Gastropods

Gastropods are invertebrate animals that make up the largest class in the phylum Mollusca. Examples of common gastropods include all varieties of snails, abalone, limpets, and land and sea slugs. There are over 35,000 existing species , as well an additional 15,000 separate fossil species. Gastropods first appeared in the fossil record during the early Cambrian period, approximately 550 million years ago.

This diverse group of animals is characterized by a soft body, made up of three main parts: the head, foot, and visceral mass. The head contains a mouth and often sensing tentacles. The lower portion of the body makes up the foot, which allows slow creeping along rocks and other solid surfaces. The visceral mass is the main part of the body, containing most of the internal organs. In addition to these body parts, gastropods possess a mantle, or fold which secretes a hard, calcium carbonate shell. The single, asymmetrical shell of a gastropod is most often spiral shaped, however, it can be flattened or cone-like. This shell is an important source of protection. Predators have a difficult time accessing the soft flesh inside, especially if there are sharp points on the outside, as there are on the shells of some of the more ornate gastropods. There are also some gastropods, such as slugs and sea hares, that do not have shells or have greatly reduced shells. Some of the shelless types that live in the ocean (i.e., nudibranchs or sea slugs) are able to use stinging cells from prey that they have consumed as a means of protection.

In addition to a spiraling of their shells, the soft bodies of most gastropods undergo 180 degrees of twisting, or torsion, during early development, when one side of the visceral mass grows faster than the other. This characteristic distinguishes gastropods from other molluscs. Torsion results in a U-shaped digestive tract, with the anal opening slightly behind the head. The torsion of the soft body and the spiraling of the shell are thought to be unrelated evolutionary events.

Gastropods have evolved to live in a wide variety of habitats. The great majority are marine, living in the world's oceans. Numerous species live in fresh water, while others live entirely on land. Of those that live in water, most are found on the bottom, attached to rocks or other surfaces. There are even a few species of gastropods without shells, including sea butterflies, that are capable of swimming. Living in different habitats has resulted in a wide variety of structural adaptations within the class Gastropoda. For example, those gastropods that live in water use gills to obtain the oxygen necessary for respiration , while their terrestrial relatives have evolved lungs to breathe.

Gastropods are important links in food webs in the habitats in which they live, employing a wide variety of feeding strategies. For example, most gastropods move a rasping row of teeth on a tongue like organ called a radula back and forth to scrape microscopic algae off rocks or the surface of plants. Because the teeth on the radula gradually wear away, new teeth are continuously secreted. Other gastropods have evolved a specialized radula for drilling through shells of animals to get at their soft flesh. For example, the oyster drill, a small east coast gastropod, bores a small hole in the shell of neighboring molluscs such as oysters and clams so that it can consume the soft flesh. In addition, some terrestrial gastropods such as snails use their radula to cut through pieces of leaves for food.

Gastropods are eaten by numerous animals, including various types of fish, birds, and mammals. They are also eaten by humans throughout the world. Abalone, muscular shelled gastropods that cling to rocks, are consumed on the west coast of the United States and in Asia. Fritters and chowder are made from the large, snail-like queen conch on many Caribbean islands. Escargot (snails in a garlicky butter sauce) are a European delicacy.

[Max Strieb ]

RESOURCES

BOOKS

Barnes, R. D. Invertebrate Zoology. 5th ed. Philadelphia: Saunders College Publishing, 1987.

Gastropoda (gastropods; phylum Mollusca) A class of molluscs, of asymmetrical form, including snails and slugs, which have a true head, an unsegmented body, and a broad, flat foot. When present, the shell is in one piece and spirally coiled, at least in young stages. The mantle cavity and visceral mass have undergone torsion, at least in the developmental stage. There is usually a well-developed radula. Gastropods inhabit a wide range of aquatic and terrestrial environments, and range from the Cambrian to the present. Fossils of coiled gastropod shells are common in marine rocks, especially those of the Cenozoic. All fossil gastropods and most modern ones have a coiled shell, which is all that remains for the identification of fossil forms. The identification of modern species is based largely on soft body parts.

Gastropoda (gastropod; phylum Mollusca) The class of Mollusca that includes snails and slugs. They have a true head, an unsegmented body, and a broad, flat foot. They appear in the Cambrian and occur in sedimentary rocks of all ages, occupying a range of aquatic and terrestrial environments. The majority of modern gastropods and all the fossil forms possess a coiled shell, which is all that is left to the palaeontologist for determining identification. The classification of living forms is based largely upon soft parts, so that similarly shaped shells developed by unrelated groups cause problems of nomenclature.

gastropod Member of the Gastropoda class of molluscs, which includes the snail, slug, whelk, limpet, abalone and sea slug. Many possess a single spiral shell produced by chemical precipitation from the mantle. Many types of gastropod live immersed in seawater, breathing through gills. Some freshwater snails, however, breathe through lungs and surface periodically for air. Sea slugs are entirely without shells.

Gastropoda A class of molluscs that includes the snails, whelks, limpets, land and sea slugs, and conches. Molluscs have a well-developed head with tentacles, a large flattened foot, and a coiled twisted shell. They occupy marine, freshwater, and terrestrial habitats; in the terrestrial and some freshwater gastropods the mantle cavity acts as a lung instead of enclosing gills.