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The giant clam (''Tridacna gigas''), known as ''pā’ua'' in Cook Islands Māori, is a clam that is the largest living bivalve mollusk. ''Tridacna gigas'' is one of the most endangered clam species. Antonio Pigafetta documented these in his journal as early as 1521. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they can weigh more than , measure as much as across, and have an average lifespan in the wild of over 100 years.〔(【引用サイトリンク】title=Giant Clam: ''Tridacna gigas'' )〕 They are also found off the shores of the Philippines, where they are called "''taklobo''", and in the South China Sea in the coral reefs of Sabah (Malaysian Borneo). ''Tridacna gigas'' lives in flat coral sand or broken coral and can be found at depths of as much as 20 m (66 ft).〔Knop, p. 10.〕 Its range covers the Indo-Pacific, but populations are diminishing quickly and the giant clam has become extinct in many areas where it was once common. ''Tridacna maxima'' has the largest geographical distribution among giant clam species; it can be found off high- or low-elevation islands, in lagoons or fringing reefs.〔Munro, p. 99.〕 Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.〔 Although larval clams are planktonic, they become sessile in adulthood. The creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which the adult clams get most of their nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesize. == Anatomy == Young ''T. gigas'' are difficult to distinguish from other species of Tridacnidae. Adult ''T. gigas'' are the only giant clams unable to close their shells completely. Even when closed, part of the mantle is visible, unlike the very similar ''T. derasa''. However, this can only be recognized with increasing age and growth. Small gaps always remain between shells through which retracted brownish-yellow mantle can be seen.〔Knop, p. 32.〕 ''Tridacna gigas'' has four or five vertical folds in its shell; this is the main characteristic that separates it from the very similar shell of ''T. derasa'', which has six or seven vertical folds.〔 As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells.〔Dame, p. 51.〕 The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.〔Gosling, p. 23.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Giant clam」の詳細全文を読む スポンサード リンク
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