Giant clam

The largest known T. gigas specimen measured 137 centimetres (4 ft 6 in). 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. The joint weight of the two shells is 230 kilograms (510 lb), which suggests that the live weight of the animal would have been roughly 250 kilograms (550 lb).[12][13]

Another unusually large giant clam was found in 1956 off the Japanese island of Ishigaki. However, it was not examined scientifically before 1984. The shell's length was 115 centimetres (3 ft 9 in) and the weight of the shells and soft parts was 333 kilograms (734 lb). Scientists estimated the live weight to be around 340 kilograms (750 lb).[12]

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  Huge maxima clam specimen in Baa Atoll, Maldives
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  Giant clam in Bunaken island, Sulawesi, Indonesia
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  Empty giant clam shell in the French National Museum of Natural History
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  Empty shell from the Aquarium Finisterrae in Spain
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  Giant clam in Waikiki Aquarium, Honolulu, Hawaii, United States

While giant clams filter-feed, symbiotic algae provide them with an essential supplementary source of nutrition.[9] These plants consist of unicellular algae, whose metabolic products add to the clam's filter food.[5] This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters.[9] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume.[14][15]

In very small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides about 65% of total carbon needed for respiration and growth; larger clams (10 g) acquire only 34% of carbon from this source.[16] A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building (hermatypic) corals.[9]

Tridacna gigas reproduce sexually and are hermaphrodites (producing both eggs and sperm). Self-fertilization is not possible, but this characteristic does allow them to reproduce with any other member of the species. This reduces the burden of finding a compatible mate, while simultaneously doubling the number of offspring produced by the process. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations be passed to further generations.[17]

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 syphonal outlet. Other clams can detect SIS immediately. Incoming water passes chemoreceptors situated close to the incurrent syphon, which transmit the information directly to the cerebral ganglia, a simple form of brain.[18]

Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract its adductor muscle. 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 100 micrometres (0.0039 in). Egg release initiates the reproductive process. An adult T. gigas can release more than 500 million eggs at a time.[19]

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.[20]

Behaviours associated with different stages of the giant clam' life cycle [21]

The fertilized egg floats in the sea for about 12 hours until eventually a larva (trocophore) hatches. It then starts to produce a calcium carbonate shell. Two days after fertilization it measures 160 micrometres (0.0063 in). Soon it develops a "foot," which is used to move on the ground; it can also swim to search for appropriate habitat.[22]

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. Free floating zooxanthellae are also 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 20 cm (8 in) .[23] It is difficult to observe the growth rate of T. gigas in the wild, but laboratory-reared giant clams have been observed to grow 12 cm (4.7 in) a year.[24]

As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. It was known in times past as the "killer clam" and "man-eating clam", and reputable scientific and technical manuals 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.[27]

In an account of the discovery of the Pearl of Lao Tzu, Wilburn Cobb said he was told that a Dyak diver was drowned when the Tridacna closed its shell on his arm.[28]

Today the giant clam is considered neither aggressive nor particularly dangerous. While it is certainly capable of gripping a person, the shell's closing action is defensive, not aggressive, and the shell valves close too slowly to pose a serious threat. Furthermore, many large individuals are unable to completely close their shells.

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau).[29] 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.[30][31][32] Recent developments in aquaculture, specifically at Harbor Branch Oceanographic Institute in Fort Pierce, Florida, and in the Marshall Islands, have succeeded in tank-raising T. gigas, both for use in home aquariums and for release into the wild.

Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea. A programme to propagate endangered giant clams for release into the wild has been ongoing since 2007. Undertaken by the Marine Ecology Research Centre (www.merc-gayana.com) based in Gaya Island just west of Sabah's capital, Kota Kinabalu, the programme successfully nurtured all seven species of the giant clams found in Malaysian waters to sufficient maturity for them to be placed in an ocean nursery for the first time during an awareness month from 22 March until 22 April 2012 in Maloham Bay. This marine awareness month had been planned to highlight and celebrate MERC's success in raising the giant clam larvae (called "spats") to juvenile stage, to highlight the importance of the giant clams and to raise awareness and support with the general public on the threats that are faced by the giant clams within the sea. During this marine awareness month, the coral restoration programme entered its final stage and attachment of 1000 one-year-old coral fragments grown at MERC's ocean nursery onto the coral reef were done throughout the month. The coral restoration programme is aimed to provide the giant clams with a suitable home surroundings when they are big enough in the future to be placed onto the reef.