Invertebrates
Giant Clam
Tridacna gigas · Bivalve mollusk — family Cardiidae, subfamily Tridacninae; the largest living bivalve
A mature giant clam is a reef landmark: a meter-plus of fluted shell holding a mantle patterned in electric blues, greens, and golds, no two animals alike. The colors come partly from iridescent cells managing light for the clam's real engine — symbiotic algae farmed in its tissues — which is why giants sit in the sunlit shallows gaping wide by day. Old stories about man-trap clams are folklore; a disturbed animal vents and closes slowly, and the largest cannot seal at all. The genuine drama is reproductive, broadcast spawning that fogs whole lagoon patches with gametes. Tridacna gigas has been fished out of much of its former range for meat and shell, and many animals divers meet today, notably in Palau, descend from hatchery restocking. Find the paired siphons, note the sunburst mantle pattern, and keep fins well clear.
どこで・いつ見られるか
「常駐（Resident）」とは、そのダイビングスポットでのほとんどのダイブで遭遇できることを意味します。「季節限定（Seasonal）」とは、記載された月に確実に見られることを意味します。それ以外の出会いはボーナスであり、計画の前提にはなりません。
| 目的地 | 信頼性 | 月 | メモ |
|---|---|---|---|
| Cairns & the Ribbon ReefsAustralia | Resident — seen on most dives | 年間を通じて | |
| PalauPalau | Resident — seen on most dives | 年間を通じて | |
| Raja AmpatIndonesia | Resident — seen on most dives | 年間を通じて | |
| Sipadan & MabulMalaysia | Resident — seen on most dives | 年間を通じて | |
| YapFederated States of Micronesia | Resident — seen on most dives | 年間を通じて |
ベストな月、一目でわかる
| 月 | 安定して出会える目的地 |
|---|---|
| January | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| February | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| March | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| April | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| May | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| June | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| July | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| August | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| September | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| October | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| November | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
| December | Cairns & the Ribbon Reefs · Palau · Raja Ampat · Sipadan & Mabul · Yap |
理解を深める
ハビタット: Sunlit reef flats, lagoon floors, and upper slopes — shallow because its symbiotic algae need strong light.
分布域: Coral Triangle, Micronesia, and the Great Barrier Reef; fished out of much of its former Indo-Pacific range, with reintroductions in several countries.
生態・行動: Sessile after settlement; gapes its patterned mantle to the sun by day, contracting and slowly closing when shadowed. Spawns by broadcast, fogging the water with gametes.
食性: Mostly sugars from symbiotic algae farmed in the mantle, topped up by filter feeding.
サイズ: 目安は 1.0 m; 確認済みの最大記録 1.3 m (shell length; the largest reliably weighed animals exceed 200 kg).
出会ったときの正しいふるまい方
Keep fins, gauges, and hands clear of the mantle and siphons; shadow the animal as little as possible and never test the closing reflex.
保全
Listed Vulnerable under a 1996 assessment widely considered out of date — recent studies argue it is far more threatened — locally extinct in parts of its range, and traded under CITES Appendix II; hatchery programs restock several countries.
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