Fauna: invertebrates - Udine Cultura

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.
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The gastropod Gibbula ardens, typically grazing Posidonia leaves