A cockle is an edible, marine bivalve mollusk. Although many small edible bivalves are loosely called cockles, true cockles are species in the family Cardiidae. True cockles live on sandy, sheltered beaches throughout the world. The distinctive rounded shells are bilaterally symmetrical and are heart-shaped when viewed from the end. Numerous radial, evenly spaced ribs are a feature of the shell in most but not all genera (for an exception, see the genus Laevicardium, the egg cockles, which have very smooth shells).
The word "cockle" is also applied to some other edible bivalves which are not in the family Cardiidae. Those most frequently called cockles include species in the genus Donax, sometimes called "thumb shells", and in several marine clams of the family Canidae (previously known as Psammobiidae), some of which are also called "razor clams." Species in the genus Ensis also bear the common name "razor clam", but are in a different family, Enidae. The more general term "clam" is used technically to mean any member of the molluscan class Bivalvia that is not a squid, snail, or mussel.
The cockleshell is rounded and symmetrical with a deep, pronounced waist in the outer edge (visible when looking at the shell from the end). The shell of most cockles is smooth, but many have growth lines and there are prominent ridges across the surface of the shell in some genera such as Clinocardium and Acrosterigma. In others, such as Glycymeris the shell is sculpted with a rounded "keel" in relief at the midpoint of each valve, which gives a sloped appearance to its profile. Cockles have a large muscular foot and a pair of strong siphons which flow straight out from the mantle cavity.
The visible part of the soft body of a cockle is generally black or dark brown, and this distinguishes most cockles from the similar Venusshells, which may be very colorful. The shell is composed mostly of calcium carbonate, and is somewhat brittle; brittleness increases with water erosion.
Cockles are capable of 'jumping' by bending and straightening the foot. This action appears to reduce the risk of being dislodged by a passing fish or crab, an ability that is also shared by several other bivalve species including the venus shells. The jumping action has earned them the name "noodle clam" among Chinese cooks.
They are sensitive to the slightest changes in water salinity and temperature, and when these change beyond their limits they can no longer remain in equilibrium with that environment. If conditions become unfavorable in one area, however, adult cockles