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 ]
Barnes, R. D. Invertebrate Zoology. 5th ed. Philadelphia: Saunders College Publishing, 1987.
gas·tro·pod / ˈgastrəˌpäd/ • n. any mollusk of the class Gastropoda, often with a spiral shell, that moves by means of a large muscular foot, including snails, slugs, and whelks.