Pectinidae

Scallop (/ˈskɒləp, ˈskæləp/) is a common name that is primarily applied to any one of numerous species of saltwater clams or marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name 'scallop' is also sometimes applied to species in other closely related families within the superfamily Pectinoidea, which also includes the thorny oysters. Scallops are a cosmopolitan family of bivalves which are found in all of the world's oceans, although never in fresh water. They are one of very few groups of bivalves to be primarily 'free-living', with many species 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, while others attach themselves to stationary or rooted objects such as sea grass at some point in their lives by means of a filament they secrete called a byssal thread. The majority of species, however, live recumbent on sandy substrates, and when they sense the presence of a predator such as a starfish, they may attempt to escape by swimming swiftly but erratically through the water using jet propulsion created by repeatedly clapping their shells together. Scallops have a well-developed nervous system, and unlike most other bivalves all scallops have a ring of numerous simple eyes situated around the edge 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 to the meat of these bivalves, the adductor muscle, that is sold as seafood. The brightly coloured, symmetrical, fan-shaped shells of scallops with their radiating and often fluted ornamentation are valued by shell collectors, and have been used since ancient times as motifs in art, architecture, and design. Owing to their widespread distribution, scallop shells are a common sight on beaches and are often brightly coloured, making them a popular object to collect among beachcombers and vacationers. The shells also have a significant place in popular culture, including symbolism. Scallops inhabit all the oceans of the world, with the largest number of species living in the Indo-Pacific region. Most species live in relatively shallow waters from the low tide line to 100 m, while others prefer much deeper water. Although some species only live in very narrow environments, most are opportunistic and can live under a wide variety of conditions. Scallops can be found living within, upon, or under either rocks, coral, rubble, sea grass, kelp, sand, or mud. Most scallops begin their lives as byssally attached juveniles, an ability that some retain throughout their lives while others grow into freeliving adults. Very little variation occurs in the internal arrangement of organs and systems within the scallop family, and what follows can be taken to apply to the anatomy of any given scallop species. The shell of a scallop consists of two sides or valves, a left valve and a right one, divided by a plane of symmetry. Most species of scallops rest on their right valve, and consequently this valve is often deeper and more rounded than the left (i.e., upper) valve, which in many species is actually concave. With the hinge of the two valves oriented towards the top, one side corresponds to the animal's morphological anterior or front, the other 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 ('equivalved'), as well as symmetrical front/back ('equilateral'), 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 producing 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, a feature which is unique to and apparent in all adult scallops. 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, an important feature for distinguishing which valve is which). As is the case in almost all bivalves, a series of lines and/or growth rings originates 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 shells of most scallops are streamlined to facilitate ease of movement during swimming at some point in their lifecycles, 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 unique feature of the scallop family is the presence, at some point during the animal's lifecycle, of a distinctive and taxonomically important shell feature, a comb-like structure called a ctenolium located on the anterior edge of the right valve next to the valve's 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.