Gastropoda

Gastropods are one of the most diverse groups of animals, both in form, habit, and habitat. They are by far the largest group of molluscs, with more than 62,000 described living species, and they comprise about 80% of living molluscs. Estimates of total extant species range from 40,000 to over 100,000, but there may be as many as 150,000 species. There are about about 13,000 named genera for both Recent and fossil gastropods. They have a long and rich fossil record from the Early Cambrian that shows periodic extinctions of subclades, followed by diversification of new groups.

Gastropods have figured prominently in paleobiological and biological studies, and have served as study organisms in numerous evolutionary, biomechanical, ecological, physiological, and behavioral investigations.

They are extremely diverse in size, body and shell morphology, and habits and occupy the widest range of ecological niches of all molluscs, being the only group to have invaded the land.

Gastropods live in every conceivable habitat on Earth. They occupy all marine habitats ranging from the deepest ocean basins to the supralittoral, as well as freshwater habitats, and other inland aquatic habitats including salt lakes. They are also the only terrestrial molluscs, being found in virtually all habitats ranging from high mountains to deserts and rainforest, and from the tropics to high latitudes.

Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. They include grazers, browsers, suspension feeders, scavengers, detritivores, and carnivores. Carnivory in some taxa may simply involve grazing on colonial animals, while others engage in hunting their prey. Some gastropod carnivores drill holes in their shelled prey,

Most aquatic gastropods are benthic and mainly epifaunal but some are planktonic. A few such as the violet snails (Janthinidae) and the sea lizards (Glaucus) drift on the surface of the ocean where they feed on floating siphonophores, while others (heteropods and Gymnosomata) are active predators swimming in the plankton. Some snails (such as the whelk Syrinx aruanus) reach about 600 mm in length. There is also a very large (and poorly known) fauna of microgastropods that live in marine, freshwater and terrestrial environments. It is amongst these tiny snails (0.5-4 mm) where many of the undescribed species lie.

The first gastropod larval stage is typically a trochophore that transforms into a veliger and then settles and undergoes metamorphosis to form a juvenile snail. While many marine species undergo larval development, there are also numerous marine taxa that have direct development, this mode being the norm in freshwater and terrestrial taxa. Brooding of developing embryos is widely distributed throughout the gastropods, as are sporadic occurrences of hermaphrodism in the non-heterobranch taxa.

The basal groups have non-feeding larvae while veligers of many neritopsines, caenogastropods, and heterobranchs are planktotrophic. Egg size is reflected in the initial size of the juvenile shell or protoconch and this feature has been useful in distinguishing feeding and non-feeding taxa in both Recent and fossil taxa.

Gastropods are characterized by the possession of a single (often coiled) shell, although this is lost in some slug groups, and a body that has undergone torsion so that the pallial cavity faces forwards. They have a well-developed head bearing a pair of cephalic tentacles and eyes that are primitively situated near the outer bases of the tentacles. In some taxa the eyes are located on short to long eye stalks. The mantle edge in some taxa is extended anteriorly to form an inhalant siphon and this is sometimes associated with an elongation of the shell opening (aperture) — this is shown in the photo of the caenogastropod Conus bullatus below. The foot is usually rather large and is typically used for crawling. It can be modified for burrowing, leaping (as in conchs, Strombidae), swimming, or clamping (as in limpets). The foot typically bears an operculum that seals the shell opening (aperture) when the head-foot is retracted into the shell (see photos below). While this structure is present in all gastropod veliger larvae, it is absent in the embryos of some direct developing taxa and in the juveniles and adults of many heterobranchs. The nervous and circulatory systems are well developed with the concentration of nerve ganglia being a common evolutionary theme.

The shell is typically coiled, usually dextrally, the axis of coiling being around a central columella to which a large retractor muscle is attached. The uppermost part of the shell is formed from the larval shell (the protoconch). The shell is partly or entirely lost in the juveniles or adults of some groups, with total loss occurring in several groups of land slugs and sea slugs (nudibranchs).

Externally, gastropods appear to be bilaterally symmetrical. However, they are one of the most successful clades of asymmetric organisms known. The ancestral state of this group is clearly bilateral symmetry (e.g., chitons, cephalopods, bivalves), but gastropod molluscs twist their organ systems into figure-eights, differentially develop or lose organs on either side of their midline, and generate shells that coil to the right or left. The best documented source of gastropod asymmetry is the developmental process known as torsion.

Neritopsines
The neritopsines Nerita fulgurans (left) and Nerita versicolor (right).
In particular, the Neritopsina are placed below the Vetigastropoda in some analyses (thus they become the sister group of Vetigastropoda + Caenogastropoda + Heterobranchia). Also, the enigmatic taxon Cocculinidae is still uncertain. It may actually be a member of the Neritopsina clade, as some characters indicate. Most likely it is the sister clade to Neritopsina, though.