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Tridacna gigas, the giant clam, is the best-known species of the giant clam genus Tridacna. Giant clams are the largest extant taxon bivalve . Several other species of "giant clam" in the genus Tridacna are often misidentified as Tridacna gigas.
These clams were known to indigenous peoples of East Asia for thousands of years and the Venetian scholar and explorer Antonio Pigafetta documented them in a journal as early as 1521. One of a number of large clam species native to the shallow of the South Pacific Ocean and Indian Ocean oceans, they may weigh more than , measure as much as across, and have an average lifespan in the wild of more than 100 years. They also are 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 may be found at depths of as great 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. The maxima clam has the largest geographical distribution among giant clam species; it may be found off high- or low-elevation islands, in or . Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.
Although clams are , 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 . This method of algal farming is under study as a model for highly efficient bioreactors.
The mantle border itself is covered in several hundred to several thousand pinhole approximately in diameter. 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 Ultraviolet, which may be unique among Mollusca. These receptors allow T. gigas to partially close their shells in response to dimming of light, change in the direction of light, or the movement of an object. The optical system forms an image by sequential, local dimming of some eyes using pigment from the aperture.
A heavier giant clam was found in 1956 off the Japanese island of Ishigaki Island. The shell's length was , and it weighed dead and estimated alive.
In very small clams— dry tissue weight—filter feeding provides approximately 65% of total carbon needed for respiration and growth; comparatively larger clams () acquire only 34% of carbon from this source. A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building (hermatypic) corals.
Since giant clams cannot move themselves, they adopt broadcast spawning, releasing sperm and eggs into the water. A transmitter substance called spawning induced substance (SIS) helps synchronize the release of sperm and eggs to ensure fertilization. The substance is released through a Siphon. Other clams can detect SIS immediately. Incoming water passes chemoreceptors situated close to the incurrent syphon that transmit the information directly to the cerebrum ganglia, a simple form of brain.
Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract its adduction. Each clam then fills its water chambers and closes the incurrent syphon. The shell contracts vigorously with the adductor's help, so the excurrent chamber's contents flows through the excurrent syphon. After a few contractions containing only water, eggs and sperm appear in the excurrent chamber and then pass through the excurrent syphon into the water. Female eggs have a diameter of . Egg release initiates the reproductive process. An adult T. gigas can release more than 500 million eggs at a time.
Spawning seems to coincide with incoming tides near the second (full), third, and fourth (new) quarters of the moon phase. Spawning contractions occur every two or three minutes, with intense spawning ranging from thirty minutes to two and a half hours. Clams that do not respond to the spawning of neighboring clams may be reproductively inactive.
At roughly one week of age, the clam settles on the ground, although it changes location frequently within the first few weeks. The larva does not yet have symbiotic algae, so it depends completely on plankton. Also, free-floating zooxanthellae are captured while filtering food. Eventually the front adductor muscle disappears and the rear muscle moves into the clam's center. Many small clams die at this stage. The clam is considered a juvenile when it reaches a length of . It is difficult to observe the growth rate of T. gigas in the wild, but laboratory-reared giant clams have been observed to grow a year.
The ability for Tridacna to grow to such large sizes with fleshy mantles that extend beyond the edges of their Bivalve shell is considered to be the result of total reorganization of Bivalvia development and morphology. 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 Metamorphosis undergo.
On the black market, giant clam shells are sold as decorative accoutrements.
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 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 an account of the discovery of the Pearl of Lao Tzu, Wilburn Cobb said he was told that a Dyak people diver was drowned when the Tridacna closed its shell on his arm. Accounts by Wilburn Dowell Cobb . pearlforpeace.org 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.
Other myths focus on the huge size of giant clams being associated with long age. While giant clams do live a long time and may serve as a bio-metric for historic climatic conditions, their large size is more likely associated with rapid growth.
T. gigas has been reported as Local extinction in peninsular Malaysia, while Tridacna derasa and Hippopus porcellanus are restricted to Eastern Malaysia. These recent local extinctions have motivated the introduction of giant clams to Hawaii and Micronesia following maricultural advancements. Restocked individuals in the Philippines have successfully dispersed their own spawned larvae to at least several hundred meters away after only ten years.
Aquaculture
Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau). A large 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.Copland, J. W. and J. S. Lucas (Eds.) 1988. Giant Clams in Asia and the Pacific. ACIAR Monograph No. 9Fitt W.K (Ed.) 1993. Biology and Mariculture of Giant Clams; a workshop held in conjunction with the Seventh International Coral Reef Symposium, 21–26 June 1992, Guam, USA 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. The species is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international trade (including in parts and derivatives) is regulated.
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