Mollusca
The Mollusca, known commonly as Molluscs, is a large phylum of invertebrate animals that are
distributed in almost every habitat on the Earth, from the sea, extending from the intertidal to the
deepest oceans, to the freshwater, while many gastropods have terrestrial habitat and are abundant in
particular in leaf litter of tropical forests. Despite their incredible diversity, all molluscs have some
universal features that define the body structure: 1) a mantle that overlaps the body that is provided
with a cavity used for breathing and excretion and that secretes calcareous plates or shells and 2) the
organization of an interesting nervous systems. However in some groups, like slugs, the shell is
reduced and internalized or it can be absent in many cases.
When present, the shell is mainly formed by chitin that is a protein called conchiolin, reinforced with
calcium carbonate. A radula and a broad, muscolar foot are other characteritics of molluscs, but they
can be absent in some groups.
Terrestrial Gastropoda (Terrestrial gastropods, Land Snails)
Land snails evolved from marine ancestors and the Pulmonate are the most successful group of these
in land-based ecosystems. Thanks to the new land habitat, such as a lung for breathing, they evolved
in great number, spreading out in several different habitats. On the contrary, the operculate snail
group, fomerly known as Prosobranchia, invaded the land but confined to the humid tropics.
Although derived from marine ancestor, the land operculate snails have lost their gills.
Pulmonates never have an operculum and sexually they are hermafrodites, meaning that an individual
is both male and female. In natural ecosystems the greater part of the terrestrial gastropods
biodiversity is composed of detritivores, contributing significantly to nutrient cycling through
facilitation of decomposition and return of plant litter to the soil. Very important is also the role of
herbivores and predators component as contributors in ecosystem processes, the first component
feeding on living plant tissue, macrofungi and photosyntetic microrganisms, the second one feeding
primarily on earthworms and other small gastropods. The terrestrial molluscs are extimated to be
30.000-35.000 species, with about 500 species of slugs and more than 1000 species of semi-slugs.
Slugs evolved from snails by reducing and internalizing the shell; in fact, sometimes shells can be
reduced to a flat plate or, may be completetely absent, like in Veronicellidae (see the photo gallery).
Terrestrial gastropods can be found everywhere and in arid regions as well; in particular they occur in
any terrestrial habitat that present some source of moisture. Another feature that outline the typical
habitat for land snails is a source of calcium, that they use for the production of both eggs and shells,
even if slugs need less calcium for their survival.
For this reason, areas of limestone are particularly rich in land snails biodiversity. Besides, in the
tropics humid, limestone formations support land snails diversity because in this climate they are
subject to severe erosion caused by rainfall, humidity and biological phenomea. These erosive
processes result in particular morphologies such as blocks of limestone with steep cliffs, strewned
with caves and deep cracks, providing a refuge for the snails.
Snails are more abundant in Primary forest than the Secondary one, and can be found in leaf litter,
near to decaying logs and fallen trees, in the crevices of tree's bark and around stones covered by
mossess.
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