The gryphaeid shell is medium-sized to large (to 300 mm), thin-walled to solid, and circular to oval, with the valve margins often plicate. It is cemented by (PLEUROTHETIC on) the left valve, usually by a small attachment area on the left valve, or secondarily detached. Most species are highly INEQUIVALVE (lower left valve more convex, upper right valve lidlike [e.g., Neopycnodonte]), whereas some are EQUIVALVE (e.g., Hyotissa). The umbonal recess of inequivalve forms usually fills with solid shell matter. The shell is inflated or compressed, not gaping, and EQUILATERAL, with OPISTHOGYRATE UMBONES separated by a wide CARDINAL AREA. Shell microstructure is a mixture of ARAGONITE and (primarily) CALCITE,
and one- or two-layered, with a thin or absent calcitic PRISMATIC outer layer and a foliated calcitic inner layer; the latter has a characteristic vesicular structure that is most apparent marginally and appears spongy or foam¬like under magnification. Irregular, porous, white, CHALKY DEPOSITS of unknown cause or function are common on the interior surface. Fluid-filled cavities can be present between the shell layers. TUBULES are absent. Exteriorly gryphaeids can be distinctly colored (e.g., pink, brown, or purple-black) and are covered by a thin, nonpersistent PERIOSTRACUM. Sculpture is radially undulate or ribbed in many species, in some with scales and/or HYOTE spines; the left valve can have a deep posterior radial groove or flexure. LUNULE and ESCUTCHEON are absent. Interiorly the shell is non-NACREOUS. The PALLIAL LINE is ENTIRE and obscure. An angulate shell margin (COMMISSURAL SHELF) is usually wide and well defined, and is denticulated by an interlocking complex of anastomosing elongated, sinuous ridges and tubercles (VERMICULAR CHOMATA), limited to each side of the ligament. The HINGE PLATE is wide and EDENTATE in adults. The LIGAMENT is ALIVINCULAR and AMPHIDETIC; an internal por¬tion (RESILIUM) is large and sunken into a trigonal, subumbonal RESILIFER.
The animal is MONOMYARIAN (anterior ADDUCTOR MUSCLE absent); the posterior adductor muscle is large, posterocentral or -ventral, and round in cross section or only slightly flattened dorsally. The ventral border of the muscle scar in the left valve is elevated above the surface of the valve, whereas its dorsal border is indented. Pedal retractor, elevator, and protractor muscles are absent. Ctenidial protractor muscles (QUENSTEDT MUSCLES) insert below the umbones. The mantle and gills can be darkly pigmented. The MANTLE margins are not fused ventrally except at the point of ventral gill attachment; SIPHONS are absent. The inner mantle folds form a PALLIAL VEIL. The mantle is not fused to the visceral mass on the right side of most species (not fused on both sides in Hyotissa), creating a space (PROMYAL PASSAGE) dorsal to the posterior adductor muscle that allows more efficient posterior flow of water. HYPOBRANCHIAL GLANDS have not been reported. The FOOT and BYSSUS are absent in the adult.
The LABIAL PALPS are large; they are fused medially to form a hood over the mouth. The CTENIDIA are large and partially encircle the centralized adductor muscle. They are PSEUDOLAMELLIBRANCH (SYNAPTORHABDIC), HETERORHABDIC, of about equal size, and not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). CEPHALIC EYES can be present. 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 STYLE SACK extends into a tapering sacklike extension of the visceral mass (METASOMA). The MIDGUT is not coiled. The HINDGUT passes through the ventricle of the heart, and leads to a freely hanging rectum with a fleshy collarlike rim. ACCESSORY HEARTS are absent. Gryphaeids are PROTANDRIC or SIMULTANEOUS HERMAPHRODITES and produce planktonic VELIGER larvae. Differences in the PRODISSOCONCH hinge have been identified to distinguish gryphaeid larvae from those of ostreids: gryphaeids have subequal PROVINCULAR teeth and sockets that continue uninterrupted along the entire hinge line, whereas a median gap is present in ostreids. The nervous system is not concentrated but pedal ganglia and STATOCYSTS are absent in adults (associated with absence of the foot and active locomotion). ABDOMINAL SENSE ORGANS are present and asymmetrical.
Gryphaeids are marine or estuarine SUSPENSION FEEDERS. They are cemented to hard subtidal substrata (e.g., rocks, dead coral, shipwrecks), but are generally not gregarious or reef-forming. Exterior surfaces of the valves are often encrusted by epibionts and (in larger species) bored by sponges, polychaete worms, and smaller bivalves. In the Indo-Pacific, a pearlfish (Onuxodon parvibrachium (Fowler, 1927), Carapidae) has been reported to inhabit the MANTLE CAVITY of Hyotissa hyotis. The latter species has been introduced to Florida, probably via commercial ship traffic (ballast water).
The family Gryphaeidae is known since the Triassic, is represented by 2 living genera and only ca. 5 species, and is distributed in tropical and subtropical seas. The characteristic vesicular shell structure, most visible near the inner shell margins, is the source of the common name "foam oysters" for this family.