Scallops swim by clapping shells and watch with mantle eyes
Scallop names many marine bivalves in family Pectinidae—and sometimes cousins in Pectinoidea. Cosmopolitan in the world’s oceans but never in fresh water, they are among the few mostly free-living bivalves, able to swim short bursts and even migrate across the seafloor.
A minority cement to rock as adults; others hitch to seagrass at times. Most rest on sand yet flee starfish by jetting erratically—clapping valves to shove water. Unlike most bivalves they sport a well-developed nervous system and a ring of numerous simple eyes along the mantle edge. Humans prize the adductor muscle as seafood and farm some species; collectors chase bright, symmetrical, fluted fans that have styled art and architecture for centuries.
The family name Pectinidae traces to Latin pecten, a comb, after a comb-like structure beside the shell's byssal notch. Every adult has a straight, toothless hinge flanked by two flat ears, a feature unique to scallops. Most lie on the right valve, so that side tends to be deeper, while the upper left one can even be concave. Inside sits a single central adductor, bigger than an oyster's, split into a fast striated part that powers swimming and a smooth catch part that holds the valves shut for long spells at little energy cost.
The eyes, up to 200 of them and each about a millimetre across, focus light with a concave mirror of guanine crystals rather than a lens, and each has a double-layered retina. Yet like every bivalve a scallop has no true brain, only three paired yellowish ganglia, the visceral pair by far the biggest. It strains plankton without siphons, and most of its species live in the Indo-Pacific, typically between the low-tide line and 100 metres down.
Source: Scallop
