Gastropods (Gastropoda) are a highly diverse group of molluscs that include between 60,000 and 80,000 living species. Gastropods account for nearly 80 percent of all living molluscs. Members of this group include terrestrial snails and slugs, sea butterflies, canoe shells, conchs, whelks, limpets, periwinkles, oyster borers, coweries, nudibranchs, and many others.
Gastropods are not only diverse with respect to the number of species alive today, they are diverse in terms of their size, shape, color, body structure and shell morphology. They are diverse in terms of their feeding habits —there are browsers, grazers, filter feeders, predators, bottom feeders, scavengers and detritivores among the gastropods. They are diverse in terms of the habitats in which they live—they inhabit freshwater, marine, deep sea, intertidal, wetland and terrestrial habitats (in fact, gastropods are the only group of molluscs to have colonized land habitats).
During their development, gastropods undergo a process known as torsion, a twisting of their body along its head-to-tail axis. This twisting means that the head is between 90 and 180 degrees offset relative to their foot. Torsion is the result of asymmetrical growth, with more growth occurring on the left side of the body. Torsion causes the loss of the right side of any paired appendages. Thus, although gastropods are still considered to be bilaterally symmetrical (that's how they start out), by the time they become adults, gastropods that have undergone torsion have lost some elements of their "symmetry". The adult gastropod ends up configured in such a way that its body and internal organs are twisted and the mantle and mantle cavity are above its head. It should be noted that torsion involves the twisting of the gastropod's body, it has nothing to do with the coiling of the shell (which we'll consider next).
Most gastropods have a single, coiled shell, although some molluscs such as nudibranchs and terrestrial slugs are shell-less. As stated above, the coiling of the shell is not related to torsion and is simply the way the shell grows. The coil of the shell usually twists in a clockwise direction, so that when viewed with the apex (top) of the shell pointing upward, the opening of the shell is located on the right.
Many gastropods (such as sea snails, terrestrial snails, and freshwater snails) have a hardened structure on the surface of their foot called an operculum. The operculum serves as a lid that protects the gastropod when it retracts its body within its shell. The operculum seals the shell opening to prevent desiccation or deter predators.
The various gastropod groups feed in different ways. Some are herbivorous while others are predators or scavengers. Those that feed on plants and algae use their radula to scrape and shred their food. Gastropods that are predators or scavengers use a siphon to suction food into the mantle cavity and filter it over its gills. Some predatory gastropods (the oyster borers, for example) feed on shelled prey by boring a hole through the shell to locate the soft body parts inside.
Most marine gastropods breath via their gills. Most freshwater and terrestrial species are an exception to this rule and breath instead using a rudimentary lung. Those gastropods that breath using a lung are called pulmonates.
Gastropods are divided into the following basic groups:
- Caenogastropoda - The predominant members fo this group are sea snails, but the group also includes a few species of freshwater snails, land snails, and (non-snail) marine gastropod molluscs. Caenogastropoda exhibit torsion, have a single auricle in tehir heard and one pair of gill leaflets.
- Heterobranchia - The Heterobranchia are the most diverse of all gastropod groups. This group includes many terrestrial, freswater, and marine snails and slugs.
The earliest gastropods are thought to have evolved in marine habitats during the Late Cambrian. The earliest terrestrial gastropods were the Maturipupa, a group that dates back to the Carboniferous Period. Throughout the evolutionary history of the gastropods, some subclades have gone extinct while others have diversified.