Гигант деңіз қабыршағы: Түрлер, өмір сүру ортасы және қауіптер

Species of bivalve

Tridacna gigas, the giant clam, is the most well known species of the giant clam genus Tridacna. Giant clams are the largest living bivalve mollusks. Several other species of "giant clams" in the genus Tridacna, are often misidentified as Tridacna gigas. Known to indigenous peoples in the area for thousands of years, Antonio Pigafetta also 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. They are also found off the shores of the Philippines and in the South China Sea in the coral reefs of Malaysia. The giant clam lives in flat coral sand or broken coral and can be found at depths of as much as 20 m (66 ft). 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. 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 can 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 senstive 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.

The largest known T. gigas specimen measured , and it weighed 230 kg (510 lb) dead and estimated 250 kg (550 lb) 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. This enables giant clams to grow as large as one meter in length even in nutrient poor coral reef waters.]] The fertilized egg floats in the sea for about 12 hours until eventually a larva (trochophore) hatches. It then starts to produce a calcium carbonate shell. Two days after fertilization it measures Soon it develops a "foot," which is used to move on the ground; it can also swim to search for appropriate habitat. Two evolutionary explanations were historically 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.

The main reason that giant clams are becoming endangered is likely to be intensive exploitation by bivalve fishing vessels. Mainly large adults are killed, since they are the most profitable. Even in countries where giant clams are easily seen, stories depict giant clams as aggressive beings. For instance, a Polynesian folk tale involves a monkey's hand being bitten off, and a Maori legend involves an attack on a canoe. Starting from the 18th century, claims of danger had surfaced to the western world. In 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 practice, their abductor muscle's slow speed and the need to force water out of their shell 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. Restocked individuals in the Philippines have successfully dispersed their own spawned larvae at least several hundred meters away after only 10 years.