Scallop, also fan shell or pecten, common name applied to any of the marine, filibranchiate (thread-gilled), bivalve mollusks closely related to clams and oysters (see Mollusk). Scallops are found in deep and shallow water in most seas, on sandy or muddy bottoms. About 360 species are known. Scallops bear ridged shells, about 5 cm (about 2 in) long, with the lower valve usually larger than the upper. The ridges radiate out along each valve in the shape of a fan. The edges of the shell are sharp and undulating, or scalloped, giving these animals their common name, and the shell is broad and flattened at the hinge. The shell of the scallop is opened and closed by a single large muscle, the adductor muscle, which is used as food in the United States. The mantle bears numerous iridescent, primitive eyes, sensitive to changes in light intensity, along a fold on its border, called the velar fold. Scallops rarely attach themselves to rocks; they actively swim by rapidly opening and closing their shells, or rest on the ocean bottom on the lower valve. The animals are hermaphroditic. The weathervane scallop is a commercially important scallop of the Alaska coast.

Scientific classification: Scallops make up the families Pectinidae, Entoliidae, and Propeamussiidae of the order Ostreoida. The weathervane scallop is classified as Pecten caurinus.
 

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