prismatica. This animal inserts part of its body inside a piece of 

Posidonia root, which it carries around and uses as a shelter when needed. 

Another peracarid associated with fascicles is Limnoria mazzellae, a species 

dedicated to the Posidonia botanist Lucia Mazzella. This species is a fascicle 

borer like the eunicid polychaetes mentioned above. It burrows complex 

tunnels in recent fascicles (0-1 years old), starting from the fascicle tip. 

Especially in summer, the tunnels host entire families made up of two or more 

adults and several juveniles. The species, which is particularly abundant in 

summer in the superficial areas of meadows subjected to stronger wave 

action, is an example of speciation associated with Posidonia. This species is 

different from other Limnoria species - usually woodborers - probably 

precisely in order to adapt to such a particular micro-environment like that 

offered by the fascicles of this seagrass. Direct development, which is 

common to all isopods, is the adaptation that undoubtedly favoured this 

process of speciation. 

Peracarids dominate very particular micro-environments which seasonally 

form in meadows, i.e, debris mounds, 

which are accumulations of Posidonia 

leaves, bundles and propagules 

collecting in clearings and channels or 

along the upper and lower margins of 

meadows. These microhabitats are 

quite ephemeral, as they are linked 

with extensive leaf shedding in autumn 

and with local wave action, which 

favours their formation and sudden 

destruction (storms, bottom currents). 

The little research available on debris 

mounds shows that they host dense 

populations of gammarid amphipods Common octopus (Octopus vulgaris) 

67

68 of the genus Gammarus, with species that are not found on leaves (G. 

aequicauda, G. subtypicus, G. crinicornis), presumably specifically 

associated with this type of habitat. Other typical species are the amphipods 

Atylus spp. and Melita hergensis, and the isopods Idotea hectica and I. 

baltica. These small detrivorous crustaceans play an important role in the 

fragmentation of leaf debris, which is essential for recycling into the system a 

source of carbon which would be wasted if unused. By reducing the size of 

debris, these organisms favour its further degradation by bacteria and fungi, 

and make it available to other detrivores like sea cucumbers, as we shall see. 

Most crustaceans associated with the rhizome and bottom layers are 

decapods, especially creeping forms, which become more abundant and 

diversified here than in other areas of canopies. Once again, the prevailing 

species of hermit crabs at rhizome levels are Cestopagurus timidus and 

Clibanarius erythropus, which use the empty shells of dead gastropods 

associated with Posidonia. 

Other frequent species are Athanas nitescens, Pisidia longimana, Alpheus 

dentipes, Processa edulis and Galathea spp. Among crabs, there are several 

species of portunids, xanthids (mud crabs) and majids (spider crabs), which 

may sometimes be quite large (Macropipus spp. and Maja spp.). Creeping 

decapods are generally detrivorous and feed on leaf debris and its epibionts, 

and actively move about on the bottom, between rhizomes and leaves. There 

The decapod Processa sp. on a meadow of Cymodocea nodosa Sea cucumber (Holothuria tubulosa) 

are also species preying on other invertebrates (Processa spp., Galathea spp.), 

some of which, like Dromia personata and Scyllarus arctos, come from nearby 

habitats. These animals are in turn preyed on by fish (mullet, scorpionfish) and 

cephalopods (common octopus, white-spotted octopus) and play an 

important role in the food-chain of Posidonia meadows. 

Among the most typical vagile animals found in rhizomes, the most abundant 

and frequent are echinoderms, especially sea urchins and sea cucumbers. The 

most typical sea urchin species are the common edible sea urchin 

(Paracentrotus lividus) and the purple sea urchin (Sphaerechinus granularis), 

whose spines exhibit colours ranging from deep purple to white. Both species 

graze on rhizomes (although the young of Paracentrotus may actually climb 

leaves in order to chew on them), removing debris and plant epiphytes and 

leaving their typical bite-marks on fascicles. 

Sea cucumbers are typically found along the margins and boudaries of 

meadows, clearings, intermatte channels and in debris mounds. They 

relentlessly devour sediment and fine leaf debris, from which they first obtain 

the energy required to survive, and then expel them in the form of 

characteristic sediment cordons compacted by their intestinal mucus. Their 

implacable feeding makes sea cucumbers the greatest bio-disturbers of 

meadows, as they dislocate great quantities of sediment daily. The most 

common species on Neptune grass are Holothuria polii and H. tubulosa. As 

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adults, these species do not have 

natural predators and live in a peculiar 

type of symbiosis with the pearlfish 

Carapus acus, which lives in their 

posterior intestine. 

Other occasional visitors to Posidonia 

meadows are the red starfish 

Red starfish (Echinaster sepositus) 

Echinaster sepositus, the spiny starfish 

Marthasterias glacialis, and the snake 

star Ophidiaster ophidianus, which are 

usually found on hard substrates 

associated with photophilic algae. 

Also the sand-burrowing brittle star 

Acrocnida brachiata and the small 

brittle star Amphipholis squamata find 

the ideal habitats for their cryptic life in 

the rhizomes of Posidonia meadows. 

All these organisms actively prey on 

bivalves and sea urchins associated 

with rhizomes and sediments. 

In systems formed by other small 

seagrasses, rhizomes are always 

hypogeal (growing under the surface), 

and therefore specific layers cannot be 

identified. Small seagrasses colonise 

Banded dye-murex (Hexaplex trunculus) incoherent sandy and muddy 

sediments (lagoons, estuaries and 

ports), and host many species typical of these environments. 

Epifauna associated with the seabed is therefore restricted, as is often the case 

in soft beds. There are few decapod crustaceans - except for some hermit 

crabs - and echinoderms - apart from the green sea urchin Psammechinus 

microturbeculatus and a few brittle stars. The gastropods Bolinus brandaris and 

Hexaplex trunculus are the most numerous mollusc species living on seahorse 

grass and common eelgrass. 

Limited research has also been carried out on the fauna associated with 

another small seagrass, Halophila stipulacea. Its fauna is very similar to that 

living on seahorse grass, which is influenced by local sedimentary and 

ecological conditions. The most abundant species are gastropods Bittium 

reticulatum and amphipods Caprella acanthifera and Gammarella fucicola. 

Infauna of mattes, clearings and swards. As described in the previous 

chapters, mattes are bioconstructions typical of Posidonia systems. They 

form as a result of the horizontal and vertical growth of this plant’s rhizomes, 

of sediments and organic debris accumulating on the bottom, and of 

entwining roots and rhizomes. 

Mattes are therefore particular substrates which have the characteristics 

of both hard substrates (roots, hypogeal portion of rhizomes, calcareous 

remains of organisms, pebbles, etc.) and soft ones. Their compactness 

and penetrability change according to the climatic conditions affecting 

the meadow in terms of sedimentation and wave action. Mattes may 

develop from a few centimetres to up to a few metres and depend on type 

of sediment; their degree of compactness may favour colonisation by 

fauna. 

The complexity and compactness of mattes deeply affect sampling and study 

of this habitat, which are particularly difficult. This is one of the reasons why 

matte fauna is the least studied and known of all areas of Posidonia. 

Fauna living in mattes belongs to infauna, i.e., benthic fauna living in the 

substrate. Mattes persist long after bundles and entire portions of 

meadows have died, and are thus known as dead mattes. Due to fine 

sediments and the compacting action of rhizomes, mattes are oxygenated 

only in the upper sediment layers, generally the top 5-10 cm: below this 

Soft venus (Callista chione), a bivalve mollusc typical of Posidonia mattes 

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level, conditions gradually become anoxic (lacking oxygen). This influences 

the fauna, which lives in the upper portion of mattes and dramatically 

decreases with depth. The only exceptions are some tube-dwellers and 

borers, which may penetrate the deepest layers and re-emerge outside, i.e., 

on the sediment surface. 

Matte infauna is mainly composed of polychaetes and a few other groups, 

like molluscs - especially bivalves - and a few decapods and echinoderms. 

There are also large meiofauna groups, like nematodes and harpacticoid 

crustaceans, especially in the top 1-2 cm of sediment. 

The polychaete species living in mattes are typical of sandy-muddy 

sediments, with burrowing detrivores feeding on sediment, like capitellids 

(lugworms) and maldanids (bamboo worms), and superficial detrivores like 

spionids, paraonids, cirratulids (fringe worms), lumbrinerids and nereids. 

More than 180 polychaete species live in mattes. 

Among molluscs, the most numerous are bivalves, which are typical of soft 

bottoms, and scaphods (tusk shells). Other animals that prefer to live in these 

environments are some edible species like warty venus (Venus verrucosa) and 

soft venus (Callista chione), which are harvested by means of fishing methods 

that destroy mattes and meadows. Ubiquitous species associated with 

mattes also include Plagiocardium papillosum, Tellina balaustina, Lucinella 

divaricata, Glans trapezia, Venericardia antiquata, and the tusk shell Antalis 

vulgare. 

Although gastropods are less frequent, some carnivorous species live partially 

burrowed in sediments and feed on bivalves, like Tectonatica filosa, Lunatia 

poliana and Nassarius (Hinia) incrassata. 

There is only one decapod species typical of mattes, Upogebia deltaura, a 

mud lobster that burrows deep, winding tunnels inside sediments. Although 

matte populations are not very different from those living in dead mattes, it 

would be worth improving the scanty research that has so far been carried 

out on them. 

Particular environments in Posidonia meadows are pockets of debris found in 

clearings and channels, discontinuous meadow structures generally 

surrounded by exposed, well-developed mattes. In these habitats, sediments 

are rather coarse and mainly composed of organic debris, i.e., deriving from 

the calcareous shells of organisms living in meadows themselves, like 

molluscs, echinoderms, bryozoans and corals. These are the erosion areas of 

meadows, with the strongest bottom currents and greatest wave action, 

which produce the typical ripple marks on the sea floor. Due to the particular 

dynamic conditions, debris accumulates in clearings and channels. Infauna 

living in these pockets of coarse sediment is made up of species generally 

found in coastal debris, the most impressive of which are bivalves of the 

genus Glycymeris and Tellina, the groove burrowing sea urchin Brissus 

unicolor and the purple heart sea urchin Spatangus purpureus, whose empty 

shells often lie on sediment. 

Fauna associated with the sediments of meadows formed of small 

seagrasses is composed of species characteristic of incoherent seabeds 

whose composition generally depends on the particle size of the 

sediments themselves. However, the presence of plants and hypogeal 

roots and rhizomes do influence the characteristics of sediments to a 

certain extent. 

For instance, surface meadows of mixed seahorse grass and dwarf eelgrass, 

with tufts of 2000 bundles/m 2 may produce a thick, compact step of 

sediment, roots and rhizomes, called a sward which, despite the shallow 

water, contains high percentages of mud. A few centimetres deep in swards 

(0-5 cm) conditions become anoxic. In this environment, fauna, mostly 

composed of polychaetes, lives in the superficial layer of sediment, and only 

a few species adapted to very low oxygen levels can survive at greater 

depths, like capitellids (Heteromastus filiformis, Capitella sp.) and the bivalve 

Lucinella divaricata, whose mantle tissues contain symbiontic 

chemosynthetic bacteria. 

Purple heart sea urchin (Spatangus purpureus), typically found in intermatte clearings 

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■ Sessile fauna 

Sessile animals are those living permanently attached to the substrate. In the 

sea, they are quite numerous on rocky bottoms and, more generally, on any 

substrate hard enough to enable them to adhere. On sandy and muddy 

seabeds, sessile organisms may be found only on the very small hard 

substrates (stones, shells, etc.) scattered on sediments. Seagrass leaves and 

rhizomes offer sessile fauna very particular substrates, which often require 

special adaptations. In particular, seagrass leaves fluctuate and continually 

renew themselves and, due to their small size, can only be colonised by 

minute organisms. Although rhizomes are slightly stabler, they are more 

selective than the rocky floors which sessile fauna usually inhabits. Due to 

their adaptations, these organisms living on seagrasses are called epiphytes, 

but are different from those found on other substrates. This is particularly true 

of Neptune grass leaves, less for its rhizomes and for other seagrasses, whose 

fauna is less characteristic. 

Seagrasses therefore play an important role in the ecology and evolution of 

sessile fauna. Hard, rocky substrates make up only a small portion of littoral 

seabeds, which are generally composed of sedimentary rocks, inhospitable 

habitats for these types of animals. Sessile organisms can colonise these 

environments only in two ways: by “jumping” from one small island of hard 

Electra posidoniae, an epiphytic bryozoan exclusive to Posidonia leaves 

substrate to another – avoiding direct 

contact with sediments - or by putting 

up with the hostile habitat and 

developing adaptations that prevent 

them from sinking into the sediment. 

The latter solution is used by large 

animals, like burrowing sea anemones 

and sea pens, and the former has been 

adopted by small, opportunistic 

species with short life-cycles. This, 

however, has led to ecological 

speciation for epiphytes of living 

organisms, like molluscs (some Epiphytic hydrozoans on Posidonia leaves 

hydroids, for instance, live exclusively 

on bivalves, gastropods, or hermit crabs in gastropod shells) or seagrasses. In 

the Mediterranean, Posidonia oceanica is the seagrass that forms the most 

extensive and stable meadows, thus offering good opportunities for avoiding 

sedimentary environments. It is not surprising then, that the only true 

specialised epiphytes live on Neptune grass. Generally, sessile species living 

on seagrasses are colonial: colonies are created by budding from a founding 

individual developing from a single, minute larva colonising the substrate. 

Colonies represent efficient strategies to monopolise suitable substrates, once 

they have been located. Seagrass epiphytes usually belong to several 

taxonomic groups, from the simplest, like protozoans, to the most highly 

evolved, like chordates. 

Protozoans are unicellular organisms that were classified as animals in the 

past and are now ascribed to other kingdoms: they are generally microscopic, 

although some species are visible to the naked eye. Most of those living on 

seagrasses belong to the class of forams (foraminiferans). 

Porifers or sponges are the most primitive animals, almost exclusively sessile 

and of various sizes and shapes. There are two types living on seagrasses: 

calcisponges, with a few, tiny species, and demosponges, which include most 

porifers. 

Cnidarians are a large group of animals with stinging cells (cnidocysts) and 

two different body forms: polyp and medusa. Polyps are generally sessile, and 

medusae, such as jellyfish, are free-swimming. Polyps may live alone or in 

colonies, according to species. On seagrasses, hydrozoans are the most 

numerous species (with polyps and, less frequently, with medusae), and there 

are also some anthozoans (corals). 

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Although most polychaete annelids are vagile, some are sessile and live in 

tubes that they secrete themselves and which secure them to the substrate. 

Tubes may have mucus, mud, parchment and rubber textures, like those of 

feather duster worms (sabellids), and may even be calcareous, like those of 

serpulids and spirorbids. Some of these families are occasionally found on 

seagrasses. 

Surprisingly, arthropods, which in the sea include mainly crustaceans, are 

represented here by sessile species, like barnacles (cirripeds) and the wellknown 

Poli’s stellate barnacle. They may also be found on seagrasses. 

Almost all bryozoans (also known as moss animals) are colonial, and each 

colony houses many tiny individuals called zooids. Colonies are sometimes 

very large and are typically sessile, encrusting or erect. Seagrasses host 

species of three orders: cyclostomes, with calcitic, tubular zooids; 

cheilostomes, with box-shaped calcitic zooids; and ctenostomes, with noncalcified 

sac-shaped zooids. 

Even chordates include sessile species: tunicates belonging to the ascidians 

(sea squirts). There are both solitary or “simple” and colonial species. The 

minuscule individuals that form their colonies are called ascidiozooids. If they 

are joined at the base by a single stolon, they are called social ascidians; if 

they are all enclosed in one tunic, they are known as compound ascidians. 

Some species colonise seagrasses. 

Aglaophenia harpago, an epiphytic hydrozoan, on Posidonia leaves 

Epiphytic fauna on Posidonia oceanica. Although several animals are 

epiphytic on the leaves of Neptune grass, only cnidarians and bryozoans include 

species which are so highly specialised that they cannot be found on any other 

substrate. They are called exclusive characteristic species. Their level of 

specialisation is such that they dominate over the many other leaf colonisers, 

and sometimes even make up 95% of the entire epiphytic population. As 

Neptune grass is a Mediterranean endemic, these species are also endemic, 

although their distribution is similar throughout the Mediterranean so that, apart 

from a few exceptions, epiphytic fauna exclusive to Neptune grass is always the 

same, independently of the geographic area within the Mediterranean. 

Bryozoans are generally the most numerous animals on the surface of leaves, 

with the exclusive species Electra posidoniae. This is a cheilostome forming 

ribbon-shaped, slightly calcitic, encrusting colonies. 

Other bryozoans exclusive to Neptune grass leaves are cheilostomes that 

produce encrusting colonies which are, however, round instead of ribbonshaped, 

like Collarina balzaci, Fenestrulina joannae and Ramphostomellina 

posidoniae. The last species is so far known to inhabit only the Aegean Sea, 

and may therefore be an exception to the general rule that epiphytic fauna on 

Posidonia is found throughout the Mediterranean. 

Many other bryozoans are found on leaves, although not exclusive to them. 

Among ctenostomes there is Mimosella verticillata, M. gracilis and Pherusella 

tubulosa. Cheilostomes are represented by species of the genus Aetea (A. 

anguina, A. lepadiformis, A. sica, A. truncata), Celleporina caliciformis, 

Chorizopora brongniartii, Fenestrulina malusii, Haplopoma impressum and 

Microporella ciliata. Cyclostomes are Disporella hispida, Patinella radiata and 

Tubulipora plumosa. 

Hydroids may also be quantitatively and qualitatively very important. Four 

species are typical and exclusive: Aglaophenia harpago, with feather-like 

colonies about 15 mm high, Orthopyxis asymmetrica (= Campanularia a.), with 

stolon-shaped colonies a few millimetres high, Pachycordyle pusilla, which 

also has stolon-like colonies with naked polyps, and Sertularia perpusilla, with 

erect colonies up to 7 mm high. A. harpago and P. pusilla are not precisely 

exclusive, as they have also been found on seahorse grass. There is also a 

Sertularella species that may be exclusive to Posidonia leaves, but experts still 

need to describe it formally. 

From the ecological-evolutionary viewpoint, an interesting case is afforded by 

Monotheca obliqua. This species has pinnate colonies as high as 40 mm, and is 

found on various substrates and on Posidonia leaves. Some morphological 

differences have led experts to distinguish a typica form - ecologically 

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distributed on hard sea floors between 5 and 30 m in depth - and a posidoniae 

form, exclusive to Posidonia leaves. Habitat specialisation may lead to genetic 

isolation of the two forms, and Monotheca posidoniae, as it is sometimes called, 

may be an instance of initial speciation. 

Several other species of hydroids are frequently found on Posidonia leaves, 

although these are not their favourite habitats. Among the most commonly listed 

species in literature there are Aglaophenia picardi, Antennella secundaria, 

Campanularia hincksi, Clytia hemisphaerica, Dynamena disticha, Eudendrium 

simplex, Halecium pusillum, Obelia dichotoma and O. geniculata. Hydroids 

typically found on Posidonia do not have a medusa phase, and this is viewed as 

an adaptation enabling them to stay inside the habitat for which they are 

specialised: medusae could easily float away with the current and end up in 

areas with no Posidonia. 

However, whenever dealing with biodiversity, there are odd exceptions, and one 

of these is the root-arm medusa Cladonema radiatum. Its polyps are joined in 

small, simple, slightly ramified colonies rising from a creeping hydrorhiza. 

Although they are not exclusive to Posidonia leaves, large numbers of these 

medusae are frequently found in these environments. Umbrella-shaped 

medusae about 3-4 mm wide are not free-swimming, and creep on leaves by 

adhering with special stalked buttons placed on their tentacles. Olindias 

phosphorica and Scolionema suvaense have similar adaptations. 

Anthozoans also have a typical species exclusive to Posidonia leaves: the small 

sea anemone Paractinia striata, whose polyp is brownish with longitudinal 

stripes. For the sake of precision, sea anemones should not be included in 

sessile fauna because, although their basal disc does adhere to the substrate, 

it is not secured in one place, thus enabling the animal to move ever so slightly. 

P. striata’s specialisation to life on Posidonia leaves is due to its large basal disc 

and flattened body, characteristics this animal shares with other typical sessile 

epiphytes. 

Other species are sessile forams like Cibicides lobatulus, Iridia serialis and 

Rosalina globularis. Although other animals colonise Posidonia leaves, 

sometimes even in large numbers, they are not characteristic of them. The most 

frequent are spirorbids (calcareous tubeworms) and sea squirts. 

The former include both clockwise (Pileolaria militaris, Simplaria pseudomilitaris) 

and anti-clockwise spiralling tubes (Janua pagenstecheri, Neodexiospira 

pseudocorrugata), and are generally smaller than 2 mm in diameter. 

Sea squirts always have representative compound species on leaves: the most 

common is Botryllus schlosseri, a colourful, jelly-like animal. Botrylloides leachi, 

which is more frequent on other seagrasses, is easily identified, because its 

ascidiozooids are patterned in a linear instead of rosette-shaped assemblage. 

Epiphytes on Posidonia leaves live on an ever-changing substrate, because 

although the leaves grow from their base, their tips break off continually, due 

both to mechanical actions and grazing by herbivores. New leaves grow inside 

the bundles, while the older, outer ones are gradually shed, and this influences 

the structure and dynamics of the epiphytic community. 

Typical exclusive species usually colonise internal and therefore the youngest 

leaves, to avoid competition with more aggressive species colonising the outer 

leaves, which are the oldest and also the most highly populated. This 

phenomenon has been observed for the bryozoan Electra posidoniae and for 

hydroids Aglaophenia harpago, Monotheca obliqua, Orthopyxis asymmetrica 

and Sertularia perpusilla. Experimental studies reveal that the planulae (very 

young larvae) of these hydroids colonise only green leaves, i.e., those which are 

not already inhabited by other organisms and are therefore young. 

Similarly, typical exclusive species colonise leaves starting from their base, so 

as to occupy new areas of the leaf as they form. This is true of the bryozoan 

Fenestrulina joannae and hydroids Monotheca obliqua, Orthopyxis 

asymmetrica and Sertularia perpusilla. However, the exclusive hydroid 

Aglaophenia harpago is generally found on leaf tips, like the non-exclusive 

bryozoan Aetea truncata. Central leaf parts are colonised by the exclusive 

bryozoan Electra posidoniae and by the non-exclusive hydroid Antennella 

Root-arm medusa (Cladonema radiatum) 

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secundaria. Leaf tip colonisation by 

non-specialised species is an example 

of acrophilia, i.e., the tendency of 

passive filterers to colonise positions 

that give them easy access to food 

suspended in the water column. To 

monopolise the available surface more 

rapidly, the colonies of many species 

develop parallel to the leaf margins, so 

Bryozoans 

that they can expand along the length 

of the leaves. This is clearly visible in 

the case of Electra posidoniae and Sertularia perpusilla. 

There are also differences in colonisation between the two sides of the leaf 

blades, that on the outer side generally being slower. Perhaps this is due to the 

fact that the outer sides are less protected and more exposed to abrasion, which 

may jeopardise the adherence and development of colonising organisms. 

Many hydroids, both exclusive (Monotheca obliqua, Orthopyxis asymmetrica) 

and non-exclusive (Antennella secundaria), prefer the inner side, like some nonexclusive 

bryozoans (Aetea, Mimosella). The exclusive hydroid Sertularia 

perpusilla apparently prefers the outer side. When colonising flexible and 

ephemeral substrates like those offered by Posidonia leaves, some adaptations 

are important, and are usually found in exclusive species. 

Substrate flexibility inevitably requires epiphytic colonies - generally long and 

extensive - to be just as flexible. And this is undoubtedly a problem for 

cheilostomes: it is no surprise that the zooids of Electra posidoniae are only 

slightly calcified, and the delicate peduncles of the erect zooids of Aetea anguina 

and A. sica have special rings that make them flexible. Fenestrulina joannae is an 

encrusting species with calcitic zooids that develops better in deep, calm 

waters, where wave action is restricted. Conversely, continual leaf fluctuation 

requires epiphytes to be robust, as they may otherwise become easily worn by 

continuous mechanical action, or scratched by leaf rubbing, and therefore 

hydroids have particular thickened or reinforced structures, especially when 

living in environments affected by strong wave action. 

Due to leaf shedding, the lives of hydroids colonising Neptune grass are shorter 

than those living on other substrates. The outer leaves become brown and fall 

away, and the hydroids living on them are short-lived too, because shed leaves 

roll and accumulate on the bottom, where environmental conditions are very 

hostile. Precocious reproduction is the only means for maintaining the species. 

Sexual reproduction is unusual, and species rely on vegetative reproduction. 

One of the most abundant and 

strictly epiphytic species, Orthopyxis 

asymmetrica, has a very short sexual 

reproduction period in summer, with 

the production of a few gonothecae 

(sexual organs). Conversely, like other 

hydroids, it has well-developed stolons: 

a suitably modified part of the colony 

can grasp a nearby leaf and adhere to 

it, detaching itself from its original Epiphytic hydroids on Posidonia leaves 

colony. These propagatory stolons are 

produced in great quantities in all seasons. In Monotheca obliqua, O. 

asymmetrica and Sertularia perpusilla, the stolons look like flexible elongations 

with rounded tips; in Aglaophenia harpago and in the still undescribed 

Sertularella species, they have more complex shapes, hooked and claw-like, 

respectively. Widespread stolonisation gives rise to concentration, and species 

may be abundant in one area and totally absent in another nearby. 

Persistence and restricted reproduction enable many Posidonia epiphytes to be 

found all year round. This is the case of some hydroids (Aglaophenia harpago, 

Monotheca obliqua, Sertularia perpusilla) and bryozoans (Mimosella verticillata, 

Chorizopora brongnartii, Electra posidoniae). Just as many, however, are present 

only in certain seasons: spring (Clytia hemisphaerica, Disporella hispida, 

Dynamena disticha, Pachycordyle pusilla), summer (Eudendrium simplex, 

Halecium pusillum, Paractinia striata) and autumn (Orthopyxis asymmetrica), 

although there is no apparent correlation between seasons and specialisation 

levels for Posidonia leaves. We must also emphasise the fact that the very few 

studies on this subject do not allow us to draw final conclusions. 

Leaf areas vary with depth, and are larger near the surface. Similarly, epiphytic 

communities also vary quantitatively and qualitatively with depth. Epiphytes are 

more abundant in shallow water, although the number of species is smaller. 

Larger numbers of species in deep water are mostly composed of ones with 

great ecological tolerance, and species typical and exclusive to Posidonia are 

more numerous at low and intermediate depths. This observation supports the 

theory that P. oceanica originated in shallow water and is today found at greater 

depths only because the sea level fell during the last glaciation. More 

specifically, Aglaophenia harpago, Collarina balzaci, Electra posidoniae, 

Orthopyxis asymmetrica and Sertularia perpusilla prefer depths of less than 15 

m. Monotheca obliqua has wide bathymetric distribution, and species that live 

at greater depths are Antennella secundaria, Clytia hemisphaerica, Halecium 

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pusillum and Pachycordyle pusilla. However, these general tendencies may be 

contradicted locally, if affected, for instance, by strong wave action. The 

bathymetric distribution of epiphytes is mainly due to wave action, and this is 

why many hydroids colonise the protected base of leaves when in shallow 

water and the more exposed leaf tips in deep water. 

Study of epiphytic fauna on P. oceanica is associated with very interesting 

scientific aspects concerning issues of adaptation and evolution, which are 

important in applied ecology. Epiphytes as indicators are sensitive to natural 

and anthropic disturbances, and are affected by environmental alterations due 

to the deterioration of water quality sooner than the plant hosting them. 

Epiphyte fauna of the rhizomes of Posidonia oceanica. The epiphyte fauna 

living on the rhizomes is more heterogeneous and generalist than that on the 

leaves, being composed of species which also populate the hard littoral and 

circumlittoral substrates. It is thus a community with high specific richness but 

generally lower abundance. With rare exceptions, the species differ from those 

on the leaves. Nonetheless, hydroids and bryozoans are also the dominant 

groups on the rhizomes. 

The most common hydroid is Sertularella ellisii (= S. gaudichaudi). Its colonies, 

which may reach a height of 50 mm, have erect simple or branched 

hydrocauli, with a characteristic zigzag pattern. S. ellisii is also found in a 

The bryozoan Margaretta ceroides 

variety of other environments, from algal beds to coral and underwater caves, 

and from the surface to a depth of about 100 m. Depending on habitat, it has 

an extremely varied appearance - to the extent that several different varieties 

were described in the past. The many other species of hydroids found on the 

rhizomes include Cladocoryne floccosa, Kirchenpaueria pinnata, Sertularia 

distans and Aglaophenia picardi - the last two species being among the few 

examples of hydroids which colonise both rhizomes and the basal part of 

leaves. 

As well as in rocky environments, many species of bryozoans are found on 

detritus beds with small hard substrates scattered in the sediment. Their 

abundance on the seagrass rhizomes should therefore come as no surprise, 

as they are generally affected by sedimentation to a varying degree. If the 

sedimentation level is high, some ctenostomes may be abundant, such as 

Nolela stipata, N. dilatata, Bowerbankia imbricata, Amathia lendigera and 

Pherusella tubulosa - the last also being common on the leaves. The small 

cheilostomatan Aetea truncata may be found on both leaves and rhizomes, 

but it is mainly the large cheilostomatans which characterise the rhizomes, 

especially where sedimentation is not excessive - these are again species 

more often found on coral and/or detritus beds. One of the most significant 

examples is Margaretta cereoides, with its pinkish or beige tree-like colonies 

up to 5 cm tall, extensively branched and weakly attached to the substrate; 

the branches are stick-like and are connected by a short narrow horny 

peduncle that makes them supple. As well as living at the base of seagrass 

rhizomes this species is also found on detritus beds, mostly at depths of 

between 10 and 50 m. 

Reteporella grimaldii prefers beds of coral. Its colonies, up to 10 cm in height, 

are composed of erect laminar expansions, wavy and folded in varying ways; 

the laminae are reticulated like nets and are exceedingly fragile. They are 

brilliant pink or orange in colour, but quickly fade when dry. Turbicellepora 

magnicostata and Calpensia nobilis have encrusting colonies which may form 

a thick cover around the rhizomes; the latter species is more abundant in the 

southern Mediterranean. 

Miniacina miniacea is a small (about 1 cm) colony-forming species which at 

first sight may be mistaken for a bryozoan instead of a foraminiferan. Its pinkcoloured 

colonies are irregularly branched in shape. They are found on coral 

and in caves and other rocky environments, but on seagrass rhizomes they 

may be so abundant that the calcareous skeletons of dead colonies 

occasionally pile up in large quantities on the beaches facing large seagrass 

meadows, forming eye-catching pink bands on the foreshore. 

83

84 

The tubicolous polychaete Sabella spallanzanii 

Among the other epiphyte invertebrates 

on Posidonia oceanica rhizomes, the 

poriferans, or sponges, have the 

largest number of species. Despite 

this, the association living in seagrass 

meadows is heterogeneous, and 

represented by an impoverished 

selection of those of the hard littoral 

beds: for example, the insinuating and 

endolithic species are missing. There is 

therefore no typical poriferan species of 

seagrass meadows. Among the more 

frequently found species are some The sea anemone Alicia mirabilis 

calcisponges, such as Leucosolenia 

botryoides and L. variabilis, which have the appearance of branched and whitish 

tubular nodules, and the barrel-shaped Sycon raphanus. Thanks to their small 

size (1-2 cm) they may also sometimes settle on leaves, especially in waters rich 

in suspended solids. Among the numerous demisponges, Mycale contarenii 

appears to prefer shallow depths and has also been found on leaves. 

Hymeniacidon perlevis is also an encrusting species, and is reddish-orange in 

colour. A common yet curious species is Chondrilla nucula, with its appearance 

of spherical or long cushions, united in groups that may be over 20 cm long. 

These cushions have a smooth surface and vary in colour between purple and 

greenish-brown, because of the presence of photosynthetic cyanobacteria 

symbionts (zoocyanelles). Lastly, Calyx nicaeensis is worth mentioning: it has a 

typical cup shape, is 5-20 cm tall, of fibrous consistency and brown in colour. 

Very common until the 1960s and 1970s, it has now become extremely rare, The 

reasons for this are unknown, but are perhaps linked to a disease. 

The anthozoans living on Neptune grass rhizomes include three species which 

prefer deeper meadows. The first is the sea anemone Alicia mirabilis, with its 

powerful sting: the contracted polyp has the appearance of a small nodule, 

but when it is expanded it may reach more than 50 cm in height, exhibiting a 

yellowish-green column with clusters of yellow or orange vescicles and 

numerous tentacles. The small specimens may also adhere to the leaves. The 

second is the gorgonid Eunicella singularis, with its erect colonies, up to 50 cm 

tall, and with narrow, parallel branches which give it its characteristic 

candelabra shape; it is dirty-white or yellowish-grey in colour, due to the 

presence of micro-algal symbionts (zooxanthellae). The third and last species 

is the colony-forming madrepore Cladocora caespitosa. The colonies form 

85

86 

large, globular, encrusting cushions around 20 cm in diameter on average; the 

polyps are greenish-brown in colour, again in this case due to zooxanthellae. 

While the vast majority of sessile animals feed by filtering or capturing 

particles of organic matter suspended in the water, the latter two species, like 

the previously-mentioned Chondrilla nucula, are able to integrate their diet 

with the products of their micro-symbionts, which mainly provide 

carbohydrates, while filtering supplies proteins and lipids. 

The sessile polychaetes that are more easily found on seagrass rhizomes 

belong to the sabellid and serpulid families. The former are represented by a 

species that is well-known to all scuba-divers and aquarium lovers: Sabella 

spallanzanii, with its characteristic plumose gills coiled in spiralling bright 

colours - commonly yellowish-brown, striped with purple, brownish-orange, 

blue, and sometimes white. Two other common species are Bispira mariae, 

with its two gill lobes coiled in a spiral, and Sabella pavonina, with funnelshaped 

gill tufts, not spiralled. Various serpulids, none of them characteristic, 

may settle on rhizomes: among the most eye-catching species are Serpula 

vermicularis, with its rugose or often carenated pink or yellowish tube, around 

5 cm long; Protula tubularia, with its smooth cylindrical tube, pure white and 

up to more than 20 cm in length; and Salmacina dysteri, which usually 

appears as friable clusters of tiny smooth white tubes, each one a maximum 

of 6-7 mm long. 

The colonial tunicate Aplidium conicum The colonial tunicate Didemnum fulgens 

A cirripede which may occasionally be found on seagrass rhizomes is Verruca 

spengleri (in the past confused with V. stroemia, a similar species from outside 

the Mediterranean). It is identified by its irregularly shaped greyish or brownish 

shell about 5 mm in diameter. 

Various species of ascidians may colonise seagrass rhizomes. Compound 

ascidians are represented both by massive forms like Aplidium conicum or, 

more commonly, by encrusting forms such as Diplosoma listerianum, 

Didemnum fulgens and D. coccineum. The social ascidian Clavelina 

lepadiformis may occasionally be found, and is extremely widespread in 

various other environments. Among the simple ascidians, Phallusia 

mammillata and Halocynthia papillosa are quite common. The former, 

commonly called sea squirt, has a thick cartilaginous tunica of a pale greyishwhite 

colour, and is up to more than 15 cm in length. The latter, called sea 

potato, is slightly smaller and is carmine red above and yellow-orange below. 

Epiphyte fauna of other phanerogams. The epiphyte fauna of the other 

phanerogams has many fewer and rarer species than that of Posidonia 

oceanica: it is almost impossible to find a belt of Neptune grass without 

epiphytes, whereas this situation is common for the other phanerogams. 

Perhaps also for this reason, knowledge of their epiphytes is scarce: most 

studies have been carried out on Cymodocea nodosa, something is known 

87

88 

about Nanozostera noltii, very little 

about Zostera marina, and nothing at 

all on the Mediterranean specimens of 

Halophyla stipulacea. 

Contrary to what has been described 

for P. oceanica, there are no truly 

exclusive species on the leaves of the 

other marine phanerogams, but at most 

preferential ones. It is significant that 

The hydroid Laomedea angulata 

even when P. oceanica forms mixed 

meadows with other phanerogams, the 

relative epiphyte populations remain separate, composed of various species, 

demonstrating stringent parallelism in their morpho-functional adaptations to 

the environmental gradients, first and foremost hydrodynamics. The rhizomes of 

other phanerogams are generally rather inconspicuous compared with those of 

Neptune grass, and are often completely or partly buried in the sediment. They 

are therefore scantily colonised, often by the same species as those found on 

the leaves. In these cases, the phenomenon of dwarfism may occur, with a 

reduction in the sizes of the colonies on the leaves compared with those on the 

rhizomes: the best examples are found among the hydroids, particularly in 

colonies of Eudendrium, which maintain their erect bearing on rhizomes, 

whereas on leaves they are prostrate and lack vertical branching. 

Another important ecological aspect is the fact that other phanerogams, 

especially Z. marina and N. noltii, are more euryhaline than P. oceanica and 

can therefore penetrate into brackish lagoons: the epiphyte fauna may thus be 

composed of species typical of these environments, already adapted to living 

in severe environments and capable of colonising various types of substrate. 

Despite all these differences, the principal groups of epiphytes are once again 

cnidarians and bryozoans. The hydroids are without doubt the best studied 

group. The species counted on various phanerogams are relatively numerous, 

so that a few general comments are possible. 

The largest number of epiphyte species have been found on Cymodocea 

nodosa (which is the most frequently studied species). Also worthy of mention 

are Aglaophenia harpago and Pachycordyle pusilla, two species which live 

mainly on the leaves of P. oceanica and only occasionally on C. nodosa. The 

majority of other species have a wider ecological range: many are common 

fouling constituents, such as Clytia hemisphaerica and Obelia dichotoma, or 

are epibionts of various organisms, like Campanularia volubilis. The largest 

number of species, and also the most abundant, have been observed in 

autumn; a change in the dominant 

species with season has also been 

found: Clytia hemisphaerica in winter 

and spring, Campanularia volubilis in 

summer, and Laomedea angulata in 

autumn. L. angulata is the only hydroid 

which shows a certain preference for 

seagrass leaves, while the closely 

related L. calceolifera prefers rocky 

beds: however, the two species have The sea anemone Paranemonia cinerea 

occasionally been found together on 

phanerogams. L. angulata is the only one found on all studied phanerogams, 

but it is particularly frequent on C. nodosa. It has unbranched colonies, around 

1 cm in height, sub-transparent to white in colour, and exhibiting a stolon 

regenerating capacity comparable to that of the species exclusive to P. 

oceanica. Another species found on various phanerogams, although more 

common on other substrates, is Ventromma halecioides, with its pinnate and 

erect colonies which may be more than 10 cm in height, but which remain 

smaller and more delicate on seagrasses. 

When phanerogams penetrate brackish waters, the above-mentioned species 

may be joined by Pachycordyle napolitana, the colonies of which, smaller than 

1 cm, can also be recognised on the typical lagoon phanerogams of the genus 

Ruppia. If salinity is particularly low, the only epiphyte hydroid found (for 

example, on N. noltii) is Cordylophora caspia, which is similar to the previous 

species but able to tolerate almost fresh water, where it colonises the leaves of 

Potamogeton with unbranched colonies, 0.5-1 cm tall. As well as hydroids, 

another cnidarian may be found on lagoonal phanerogams which offers a cue 

for illustrating a case of ecological vicariance: the anthozoan Paranemonia 

cinerea. This is a small sea anemone (around 1 cm in diameter and 6 mm in 

height), whose olive-green colour with lighter longitudinal stripes allows it to 

achieve excellent camouflage on the leaves of Zostera and Ruppia on which it 

settles. On these phanerogams, P. cinerea assumes the same role that 

Paractinia striata has on Posidonia. 

The case of the bryozoan Electra pilosa is even more interesting, as it replaces the 

closely related E. posidoniae on Cymodocea and Zostera, occasionally also 

penetrating into lagoons. Another encrusting cheilostomatan, Tendra zostericola, 

which prefers to colonise the leaves of Zostera, is more strictly lagoonal. 

Spirorbids and ascidians also occasionally colonise other phanerogams, often 

with the same species as those found on Neptune grass leaves. 

89

Fauna: vertebrates 

PAOLO GUIDETTI 

■ Introduction 

Tackling the subject of vertebrates 

associated with marine phanerogams 

essentially means discussing the fish 

fauna associated with the submerged 

meadows that these plants form in 

coastal habitats. 

There are no large herbivorous 

mammals or reptiles (such as dugongs 

or turtles) living in close association 

with and feeding on seagrass 

meadows in the Mediterranean, as Black seabream (Spondyliosoma cantharus) 

happens in other parts of the world. 

The fish fauna particularly associated with Posidonia oceanica has been 

studied by many marine biologists, mainly along the French, Italian and 

Spanish coasts. But the fish fauna associated with other marine phanerogams 

has received little attention, although interest has been growing in recent 

years. 

As mentioned in the previous chapters, P. oceanica differs from the other 

Mediterranean marine phanerogams - the most common are Cymodocea 

nodosa and Nanozostera noltii, which may form mixed meadows at shallow 

depths. It has greater structural complexity (e.g., in terms of three-dimensional 

structure and height of leaf fronds, the system of rhizomes and mattes, variety 

of substrates), the wider bathymetric range within which the meadows grow, 

the type of environments colonised (typically marine for P. oceanica, and 

sheltered ones, including coastal lagoons with brackish waters, for the other 

phanerogams). Also, P. oceanica has different time dynamics on an annual 

scale: it substantially maintains its architecture, whereas the leaf cover and 

density of C. nodosa and N. noltii are reduced during winter, sometimes 

drastically. As may be easily imagined, this all influences the composition and 

annual dynamics of the associated fish population, as well as the role these 

Salema (Sarpa salpa) 

91

Blotched picarel (Spicara maena) 

seagrass systems may play during 

some crucial stages in the life cycle of 

many species of fish (e.g., hosting the 

juvenile stages). 

Before describing the fish fauna 

associated with Mediterranean marine 

phanerogams, it should be noted that 

the term “associated” is used here to 

define the fish species that may be 

found, in higher or lower numbers, in 

seagrass systems and which may 

frequent them for different purposes. 

The species of fish defined as 

“associated” choose between different 

habitats in some way, selecting 

seagrass meadows for specific uses 

during their juvenile and/or adult 

stages, e.g., as a habitat in which to 

prey or graze, hide from predators, 

breed, etc. The wide variety of “uses” 

these fish make of the seagrass 

systems explains the large number of 

species that may be found there. This 

means that seagrass systems make an 

essential contribution to the 

Two-banded seabream (Diplodus vulgaris) 

maintenance of fish biodiversity in the 

Mediterranean coastal environment. 

The wide variety of fish species associated with Mediterranean marine 

phanerogams is due to the level of dependence on seagrass meadows and 

the exploitation of space: there are species which populate the water column 

above the leaf cover, species which swim just above or among the leaves, yet 

others which frequent the base of the plants, on the rhizomes or live on the 

substrate on which the plants grow. From the trophic point of view, seagrass 

meadows host herbivores, detrivores (or rather, fish which also feed on 

detritus) and carnivores of different orders and types (e.g., piscivores, and 

those which feed on zooplankton), which may stably frequent seagrass 

meadows or just visit them for brief excursions in search of prey. Others follow 

a more precise nychthemeral (day/night) cycle, and so may be observed in the 

P. oceanica meadows mainly or exclusively at night or during the day. 

■ Fishes associated with seagrass meadows 

92 93 

The species of fish which commonly occupy the column of water above the 

meadows of P. oceanica includes damselfish (Chromis chromis), a small 

planktivorous fish which may form huge shoals by day. At night, the adults 

seek refuge among the seagrass leaves. 

Although the most suitable environment for nocturnal hiding-places are the 

rocky infralittoral beds, with their many crevices, damselfish (juveniles are an 

electric-blue colour) may also find adequate shelter among the meadows of 

P. oceanica. As well as the damselfish, picarel (Spicara smaris), blotched 

picarel (S. maena), bogue (Boops boops) and saddled seabream (Oblada 

melanura), all essentially planktivorous fish, are also frequently observed in 

the water column above P. oceanica meadows. In sites particularly sheltered 

from waves, it is also possible to observe shoals of whitebait (Atherina sp.) 

above Posidonia leaves. 

The fishes that may be found on seagrass meadows also include the mullets 

(such as Liza aurata), which have a diet at least partially composed of 

detritus. Some researchers believe that mullets mainly frequent P. oceanica 

meadows in summer and then abandon them in winter, but this does not 

appear to be true everywhere. Mullets may swim in open water, sometimes 

just slightly below the surface, but they can also be observed on the seabed, 

Seabream (Dentex dentex)

especially at points where the detritus of P. oceanica and its associated 

invertebrates tend to accumulate (e.g., in channels or sandy hollows close to 

meadows themselves). 

The common dentex (Dentex dentex), one of the largest and most efficient 

predators of the Mediterranean coastal systems, is one of the biggest fishes 

that it is possible to find above the mantle of P. oceanica, and it is sometimes 

found in shoals of dozens of individuals. Seabream may reach 1 m in length, 

are very gregarious, and essentially piscivorous (i.e., they feed mainly on other 

fish). The sudden arrival of a shoal of seabream above the meadows - as 

occurs with other large predators - is often heralded by a mass dash for safety 

by the small fish (e.g., damselfish, bogue) swimming in the water column. 

It is also not rare to observe other species of predators in P. oceanica 

meadows, such as the barracuda (Sphyraena viridensis and S. sphyraena), 

greater amberjack (Seriola dumerili) and sea bass (Dicentrarchus labrax). 

Barracuda usually pause in a shoal above the seagrass leaves, they are 

considered to have an affinity for warmer waters and their increase in many 

areas of the Mediterranean is imputed by many scientists to warming of the 

seas. The two species of barracuda differ, as has recently been shown by 

researchers at the University of Genoa, by both the maximum size they reach 

and their colours. Sphyraena viridensis may grow to more than a metre in 

length, whereas S. sphyraena barely reach more than half a metre. The former 

also has a series of unmistakable dark 

transversal stripes on its body; the 

latter has a dark back and silvercoloured 

underbelly. 

Greater amberjack are in constant 

motion, almost always in shoals, 

especially when they are small. Sea 

bass are also always on the move in 

search of prey above the leaf mantle of 

Posidonia. They frequent not only Rainbow wrasse (Coris julis) 

seagrass habitats, but are also found in 

rocky environments, on sandy/muddy 

substrates, in areas exposed to the 

movement of waves, and equally in 

sheltered areas, brackish waters, and 

even inside harbours. 

Many species of fish live more closely 

associated with the leaf mantle of P. 

oceanica. These are species which 

swim slightly above and/or among the Ornate wrasse (Thalassoma pavo) 

leaves, such as many of the labrid 

family. These include the brown wrasse (Labrus merula) and green wrasse (L. 

viridis), among the largest of the Mediterranean wrasses, which may assume a 

wide variety of livery, but always tend to have a greenish livery when they live 

associated with P. oceanica. They grow to a maximum length of around 45-50 

cm, and have predatory habits (they feed on invertebrates and, to a lesser 

extent, small fish). There are also species like the peacock wrasse 

(Symphodus tinca), Mediterranean rainbow wrasse (Coris julis), Symphodus 

ocellatus and, especially in the more southerly parts of the Mediterranean, 

ornate wrasse (Thalassoma pavo). These are smaller than those of the genus 

Labrus (from 40 cm maximum length in the peacock wrasse to 12 cm in S. 

ocellatus) and characterised by pronounced differences in livery, size and 

morphology of the body in relation to sex (sexual dimorphism). Males are 

generally larger and brighter in colour, especially during the breeding season. 

A quite common labrid on P. oceanica is also the blacktailed wrasse 

(Symphodus melanocercus). This small fish (maximum length 14 cm) is a 

characteristic “cleaner” which interacts with many other species of fish, 

including the peacock wrasse, various sea breams (sparids of the genus 

Diplodus) and small serranids like the comber (Serranus cabrilla) and painted 

94 95 

Barracuda (Sphyraena viridensis)

comber (S. scriba). The blacktailed 

wrasse, like all cleaner fish, feeds on 

ectoparasites, mucus, scales, infected 

tissue and food residues, which it 

removes from the body of another fish. 

A particular spot where these cleaning 

labrids are “invited” to work by other 

fishes is called a “cleaning station”. 

Specific rituals are followed: a fish 

which wants to be cleaned invites the 

cleaner by assuming a particular 

posture, e.g., placing itself semi- 

Salema (Sarpa salpa) 

vertically or vertically, with its head 

down or up, immobile, often with its 

mouth wide open and all fins and gills open. 

Other smaller labrids (between 12 and 18 cm in length, depending on species) 

that may commonly be found on P. oceanica, again belong to the genus 

Symphodus and are essentially carnivorous fishes which feed on small vagile 

invertebrates (e.g., echinoderms, molluscs, polychaetes and crustaceans), 

which they find among the leaves, on the rhizomes and in the sediment. These 

include axillary wrasse (Symphodus mediterraneus), five-spotted wrasse (S. 

roissali), S. doderleini and S. rostratus. Instead, the grey wrasse (S. cinereus) is 

commonly found on the stretches of sandy bottom next to P. oceanica or 

where leaf litter accumulates close to the meadows. Both S. rostratus and grey 

wrasse may assume various liveries, but are often a pale green colour when 

they live in seagrass meadows. 

Many species of sparids are associated with the leaf mantle of P. oceanica. 

First and foremost is the salema (Sarpa salpa), which is the most important 

herbivore (at least as an adult) in the Mediterranean littoral system. The 

salema, which may reach a maximum length of 50 cm, often form shoals 

composed of hundreds of individuals. This gregariousness is to be found 

both in juveniles (which have an omnivorous diet) and in adults. Like many 

terrestrial herbivorous vertebrates, the salema move around in groups, and 

the moment an individual detects the possible presence of a predator, 

including a human being, the entire shoal quickly flees in a coordinated 

movement. Although the salema are also found in rocky environments (where 

they feed on microalgae), it is common to observe them grazing on P. 

oceanica, leaving the classical half-moon sign of their bite on the leaves (see 

figure on page 41). 

It is still not entirely clear, as the salema 

often graze on the epiphyte-bearing 

tips of the leaves of P. oceanica, if their 

diet is essentially herbivorous - taking 

into account that the living plant 

tissues of P. oceanica do not have a 

high energy value - or if it also includes 

a reasonable proportion of epiphytes, 

i.e., the plants (algae) and animals of 

the sessile benthos that colonise, grow 

and live on P. oceanica leaves. These 

epiphytes are particularly abundant on 

the tips of the oldest leaves. There is no Annular seabream (Diplodus annularis) 

doubt, however, that salema grazing 

can be intense enough to determine differences in the average height of the 

leaves of P. oceanica among the meadows where they are abundant (and large) 

and those where they are more or less absent. 

Among the sparids of the genus Diplodus (which includes fish commonly 

known as seabream), the one which is associated more closely to P. oceanica 

than all the others, is definitely the annular seabream (D. annularis). This is the 

smallest of the seabreams (with a maximum length of 25 cm) and is silverycoloured 

with greenish-yellow tinges, which camouflages it very well amongst 

the P. oceanica leaves. It feeds on small vagile and sessile invertebrates that it 

finds on the leaves. The other seabream, i.e., white seabream (D. sargus), 

sharpsnout seabream (D. puntazzo) and two-banded seabream (D. vulgaris) 

are often found in association with P. oceanica, although they are also very 

frequent on rocky sea-beds. These three species have a much higher 

commercial value than the annular seabream and grow much larger (the white 

and common two-banded types reach 45 cm in length, the sharpsnout may 

even reach 60 cm). Although their diets are not entirely the same (especially for 

sharpsnout), all three feed on invertebrates - in some cases also relatively 

large ones, such as adult sea urchins. 

Another sparid which is found typically associated with P. oceanica is the black 

seabream (Spondyliosoma cantharus), which may reach 60 cm in length, and 

which feeds on invertebrates, including jellyfish. Although the largest specimens 

are mainly found in rocky areas, juveniles and medium-sized adults may often 

be observed swimming in the open waters above Posidonia meadows. The 

sparids which may be found on P. oceanica once again include the gilthead 

seabream (Sparus auratus) and common pandora (Pagellus erythrinus). 

96 97

Another species commonly associated 

with P. oceanica is the brown meagre 

(Sciaena umbra). This fish, which may 

become very large (around 70 cm in 

total length), is often gregarious and 

feeds mainly on small invertebrates. 

Usually not very mobile because it 

stays in groups near shelter, if 

disturbed, it hides in rocky crevices or 

among the seagrass leaves, and even 

emits sounds. 

In the more southern parts of the 

Mediterranean, it is possible to observe Brown meagre (Sciaena umbra) 

parrotfish (Sparisoma cretense) 

swimming alone or in small groups in the vicinity of the leaf mantle of P. 

oceanica. This fish, which has a robust beak, has a diet which often includes 

leaves of P. oceanica with epiphytes on the tips. As in the case of the salema, in 

energy terms the importance of epiphytes with respect to plant tissues is still 

not well understood, as in rocky environments Mediterranean parrotfish graze 

on calcareous algae (such as Halimeda tuna, which often also bear epiphytes) 

and on small sessile invertebrates with calcareous structures (e.g., bryozoans). 

The dusky grouper (Epinephelus marginatus) is a large sedentary and territorial 

predator, which can grow to well over a metre in length and live for 50 years. It 

may be observed on P. oceanica, especially when the meadows are 

interspersed with rocky substrates which offer adequate refuge. The dusky 

grouper and brown meagre are much sought-after by spearfishermen. Their 

tendency not to escape quickly into open water, but to take refuge in hidingplaces 

close to the areas where they have been spotted mean that they are at 

particular risk of local extinction in areas where underwater fishing is 

particularly intense. This situation has prompted countries like France to 

declare a moratorium on these fish, which involves a ban on their capture by 

non-professional fishermen. 

As well as sea bass, P. oceanica meadows are frequented by another two much 

smaller serranids (a maximum of around 35 cm), already mentioned above 

because of their frequent association with the cleaner labrids. These are the 

painted comber and comber, which are also territorial piscivorous species (given 

their smaller size, their attention turns to small fish species or, in most cases, to 

juvenile forms). The brown comber (Serranus hepatus) is only reported in some P. 

oceanica meadows of the central-northern Adriatic, along the Croatian coasts. 

98 99 

Female parrotfish (Sparisoma cretense)

100 

The striped red mullet (Mullus surmuletus) may often be found in P. oceanica 

habitats as well as on rocky bottoms mixed with sand. Striped red mullet may 

be observed swimming above the leaf mantle of Posidonia, but it is certainly 

easier to find them intent on searching for the small invertebrates on which 

they feed in the sandy patches inside Posidonia meadows. They are often 

followed by a retinue of other species, like wrasse and small seabream, which 

exploit the powdery drifts raised by the red mullet to snatch any small 

invertebrate that becomes available. The red mullet (Mullus barbatus), instead, 

is more commonly associated with sandy/muddy substrates, although some 

authors report it as being present (but not abundantly, and mainly in juvenile 

stages) in the deeper meadows of Posidonia. 

The conger eel (Conger conger) and moray eel (Murena helena) are typically 

associated with rocky substrates with abundant crevices. At night, these 

voracious predators move towards the meadows of P. oceanica situated near 

the rocky seabed in search of prey. They swim at the base of the leaf fronds, 

close to the substrate. 

Other species which are to be found mainly at night in Posidonia meadows are 

the small gadids, such as the shore rockling (Gaidropsarus mediterraneus) and 

three-bearded rockling (G. vulgaris), ophidiids such as Ophidion rochei and 

Parophidion vassali and, lastly, the dogfish or smaller-spotted catshark 

(Scyliorhinus canicula). 

Syngnathus typhle, an example of perfect camouflage 

Some fish species have evolved on the basis of their association with P. 

oceanica. More specifically, these are the pipefish (Syngnathus acus, with a 

more pointed snout, and S. typhle, with its higher snout and more vertical 

profile), which feed mainly on small vagile invertebrates. These fish may be 

either pale green or brown, in this way imitating the colours of the leaves of 

living Posidonia (green) and dead (which become brown in colour). In order to 

camouflage themselves even better, the pipefish often assumes a vertical 

position, parallel to the Posidonia leaves, and undulates with the leaves set in 

motion by the waves or currents. Other famous representatives of the 

Syngnathidae which may be observed in Posidonia meadows, often attached 

by their tails to the rhizomes or leaves, are the seahorses (Hippocampus spp.). 

Various species belonging to the gobiesocids (known as cling fish) are 

reported as being associated with P. oceanica. These include the Connemara 

cling fish (Lepadogaster candollei) and Opeatogenys gracilis. The latter is a 

small green-coloured fish that adheres to Posidonia leaves, perfectly 

camouflaging itself. 

The black scorpionfish (Scorpaena porcus) is often to be found resting at the 

base of the leaf fronds or directly on the seabed where P. oceanica is growing 

and, less often, the small red scorpionfish (S. notata) and largescaled 

scorpionfish (S. scrofa). All scorpionfish are predators which lurk on the 

bottom, suddenly opening their large mouths to suck in their prey, and then 

Black scorpionfish (Scorpaena porcus) 

101

gripping them tightly between their 

teeth before swallowing them. 

Gobies may also be observed resting 

on the seabed beneath the leaf 

mantle, including the slender goby 

(Gobius geniporus), red-mouthed 

goby (G. cruentatus) and rock goby 

(G. paganellus), some blenniids 

(blennies) such as the tompot blenny 

(Parablennius gattorugine), or flatfish 

like the turbot (Bothus podas). 

Among the gobies, the quagga goby 

(Pomatoschistus quagga) should also 

be mentioned. This small fish typically 

rises from the bottom and stays in 

open water, not too far (about 1 metre) 

from the substrate. More rarely, 

Dusky spinefoot (Siganus luridus) 

triglids like the streaked gurnard 

(Chelidonichthys lastoviza) have also been observed. 

In the areas where seagrass meadows have tall mattes, which at a certain 

moment start to become eroded because of the displacement of sediment, a 

particular microhabitat may be created, with small semi-dark recesses in the 

free spaces between the rhizomes. In these microhabitats, as well as 

damselfish fry, lives the cardinal fish (Apogon imberbis), a small markedly 

sciaphilous fish (attracted by shady environments) typically associated with 

rocky substrates with plenty of crevices or underwater grottoes. In the sand 

beneath or close to the P. oceanica meadows, are other species of fish, such 

as the weever-fish (Trachinus spp.), stargazer (Uranoscopus scaber) or Atlantic 

lizardfish (Synodus saurus), which spend most of their time buried, waiting to 

ambush their prey as they pass close by. 

It should be remembered that the Mediterranean fish fauna is in continuous 

evolution and that some of the changes are due to humans, as in the case of 

the species of fish that entered the Mediterranean Sea following the opening 

of the Suez Canal and which are expanding rapidly as a result of gradual 

colonisation of new areas. The non-indigenous species which may be found 

associated with P. oceanica in some areas of the southern Mediterranean 

include the dusky spinefoot (Siganus luridus). This is a herbivorous fish and 

may be observed as it swims or grazes on the leaves of P. oceanica, 

occasionally in mixed shoals with salema or parrotfish. 

■ Temporal variations 

102 103 

A description of the fish fauna requires further elements in order to understand 

its dynamics over time. Indeed, the fish fauna of P. oceanica may change on a 

seasonal scale, but also on the briefer daily scale. During the year, changes are 

linked to the life-cycles of many of the species hosted in Posidonia meadows. 

For example, some species which exploit P. oceanica as a habitat for the 

installation or enlistment of juveniles, may demonstrate higher total 

abundances (often due to many small individuals) only in specific periods of 

the year - in Posidonia, this happens mainly in spring and early summer. 

Concerning the species composition of the fish populations as well as their 

relative abundance, large differences have not been noted between winter and 

summer at Port Cros (France), considering only the adult portion of the 

populations. Instead, off Ischia (Italy), a reduction in both number of species 

and abundance of fish, excluding those more closely associated with the 

water column, has been observed in winter. However, it is not known if these 

differences are to be attributed to effective migration from P. oceanica 

meadows in winter, or to the reduced mobility of the fish during the cold 

season. 

The changes that may occur between day and night are a great deal more 

marked. This is due mainly to the active presence at night of conger eels, 

gadids and ophidiids, all nocturnal predators which abandon their daytime 

refuges in the mattes, in sediment, or among the rocks in Posidonia meadows. 

Instead, damselfish, blotched picarel and picarel tend to sink between the 

Posidonia leaves during the night. The scorpionfish show their peak of feeding 

activity during the night and tend to rise on the leaves of Posidonia (especially 

the smaller individuals). During the day, the adults stay on the bottom or 

among the rhizomes, whereas the young slip inside the mattes or other natural 

crevices. At night, labrids are almost entirely inactive, sinking towards the 

bottom among the Posidonia fronds and remaining practically immobile. In 

general, several studies report a quite variable number of fish species 

associated with P. oceanica - in the range of 30-50. This variability is due not 

only to differences in the specific composition of the populations between 

different sectors of the Mediterranean, but is also partly attributable to the 

study methods used. 

From the point of view of overall populations, the labrids certainly dominate in 

terms of number of species, followed by the sparids. Instead, in terms of 

number of individuals, the gregarious and planktivorous species which occupy 

the water column (damselfish, blotched picarel, bogue) are dominant (they

104 

Vertical distribution of main fish families in Neptune grass meadows, by day and night 

may represent up to 90% of the 

individuals found). 

Concerning the trophic organisation 

of the fish fauna, as previously 

mentioned, the majority of fish species 

most closely associated with P. 

oceanica feed on small invertebrates 

and are thus substantially linked to the 

detritus chain rather than that of the 

herbivores. The proportion of primary 

production that is used by herbivores 

and transferred higher up the foodchain 

is therefore negligible. 

Pipefish (Syngnathus acus) 

Many invertebrates are the prey of a 

large number of fish living in P. oceanica meadows. These fish, in turn, are the 

prey of large piscivores. 

As regards juveniles, it is known that in many temperate regions seagrass 

meadows play a crucially important role, i.e., that of constituting nurseries in 

which the juvenile stages of many fish species, including many of commercial 

importance, pass the earliest periods of their life-cycle. After fecundation, the 

eggs of most coastal fish species remain in the open sea, although there are 

some (like those of the gobies) which have benthic eggs. The eggs eventually 

hatch and the larvae emerge, and, after a variable period (generally a few 

weeks), head towards the coast. The pelagic larval stage is followed by 

metamorphosis, during which the post-larvae slowly adapt to a more 

pronounced benthic stage. Depending on species, these early stages 

demonstrate clear preferences for different habitats. Being small (at times less 

than 2 cm), the juveniles are often subjected to intense predation, so they 

need habitats where they can find refuge as well as food. 

For clarification, it should be mentioned that many fish biologists define the 

transition between the pelagic and benthic stages as “settlement”. 

“Recruitment” is the stage during which the individuals of a new generation of 

a given species join the adult population. These two stages are distinct for 

species whose juveniles live in different habitats from those of the adults, but 

overlap for species whose post-larvae choose the same habitat as that of the 

adults. The above terms should not be confused with “colonisation”, the 

process by means of which a species occupies a geographical area where it 

was not previously present (e.g., species entering the Mediterranean Sea 

through the Suez Canal). 

105

At a world level, seagrass meadows appear to be particularly well adapted to 

playing the role of nurseries, especially where they grow in shallow waters, or in 

brackish environments such as coastal lagoons or river mouths. In the 

Mediterranean, many have transposed this role automatically to P. oceanica, but 

data are only available for a small number of locations in the north-western 

Mediterranean. The list of species which use P. oceanica meadows as nurseries 

includes many of little commercial interest, like various wrasses of the genus 

Symphodus (such as S. ocellatus or the axillary wrasse) and Labrus (brown 

wrasse). Among the sparids, the annular seabream and black seabream (and, to a 

lesser extent, the common seabream, Pagrus pagrus) appear to prefer Posidonia 

oceanica meadows during their juvenile stage. During this stage, the colours of 

many species are particularly appropriate for camouflage among the leaves. 

Instead, the striped red mullet is a species of commercial importance, which 

also exploits Posidonia oceanica meadows during its juvenile stages, as do 

black scorpionfish and blotched picarel. Many of the young of these species 

are present in the late spring or summer, while there are few species whose 

young are observed during colder seasons. As well as juveniles, i.e., in a 

particularly early stage of the life-cycle, many species frequent the Posidonia 

oceanica meadows as sub-adults. These are often the same species as those 

whose adults live in association with the meadows (such as salema, comber, 

axillary wrasse and ornate wrasse). 

The fish fauna associated with other Mediterranean marine phanerogams has 

certainly received less attention than the populations associated with P. oceanica. 

There is very little known about the fish fauna associated with Zostera marina. A 

study conducted some years ago on meadows of Z. marina and Nanozostera 

noltii off Rovinj (Croatia) listed more than 30 fish taxa associated with these 

phanerogams. The majority of them have already been mentioned for P. 

oceanica, while the presence of others, such as some gobiids (gobies like the 

black goby, Gobius niger) and soleids (soles), reveal the sandy or sandy-muddy 

nature of the substrates where the meadows of these lesser phanerogams grow. 

In shallow sheltered environments - both typically marine, like sheltered bays, and 

in the brackish waters of coastal lagoons - there may be mixed meadows of C. 

nodosa and N. noltii. The fish fauna associated with them has only received some 

attention in recent years and currently available studies are few and localised. 

A series of studies conducted in bays of the Gulf of Olbia (Sardinia), which have 

slightly less salty waters than those of typical marine conditions, has 

demonstrated the importance of the cover of C. nodosa and N. noltii 

(henceforth called “small phanerogams”) for the associated fish fauna. In the 

studied meadows, 23 taxa of fish fauna were counted, by monthly visual 

sampling over a full year’s cycle. Whitebait were the most frequent and 

abundant throughout the year, followed by some labrids, such as grey wrasse, 

peacock wrasse and green wrasse, and some sparids, such as gilthead 

106 107 

Two-banded seabream (Diplodus vulgaris) Green wrasse (Labrus viridis)

seabream, two-banded seabream, annular seabream and salema. Most of the 

other species were either less abundant or occasional. Medium-sized 

specimens of garfish (Belone belone) and painted comber were relatively 

frequent. These are both piscivores: the needlefish sometimes chased and 

captured whitebait; the painted combers were observed lying in wait to 

ambush the juveniles of other species. 

For the fish associated with the meadows of small phanerogams, no distinction 

can be made between the species that live on the bottom or between the 

rhizomes and those which swim among the leaves or just above the leaf mantle, 

as described for the fish fauna associated with P. oceanica. The small 

phanerogams have a less complex structure, with lower and more delicate leaves 

and a looser system of rhizomes that does not form mattes. There is therefore no 

shadier environment at the base of the leaves, so there are no habitats suitable for 

particularly sciaphilous species (e.g., cardinal fish). Moreover, unlike P. oceanica, 

small phanerogams only grow on sandy/muddy substrates. This means that, 

among the fish which may be found associated with meadows of small 

phanerogams, especially when they are growing in very sheltered coastal 

environments and with an influx of freshwater, there are also species typically 

associated with sandy or muddy-sand substrates, such as striped seabream 

(Lithognathus mormyrus), black goby and Bucchich’s goby (Gobius bucchichi) - 

the latter species often associated with sea anemones (Anemonia viridis). 

Concerning the temporal dynamics of 

the fish fauna, meadows of small 

phanerogams have a more variable 

structure during the year than those of P. 

oceanica. For example, the small 

phanerogam meadows studied in the 

Gulf of Olbia showed a clearcut annual 

cycle in the density of leaf fronds, which 

fell from almost 2000 fronds/m2 in 

summer to less than 1000 in winter. This 

involved a notable reduction in the 

complexity of the habitat and fewer hiding-places for the fish. There was also a 

change in water temperature, between 25 °C in summer and 13-14 °C in winter. 

These alterations in environmental conditions and leaf density were correlated to 

important variations in the fish fauna during the year. Species richness between 

summer and winter dropped from 15-18 to 3-4. The total number of fish does not 

change much overall, but only because the vast majority of individuals are 

whitebait, for which the protection against predators offered by shallower depths 

(where in any case they become more vulnerable to other predators, such as 

seabirds) is perhaps more important than the plant cover. Instead, if the 

abundance of fish is studied without the contribution of planktivorous fish, the 

picture is very different. The average number of fish counted on the standard 

surface area of 150 m2 Saddled seabream (Oblada melanura) 

varies from about 20-100 specimens in summer-autumn 

(when leaf density is high), to a few individuals (even less than 5) between 

December and April (when leaf density is at its minimum). The species whose 

abundance increases or diminishes in relation to the increase or reduction of leaf 

density are the peacock wrasse, grey wrasse, green wrasse, striped red mullet, 

annular seabream, two-banded seabream, salema and gilthead seabream. 

An important fact is that the vast majority of specimens observed were 

juveniles or sub-adults. This suggests that the small phanerogams may play a 

more specific role as nurseries than P. oceanica. All the individuals observed 

belonging to commercially important species such as common seabream, 

gilthead seabream, striped seabream and saddled bream, plus a relatively 

significant proportion of striped red mullet, two-banded seabream and white 

seabream, were juveniles. On the whole, striped red mullet and annular 

seabream appear to use these meadows of small phanerogams, like the P. 

oceanica meadows, for installing the juveniles and initial sub-adult stages. 

The shallow depths and the leaf mantle render these habitats particularly 

suitable for juvenile fish. 

108 109 

Striped seabream (Lithognathus mormyrus)

Among fish species associated with 

the small phanerogam meadows, 

those which also maintain a 

conspicuous proportion of the adult 

population are the green wrasse (20% 

of adults), grey wrasse (12%), striped 

red mullet (7%) and annular seabream 

(6%). Grey wrasse males, in particular, 

have been observed building their 

Peacock wrasse (Symphodus tinca) 

nests in the meadows, using drifting 

fronds of macroalgae, small pebbles 

and leaves of small phanerogams. Many of these fishes can be observed 

beside heaps of small pebbles inside the meadows. Juveniles and adults of 

striped red mullet were almost always observed while searching for prey by 

probing the sand with their barbels at the base of the leaf fronds or in the 

patches of sand within or at the edges of the small phanerogam meadows. 

Studies conducted on the fish populations associated with the meadows of 

C. nodosa, N. noltii and Zostera marina in the Lagoon of Venice have 

demonstrated a very different situation to that of the Gulf of Olbia. Species 

richness was of the order of 30-40 associated taxa. The dominant fish 

species were the black-striped pipefish (Syngnathus abaster), pipefish, 

whitebait (Atherina boyeri) and grass goby (Zosterisessor ophiocephalus). It 

should be noted that most of the species counted were almost always adults. 

From this viewpoint, the phanerogam meadows of the Lagoon of Venice 

would seem to play an important role in terms of habitat where reproduction 

takes place (especially for the pipefish and grass goby), whereas the role of 

nursery appears to be marginal. In general, the reduction in mortality due to 

predation of juveniles is often attributed to the nursery function of the 

phanerogams, thanks to the protection given by the leaf mantle. However, 

limitations to the architectural complexity and homogeneity of habitat do 

exist, beyond which there is no longer an optimal balance between reduction 

of the risk of predation and optimisation of searching for one’s own prey, 

often ensured in habitats of a different type in proximity to the one which 

offers the best protection from predators. 

Compared with other habitats, the homogeneity of the phanerogam meadows 

in the Lagoon of Venice may be the reason for the scarcity of juvenile fish, 

which are more frequent in areas where the phanerogams are laid out in 

patches on loose unvegetated sea-beds. It should also be noted that these 

phanerogam meadows host abundant adults that prey on juveniles, such as 

grass goby and pipefish, which can 

feed on both vagile invertebrates and 

fish larvae and juveniles. 

As well as the native phanerogams of 

the Mediterranean Sea, there has been 

the addition of another species which 

arrived through the Suez Canal - 

Halophila stipulacea. The only study 

conducted on the fish fauna associated 

with this species was carried out in Grey wrasse (Symphodus cinereus) 

eastern Sicily, in a site where the 

meadow grows at a depth of about 20 m, where 30 fish species have been 

counted. The species most frequently observed were ornate wrasse, 

saddledbream, comber and painted comber. In terms of abundance, although 

the frequency of observation was not very high, salema was the most abundant 

species on average, followed by rainbow wrasse, saddled bream and striped 

red mullet. The size structure of the fish population did not reveal many large 

individuals, while for 8 species, more than 25% of the individuals recorded 

were small specimens or juveniles, particularly black scorpionfish, saddled 

bream and common two-banded seabream. Overall, as also observed for the 

fish fauna of the small phanerogams in the Gulf of Olbia, the highest species 

richness was observed in summer and autumn, and the lowest in winter. The 

total density followed a similar seasonal trend, with maximum values in 

summer-autumn and minimum in winter. This fact is interesting because, as H. 

stipulacea maintains a highly structured and therefore stable architecture 

throughout the year, the observed seasonal differences may be attributed to 

other factors (e.g., water temperature). 

The fish fauna associated with H. stipulacea sampled in eastern Sicily 

revealed similarities and differences with respect to the assemblages 

associated with the other phanerogam species. In all phanerogam systems, 

the dominance of fishes belonging to the labrid and sparid families is 

obvious. Conversely, H. stipulacea had a fish fauna characterised by a 

scarcity of planktivorous fish and the presence of species such as the 

blackbelly rosefish (Helicolenus dactylopterus) and butterfly blenny (Blennius 

ocellaris), which are species typical of deeper loose seabed environments. 

However, this being the only available study, we cannot be sure whether 

these differences are really due to the presence of H. stipulacea or to the local 

characteristics of the studied site, which may be influenced by the special 

conditions of the nearby Strait of Messina. 

110 111

Fauna: invertebrates - Udine Cultura

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