Tridacna is a genus of large and gigantic saltwater clams, marine bivalve mollusks in the subfamily Tridacninae, the giant clams. They have heavy shells, fluted with 4–6 folds. Mantle is brightly coloured. They inhabit shallow waters of coral reefs in warm seas of the Indo-Pacific region. These clams are popular in marine aquaria, and in some areas, such as the Philippines, members of the genus are farmed for the marine aquarium trade. They live in symbiosis with photosynthetic algae (zooxanthellae). Some species are used as seafood.
Systematics and phylogeny
Subgenus Tridacna (Tridacna)
- Tridacna derasa (Röding, 1798)
- Tridacna gigas (Linnaeus, 1758)
- Tridacna tevoroa Lucas, Ledua & Braley, 1990
Subgenus Tridacna (Chametrachea)
- Tridacna costata Richter, Roa-Quiaoit, Jantzen, Al-Zibdah, Kochzius, 2008
- Tridacna crocea Lamarck, 1819
- Tridacna maxima Röding, 1798( =Tridacna elongata)
- Tridacna rosewateri Sirenho & Scarlato, 1991
- Tridacna squamosa Lamarck, 1819
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An alternative older classification recognises a third subgenus Persikima containing T. derasa and T. tevoroa. Recent biochemical studies have suggested that there may exist morphologically indistinct cryptic species.
Ecology and behaviour
Tridacna clams are common inhabitants of Indo-Pacific coral reef benthic communities in shallower waters. They live in symbiosis with photosynthetic dinoflagellate algae (Symbiodinium) that grow in the mantle tissues. Light penetrates the mantle through small lens-like structures called ocelli.  They are sessile in adulthood. By day, the clams spread out their mantle so that the algae receive the sunlight they need to photosynthesize, whereas the colour pigments protect the clam against excessive light and UV radiation. They get most (70-100%) of their nutrients from the algae and the rest from filter feeding. When disturbed, the clam closes its shell. The popular opinion that they pose danger to divers who get trapped or injured between the closing sharp-edged shell is not very real, as the closing reaction is quite slow. Their large size and easy accessibility has caused overfishing and collapse of the natural stocks in many places and extirpation in some of the species. They are being sustainably farmed in some areas.
Over a hundred examples of carved Tridacna shells have been found in archaeological expedititions from Italy to the Near East. Similar in artistic style, they were probably produced in the mid-seventh century, made or distributed from the southern coast of Phoenicia. The backs and interior perimeters of the shells show animal, human, and floral motifs, while the interiors typically show recumbent sphinxes. The umbo of the shell is in the shape of a human female or bird's head. They were probably used to store eye cosmetics.
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- Murphy 2002, p. 25
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- Murphy 2002, p. 28
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