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Family Tridacnidae - giant clams
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Bivalvia |
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Environment |
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Fresh : No |
Brackish : No |
Marine : Yes |
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Synonym of subfamily Tridacninae under family Cardiidae (Ref. 3477). To about 35 cm. Marine (Ref. 94508). Shell equivalve, thick, heavy and often very large, with strongly scalloped free margins, inequilateral. Umbones ventral in position, pointing posteriorly. Free margins of the valves dorsal most in position. Byssal gape generally well developed (obsolete in genus Hippopus), internally plicate, near the umbones. Outer surface of shell with strong radial folds which may be fluted, ribbed or with transverse scales. Ligament external, set in a groove of anteroventral margin. Hinge with a single, ridge-like cardinal tooth in left valve, and 2 lateral teeth in right valve. Interior of shell porcelaneous. A single, large and rounded (posterior) adductor muscle scar, associated with a generally large posterior pedal retractor scar, both of them submedian in postition. Anterior pedal retractor scar small, on superior surface of the cardinal tooth. Pallial line without a sinus. Internal margins often more or less crenulated. Gills of eulamellibranchiate type, with moderately narrow, folded branchial sheets, the outer demibranch sometimes reduced. Foot either small and with a strong byssus (genus Tridacna), or relatively large and non-byssate (genus Hippopus. Exhalant siphon narrow, tubular, located mid-dorsally; inhalant siphon broad, situated high on posterior end of shell, and often provided with tentacles. Mantle margins papillate, extensively fused, provided with a ventral byssal-pedal opening and numerous, small hyaline organs with a lens. The Tridacnidae are highly specialized sedentary bivalves, living in clear shallow waters of coral reefs, with the umbones and hinge situated at the underside of the body and the free edge of shell on the upper side. Depending on the species, they live either unattached in burrows within the corals, or fastened by a strong byssus. When covered during the tide, the valves are opened and the mantle lobes are protruded, exposing to the light 2 large, undulate, and often highly coloured bands. Nutrition partly occurs by filter-feeding, and by nutrient molecules gained from the photosynthesis of symbiotic algae (zooxanthellae) housed in the greatly developed mantle tissue. Hermaphroditic animals, breeding throughout the year or during the summer only (at higher latitudes). Fertilization external, giving free-swimming planktonic larvae. Symbiosis with zooxanthellae is established shortly after settlement of the young clam onto the substrate. Growth rapid in the larger species. Tridacnidae are traditionally harvested throughout the tropical western Pacific for their shell and highly prized meat. All the soft parts are edible except for the kidney which may accumulate arsenic and heavy metals. Widespread overfishing and deterioration of coral environments in some areas have frequently caused a decline of the clam populations in the recent past years (Ref. 348). |
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