conch (kŏngk, kŏnch, kôngk) [key], common name for certain marine gastropod mollusks having a heavy, spiral shell, the whorls of which overlap each other. In conchs the characteristic gastropod foot is reduced in size and the operculum, a horny plate located on the foot and used to seal the shell opening in many gastropods, has the appearance and function of a claw. During locomotion, the operculum secures a foothold in the sand, and the conch jumps forward by means of the quick contraction of a retractor muscle called the columella muscle. Thus the conch lacks the creeping motion of most gastropods. The king conch, Strombus gigas, found in the warmer waters of the Atlantic, Caribbean, and Gulf of Mexico, has a shell 10 to 12 in. (25–30 cm) long and may weigh up to 5 lb (2.3 kg). Similar in size and distribution is the queen conch, Cassis cameo. Its shell has been used in Europe to carve cameos. Conch shells range in color from white to red; they have been used by humans to fashion a number of items, such as buttons, ornaments, or the crude trumpets made from the shell of the trumpet conch, Charonia tritonis. This conch is similar in shape to the king and queen conchs but is much more slender and reaches a length of 20 in. (50 cm). C. tritonis is found in the Gulf of Mexico and the Indian Ocean. The largest conch and also one of the largest univalves in the world is the horse conch, Pleuroploca gigantea, having a shell length of 24 in. (60 cm). It is found along the Atlantic Coast from North Carolina to Brazil. The body can retreat entirely into the shell and remain there for months if unfavorable conditions prevail. An unusual conch shell is that of the spider conch, Lambis lambis, which has leglike projections. Spider conchs are voracious carnivores, common on coral reefs. They also feed on algae, as do the king conchs. Most conchs are carnivorous, feeding on bivalve mollusks; some are scavengers as well. They inhabit tropical waters and have been used as a food source for man. The conch is classified in the phylum Mollusca, class Gastropoda, order Mesogastropoda.