The pteriid shell is medium-sized to large (to >300 mm), thin-walled to solid, and obliquely oval to circular, with anterior and posterior AURICLES (posterior usually longer). It is EQUIVALVE to strongly INEQUIVALVE (right valve less convex, byssally notched, and usually resting on it [PLEUROTHETIC on the right valve]), and compressed. The shell is strongly INEQUILATERAL (umbones anterior), with OPISTHOGYRATE UMBONES separated by a narrow CARDINAL AREA. Shell microstructure is a mixture of ARAGONITE and CALCITE, and two-layered, with a calcitic simple PRISMATIC outer layer, and an aragonitic sheet-NACREOUS inner layer. The outer layer often includes scaly lamellae that readily crack and flake, especially in dried shells. TUBULES are absent. Exteriorly pteriids are white to brown to greenish, in some individuals with darker radial stripes, and covered by a nonpersistent PERIOSTRACUM. Sculpture is variable, usually smooth or weakly commarginal; fingerlike lamellae align in radial rows in many species. LUNULE and ESCUTCHEON are absent. Interiorly the shell is thinly to thickly nacreous, with a wide ventral prismatic border. The nacre is widely variable in color, from white to gray to shades of green, blue, yellow, or rose, most famously in the Tahitian populations of the Black-lipped Pearl Oyster, Pinctada margaritifera. The PALLIAL LINE is ENTIRE, obscure, and discontinuous. The inner shell margins are smooth. The HINGE PLATE is straight and narrow, EDENTATE in adults (in Electroma) or with weak teeth reduced to a single subumbonal denticle fitting into a socket in the opposite valve, plus one set of elongated posterior ridges; teeth are stronger in juveniles, often obsolete in adults. The hinges of Pinctada and Pteria include identical components but are mirror images of one another. The LIGAMENT is ALIVINCULAR and AM-PHIDETIC, extending along the entire dorsal margin; the internal portion (RESILIUM) is set
on a subumbonal, posteriorly directed, trigonal RESILIFER.
The animal is MONOMYARIAN as an adult (anterior ADDUCTOR MUSCLE absent); the posterior adductor muscle is large and central, and concave to oval in cross section. The posterior pedal retractor muscles are large and insert close to (or within the cavity of) the posterior adductor scar; the anterior pedal retractors are small, and each divides into two portions before insertion at the anterodorsal corner of the shell. Pedal protractors and elevators are absent. The MANTLE margins are not fused ventrally; SIPHONS are absent. The inner mantle folds form a
PALLIAL VEIL. Simple PALLIAL EYES (without lens) are present on the outer folds some species. HYPOBRANCHIAL GLANDS have not been reported. The FOOT is anterior, small, and digitiform. It has a BYSSAL GROOVE; the adult is byssate.
The LABIAL PALPS are small. The CTENIDIA are large and partially encircle the central-ized muscle bundles. They are FILIBRANCH or PSEUDOLAMELLIBRANCH (ELEUTHERO-RHABDIC), usually HETERORHABDIC, and not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). CEPHALIC EYES are present in most species (on both sides in Pteria, in some Pinctada on the left side only, and absent in Electroma). Distally the tips of the ctenidia are attached to the inner mantle fold, separating INFRA-and SUPRABRANCHIAL CHAMBERS; water flow is anteroposterior. The STOMACH is TYPE III. The MIDGUT is not coiled. The HINDGUT usually passes through (dorsal to in Pinctada) the ventricle of the heart, and leads to a rectum with an ANAL FUNNEL. Pteriids are usually GONOCHORISTIC (some are PROTANDRIC HERMAPHRODITES) and produce planktonic VELIGER larvae. The nervous system is not concentrated. STATOCYSTS are present in adults, with numerous STATOCONIA. ABDOMINAL SENSE ORGANS are present and asymmetrical.
Pteriids are marine SUSPENSION FEEDERS. They are EPIBYSSATE on hard substrata (rocks, seafans, other shells), often gregariously so. Some are cryptically colored attached to marine plants (e.g., Pinctada longisquamosa on seagrass) or heavily coated by epibiotic growth. Commensal pea crabs (Pinnotheridae) or pearlhsh (Onuxodon, Carapidae) are re¬ported in some species.
The family Pteriidae is known since the Triassic, is represented by 3 living genera and ca. 60 species, and is distributed worldwide in subtropical and tropical seas, from the intertidal zone to deep waters. Species of Pinctada and Pteria are widely used for mother-of-pearl and for cultured pearl production, most commonly in the Indo-Pacific region. The Atlantic Pearl Oyster (Pinctada imbricata) was harvested during the 1500s to near extinction at certain Caribbean localities for its diminutive yellowish natural pearls, but has not been successfully used in commercial pearl culturing. The Black-lipped Pearl Oyster (Pinctada margaritijcra), known for its "black" South Seas or Tahitian cultured pearls, has become established off of southeastern Florida, with reports of single living specimens since the 1990s; its likely source of introduction is ship ballast or fouling. The muscle meat of pteriids is consumed locally in Japan and Australia. Growth rates, diseases, genetics, symbionts, predators, shell and pearl formation, and reproduction are well docu¬mented for the commercial species.