Gastropoda

Background & Life History

Gastropods are one of the most diverse groups of animals, both in form, and habitat. They are 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 [1]. They have a long and rich fossil record from the Early Cambrian that shows periodic extinctions of subclades, followed by diversification of new groups. The Class Gastropoda includes the snails, slugs, limpets, and sea hares. Gastropods have figured prominently in paleobiologic and biological studies, and have served as study organisms in numerous evolutionary, biomechanical, ecological, physiological, and behavioral investigations.

Gastropods are mainly dioecious while some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other. Marine species have veliger larvae. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced in some important groups. Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted.

Ecology & Habitat

Gastropods live in every conceivable habitat on Earth, having a worldwide distribution. They have become adapted to almost every kind of existence on earth, having colonized nearly every available medium. 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. Some of the more familiar and better-known gastropods are terrestrial gastropods (the land snails and slugs). Some live in freshwater, but the majority of all named species of gastropods live in a marine environment. In habitats where there is not enough calcium carbonate to build a really solid shell, such as on some acidic soils on land, there are still various species of slugs, and also some snails with a thin translucent shell, mostly or entirely composed of the protein conchiolin.

Their 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, this method of entry having been acquired independently in several groups, as is also the case with carnivory itself. Some gastropods feed suctorially and have lost the radula.

Morphology

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. Gastropods have a muscular foot which is used for "creeping" locomotion in most species. In some, it is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue (ganglion). 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). The foot is usually rather large and is typically used for crawling. It can be modified for burrowing, leaping, swimming, or clamping. The foot typically bears an operculum that seals the shell opening (aperture) when the head-foot is retracted into the shell. 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). Torsion takes place during the veliger stage, usually very rapidly. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell. Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

References

• Holthuis, B.V. (1995): Evolution between marine and freshwater habitats: a case study of the gastropod suborder Neritopsina. Ph.D. thesis, University of Washington
• “The Gastropoda.” Ucmp.berkeley.edu, 1999, ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php.