Fauna: invertebrates - Udine Cultura

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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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