Scallop is a common name that is primarily applied to any one of numerous species of saltwater clams or marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. The common name “scallop” is also sometimes applied to species in other closely related families within the superfamily Pectinoidea.
Scallops are a cosmopolitan family of bivalves, found in all of the world’s oceans, though never in freshwater. They are one of very few groups of bivalves to be primarily free-living; many species are capable of rapidly swimming short distances and even of migrating some distance across the ocean floor. A small minority of scallop species live cemented to rocky substrates as adults. Some others species are more simply attached, by means of a filament they secrete. The majority of species, however, live recumbent on sandy substrates, but when they sense the presence of a predator such as a starfish, they are able to escape by swimming swiftly but erratically through the water using a form of jet propulsion created by repeatedly clapping the valves of their shells together. Scallops have a well-developed nervous system. Unlike most other bivalves, they have numerous simple eyes situated around the edges of their mantles.
Many species of scallops are highly prized as a food source, and some are farmed as aquaculture. The word “scallop” is also applied simply to the meat of these bivalves when it is sold as seafood. In addition the name “scallop” is used as part of the name of dishes based on the meat of scallops, and is even applied to some dishes not containing the meat of these bivalves, dishes that are prepared in a similar fashion. The brightly coloured, symmetrical, fan-shaped shells of scallops, with their radiating, often fluted sculpture, are valued by shell collectors, and have been used since ancient times as motifs in art, architecture and design. The shell of a scallop consists of two sides or valves, a left valve and a right one, divided by a plane of symmetry. The animal normally rests on its right valve, and consequently this valve is often shaped differently than the left (i.e., upper) valve. With the hinge of the two valves oriented as shown in the diagram at right, the left side of the image corresponds to the animal’s morphological anterior or front, the right is the posterior or rear, the hinge is the dorsal or back/ top region, and the bottom corresponds to the ventral or (as it were) underside/ belly. However, as many scallop shells are more or less bilaterally symmetrical as well as symmetrical front/back, determining which way a given animal is “facing” requires detailed information about its valves
The model scallop shell consists of two similarly shaped valves with a straight hinge line along the top devoid of teeth and which produces a pair of flat wings or “ears” (sometimes called “auricles”, though this is also the term for two chambers in its heart) on either side of its midpoint. These ears may be of similar size and shape, or the anterior ear may be somewhat larger (the posterior ear is never larger than the anterior one). As is the case in almost all bivalves, a series of lines and/ or growth rings originate at the center of the hinge, at a spot called the beak surrounded by a generally raised area called the umbo. These growth rings increase in size downwards until they reach the curved ventral edge of the shell. The shell of most scallops is streamlined to facilitate ease of movement during swimming at some point in the life cycle, while also providing protection from predators. Scallops with ridged valves have the advantage of the architectural strength provided by these ridges called ribs, although the ribs are somewhat costly in terms of weight and mass. A feature that is unique to the members of the scallop family is the presence, at some point during the animal’s life cycle, of a distinctive shell feature, a comb-like structure called a ctenolium located on the anterior edge of the right valve next to the byssal notch. Though many scallops lose this feature as they become free-swimming adults, all scallops have a ctenolium at some point during their lives, and no other bivalve has an analogous shell feature. The ctenolium is found in modern scallops only; the ancestors of modern scallops, the entoliids, did not possess it.