Giant clam

Young T. gigas are difficult to distinguish from other species of Tridacninae. Adult T. gigas are the only giant clams unable to close their shells completely, allowing part of the brownish-yellow mantle to remain visible. Similar to coral matrices composed of calcium carbonate, giant clams grow their shells through the process of biomineralization, which is very sensitive to seasonal temperature. The isotopic ratio of oxygen in carbonate and the ratio between strontium and calcium together may be used to determine historical sea surface temperature. Each one consists of a small cavity containing a pupil-like aperture and a base of 100 or more photoreceptors sensitive to three different ranges of light, including UV, which may be unique among molluscs. The optical system forms an image by sequential, local dimming of some eyes using pigment from the aperture. Largest specimens The largest known T. gigas specimen measured ; it weighed dead and was estimated to be alive. It was discovered around 1817 on the north western coast of Sumatra, Indonesia, and its shells are now on display in a museum in Northern Ireland. A heavier giant clam was found in 1956 off the Japanese island of Ishigaki. The shell's length was , and it weighed dead and estimated alive. File:Giant Clam (Tridacna gigas) (6058446919).jpg|Giant clam in Bunaken Island, Sulawesi, Indonesia File:Tridacna gigas 01 by Line1.JPG|Empty giant clam shell in the French National Museum of Natural History File:Tridacna gigas.001 - Aquarium Finisterrae.JPG|Empty shell from the Aquarium Finisterrae in Spain File:Living giant clam (Tridacna gigas), Waikiki Aquarium.JPG|Giant clam in Waikiki Aquarium, Honolulu, Hawaii, United States == Ecology ==

Feeding Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are supplied by zooxanthellae. This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters. Reproduction Tridacna gigas reproduce sexually and are hermaphrodites (producing both eggs and sperm by one clam). While self-fertilization is not possible, having both characteristics does allow them to reproduce with any other member of the species as well as hermaphrodically. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations be passed to further generations. Historically, two evolutionary explanations have been suggested for this process. Sir Yonge suggested and maintained for many years that the visceral-pedal ganglia complex rotate 180 degrees relative to the shell, requiring that they develop and evolve independently. Stasek proposed instead that the growth occurs primarily in a posterior direction instead of the more typical direction of ventral in most bivalves, which is reflected in the transitional stages of alternative ways of growing that juveniles undergo. == Human relevance ==

s of the Église Saint-Sulpice in Paris, carved by Jean-Baptiste Pigalle as a paint holder The main reason that giant clams are becoming endangered is likely to be intensive exploitation by bivalve fishers. Mainly large adults are killed because they are the most profitable. Even in countries where giant clams are easily seen, stories incorrectly depict giant clams as aggressive beings. For instance, although the clams are unable to close their shells completely, a Polynesian folk tale relates that a monkey's hand was bitten off by one, and even though once past larval stage, the clams are sessile, a Maori legend relates a supposed attack on a canoe by a giant clam. Starting from the eighteenth century, claims of danger had been related to the western world. In the 1920s, a reputable science magazine Popular Mechanics once claimed that the great mollusc had caused deaths. Versions of the U.S. Navy Diving Manual even gave detailed instructions for releasing oneself from its grasp by severing the adductor muscles used to close its shell. In reality, the slow speed of their adductor muscle contraction and the need to force water out of their shells while closing, prevents them from trapping a human. A large Australian government-funded project from 1985 to 1992 mass-cultured giant clams, particularly T. gigas at James Cook University's Orpheus Island Research Station, and supported the development of hatcheries in the Pacific Islands and the Philippines. Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea. == Conservation status ==

There is concern among conservationists about whether those who use the species as a source of livelihood are overexploiting it. The numbers in the wild have been greatly reduced by extensive harvesting for food and the aquarium trade. Restocked individuals in the Philippines have successfully dispersed their own spawned larvae to at least several hundred meters away after only ten years. == See also ==