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cockle, common name applied to the heart-shaped, jumping or leaping marine bivalve mollusks, belonging to the order Eulamellibranchia. The brittle shells are of uniform size, are obliquely spherical, and possess distinct radiating ridges, or ribs, which aid the animal in gripping the sand. The mantle has three distinct apertures (inhalant, exhalant, and pedal) through which the inhalant and exhalant siphons and the foot protrude. The cockle lives in sand and mud in shallow water, often in brackish inlets. It burrows until only the siphons project, pulling in water from which the animal strains the minute planktonic organisms on which it feeds. All cockles are hermaphroditic. In order to accomplish the characteristic jumping form of forward locomotion, the large, powerful, muscular foot is bent backward beneath the shell and then straightened. In most adults, the foot is about as long as the greatest length of the shell.
Several species of cockles are considered to be good, edible clams. In the British Isles, great numbers of cockles are taken annually for food from densely populated beds. These beds have been known to migrate in units, probably in response to changes in currents. Protothaca staminea, the rock cockle, is among the best known and most widely used for food. It usually does not exceed 3 in. (7.5 cm) in length. Rock cockles are poor diggers and inhabit packed mud, or gravel mixed with sand, usually 8 in. (20 cm) below the surface. They are found on the Pacific Coast near the rocky shores of bays and estuaries. Those inhabiting the open coast during the summer months should not be eaten because they may be infected with toxin-producing organisms. P. semidecussata, the Japanese littleneck clam, is smaller but considered to be better-flavored than the rock cockle. The shell is more elongated, with a brownish to bluish banding on one end. It inhabits an environment similar to that of P. staminea and is widespread in Puget Sound, Wash.; British Columbia; and San Francisco and Tomales Bay, Calif.
Unlike the genus Protothaca, the basket cockles (Clinocardium nuttalli, or Cardium corbis) are good diggers and have a large foot. Lacking siphon tubes, basket cockles burrow only slightly beneath the surface and inhabit sand flats, particularly along the Pacific Coast. They are considered good eating clams but are too few in number to be widely marketed. They are most abundant in British Columbia and in Puget Sound, Wash., with fewer found south as far as Baja California and north as far as the Bering Sea.
The hard shell cockles, genus Chione, are found from San Pedro, Calif., S into Mexico. The giant Atlantic cockle, Dinocardium robustum (Cardium magnum), reaches 5 in. (12.5 cm) in diameter and is found along the Atlantic Coast from Virginia to Brazil. It has shells with toothed margins, strikingly colored in yellowish brown with spots and transverse stripes of chestnut or purple. Cockles are classified in the phylum Mollusca, class Pelecypoda or Bivalvia, order Eulamellibranchia.