Shell of variable form and size, in most cases inequivalve, often with radial ribs or folds; hinge margin occasionally with diverging lamellae or strong, identical teeth anteriorly and posteriorly; an anterior adductor muscle absent; mantle margin in most cases with a broad internal fold and with tentacles, frequently also with eyes; lip margins folded; foot often without byssus, sometimes rudimentary; gill lamellae smooth or pleated; the cerebral ganglia are displaced more or less far backward, and may even be fused with the visceral ganglia. The habits are variable; a few groups are cemented by one valve, others are able to swim by opening and closing the shell.

The pectinid shell is small to large (to 300 mm, cemented Hinnites to 500 mm), usually thin-walled, orbicular to trigonal to oval fan-shaped, and with distinct anterior and posterior AURICLES. It is either EQUIVALVE or INEQUIVALVE (right valve more convex and resting on or cemented by it [PLEUROTHETIC on the right valve]), compressed to inflated, usually not gaping, and with a BYSSAL NOTCH below the auricle in the right valve (accompanied by a shallow SINUS in the left valve). As the animal grows, the byssal notch fills with shell material, leaving behind a distinct track (BYSSAL FASCIOLE). A comb-like series of denticles (CTENOLIUM) is found, at least in early growth stages, along the ventral margin of the byssal notch of the right valve. This structure, recognized as a SYNAPOMORPHY of the family, separates the byssal strands, prevents their rotation and breakage, and mechanically strengthens their attachment. In some species, an analogous tooth (PSEUDOCTENOLIUM) is present, formed from external sculptural elements on the right margin of the byssal notch. The shell is EQUILATERAL, with the UMBONES central and ORTHOGYRATE, but often with one auricle larger than the other. Shell micro structure is a mixture of ARAGONITE and CALCITE, two- or three-layered, with a foliated calcitic outer layer, an aragonitic CROSSED LAMELLAR middle layer (absent in some), and a foliated calcitic inner layer. The right valve of early postmetamorphic juveniles can be of PRISMATIC calcite (i.e., "prismatic stage"), conveying additional flexibility to the right larval shell, but disappears with the onset of radially ribbed sculpture, TUBULES are apparently absent. Exteriorly pectinids are often distinctively and brightly colored, covered by a nonpersistent PERIOSTRACUM. Sculpture is most often composed of radial ribs but can be smooth or cancellate, or differing on the two valves, LUNULE and ESCUTCHEON are absent. Interiorly the shell is non-NACREOUS and often reflects the color(s) of the exterior. The PALLIAL LINE is ENTIRE. The inner shell margins are denticulated by the exterior ribs, in some cases extending onto the inner surface. The HINGE PLATE is straight and EDENTATE in adults, but in some, one to several hinge teeth or auricular ridges (CRURAE) radiate from the umbo on both sides. The dorsal edge of the right valve overlaps that of the left valve in many species. The LIGAMENT is ALIVINCULAR and AMPHIDETIC; an internal portion (RESILIUM) has a nonfibrous core and sits on a central trigonal RESILIFER. A secondary external ligament of fused periostracum unites the valves dorsally.
The animal is MONOMYARIAN (anterior ADDUCTOR MUSCLE absent); the posterior adductor muscle is large and central. Of the pedal retractor muscles, the anterior pair is absent in all and in some only the left posterior is present. Pedal elevator muscles are present, but pedal protractors have not been reported. The MANTLE margins are not fused ventrally; SIPHONS are absent. The inner mantle folds form a PALLIAL VEIL (velum) with guard tentacles; the edges of the velum can be appressed in some species at certain areas to form an EXCURRENT APERTURE. The middle folds bear extensible sensory tentacles and complex PALLIAL EYES (invaginated, with lens). Pectinid eyes are generally blue in color, 1.0-1.5 mm in diameter, and present at the mantle margins of both valves, HYPOBRANCHIAL GLANDS have not been reported. The FOOT is anterior, small to mediumsized, and is suckerlike distally, functioning as a cleansing organ for the adjacent MANTLE CAVITY; it has a BYSSAL GROOVE and in some (e.g., Caribachlamys) produces a BYSSUS in the adult. Other pectinids (e.g., Euvola) are free-living and one (Hinnites) is secondarily cemented (byssally attached as a juvenile) by the right valve.
The LABIAL PALPS are small to medium-sized. The CTENIDIA are large and partially encircle the centralized muscle bundles. They are FILIBRANCH or PSEUDOLAMELLIBRANCH (in both cases ELEUTHERORHABDIC),HETERORHABDIC,and are notinserted into (or fused with) the distal oral groove of the palps (CATEGORY ill association). Water flow is anteroand ventroposterior. The LIPS are hypertrophied and arborescent, interdigitating to create a series of pores over the mouth, allowing exit of excess water without loss of food. The STOMACH is TYPE IV; the MIDGUT is not coiled but can form a wide loop. The HINDGUT passes through the ventricle of the heart, and leads to a freely hanging rectum. Pectinids are usually HERMAPHRODITES (mostly PROTANDRIC; GONOCHORISTIC species also occur) and produce planktonic VELIGER larvae. The nervous system is unusually concentrated, with a complex, fused parietovisceral ganglion that is the largest and most intricate of all Bivalvia; in Pecten, the pedal ganglia are closely associated with the cerebropleural ganglia, STATOCYSTS (with STATOCONIA) are present in adults and are asymmetrical in some species, ABDOMINAL SENSE ORGANS are present and occasionally unpaired.

Pectinids are marine SUSPENSION FEEDERS. Most species of scallops are EPIBYSSATE on hard substrata (e.g., rocks, kelp, or pilings); some are free-living. Common predators include sea stars, whelks, and octopuses. True scallops are perhaps best known for their swimming behavior, achieved by clapping the valves together (by contraction of the adductor muscle), and in some cases by pursing the margins of the pallial veil together, resulting in a form of jet propulsion. Under normal conditions, water jets from right and left dorsal water channels near the auricles, propelling the scallop "forward" with the ventral edge leading; this behavior is also used for expelling PSEUDOFECES. A more rapid escape response jets water from all around the valve margin, sending the shell "backward," usually with the dorsal edge leading. Some species can intentionally detach the byssus and swim away to relocate when danger threatens. Although most scallops are poor swimmers, using the response solely for escape, others are rapid swimmers; Amusium pleuronectes (Linnaeus, 1758) has been clocked at 73 cm/sec. Certain species (e.g., Argopecten irradians (Lamarck, 1819), Placopecten magellanicus (Gmelin, 1791), and Australian Pecten fumatus (Reeve, 1852)) are locally abundant and have been targeted by commercial harvesting, with P. magellanicus providing one of the most valuable shellfisheries along the U.S. eastern seaboard. Today, nearly 60 species of true scallops form the basis of successful aquaculture practices in at least 15 countries. Larger species can harbor diverse epizoic and/or boring communities; pea crabs (Pinnotheridae) have been reported as commensal in the mantle cavities. Pectinids have been widely used symbols in art and heraldry throughout history, particularly in Greek, Roman, and Medieval Europe.
The family Pectinidae is known since the Triassic and is represented by ca. 50 living genera and ca. 400 species, inhabiting intertidal to hadal depths (ca. 7,000 m) from the tropics to polar seas. In the Florida Keys, true scallops are common in shallow-water seagrass beds and under rocks in reefal environments.