Fauna: invertebrates - Udine Cultura

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
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Turban snail (Tricolia tenuis) grazing on a
Posidonia leaf
Cuttlefish (Sepia officinalis)