Mollusca (Phylum) > Bivalvia (Class) > Heterodonta (Subclass) > Euheterodonta (Infraclass) > Veneroida (Order) > Cardioidea (Superfamily) > Cardiidae (Family) > Tridacninae (Subfamily) > Tridacna (Genus)

Le tridacne géant ou bénitier géant (Tridacna gigas) est le plus gros mollusque bivalve. Cette espèce comestible est protégée car elle est en danger d'extinction.

Répartition
Ils habitent la mer Rouge et les océans Indien et Pacifique.

Biotope
Fonds récifaux coralliens peu profonds de 50 cm à 15 mètres.

Comportement
Lorsqu'il est jeune, l'animal sécrète un byssus, touffe de filaments qui passe par l'ouverture de la coquille et par laquelle il se fixe au fond marin, la charnière dirigée vers le bas. À mesure que la colonie grandit, des coraux, des éponges, des algues la recouvrent ou l'entourent, la dissimulant sous leur masse, n'en laissant dépasser que le bord. Les valves légèrement écartées laissent entrevoir le manteau brillamment coloré en vert, en rouge, ou en bleu. Les bords de ce manteau sont hérissés de protubérances enfermant les organes hyalins, sortes de lentilles qui concentrent la lumière dans les profondeurs des tissus et y favorisent, par la photosynthèse, la multiplication d'algues microscopiques. Absorbées par les globules blancs, ces algues constitueraient la nourriture principale du mollusque, dont le tube digestif est fortement réduit chez l'adulte. Entre les lobes du manteau s'ouvrent deux tubes, les siphons inhalant et exhalant par lesquels pénètre et sort un courant d'eau mettant la cavité palléale en communication avec le milieu ambiant.

Reproduction
La majorité des espèces sont hermaphrodites simultanés, ils deviennent mâles et peuvent se reproduire entre 2 et 6 ans ; puis par la suite quand ils ont atteint leur taille adulte, ils peuvent produire des ovocytes. Leur reproduction se passe en deux temps : en premier les gamètes mâles sont lâchés en pleines eaux puis viennent en deuxième les ovocytes. La fécondation donne naissance à une larve qui va mener une vie planctonique pour s'installer au bout d'un certain temps sur un support où elle évoluera en bivalve.

Particularité
Il s'agit du plus grand coquillage du monde : sa coquille peut mesurer 1,5 m et peser 250 kg. Elle ne possède pas de dessin concentrique (écaille). Ils doivent leur nom de bénitier à leur utilisation traditionnelle dans les églises catholiques pour contenir l'eau bénite aux portes d'entrée. Jules Verne dans Vingt mille lieues sous les mers décrit le salon du Capitaine Némo, dans le Nautilus, avec une fontaine constituée à partir d'un tridacne géant délicatement dentelé, d'un diamètre de deux mètres et éclairé par le dessous.

Synonymes
Chama gigantea (Perry, 1811)
Chama gigas (Linnaeus, 1758)
Tridacna (Tridacna) gigas (Linnaeus, 1758)

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The giant clam, Tridacna gigas (known as pa'ua in Cook Islands Maori), is the largest living bivalve mollusk. Tridacna gigas is one of the most endangered clam species. Antonio Pigafetta 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 200 kilograms (440 lb), measure as much as 120 cm (47 in) across, and have an average lifespan in the wild of 100 years or more. They are also found off the shores of the Philippines, where they are called taklobo, and in the South China Sea in the coral reefs of Sabah (Malaysian Borneo). Tridacna gigas lives in flat coral sand or broken coral and can be found at depth 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. Tridacna maxima has the largest geographical distribution among giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs. Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue. Although larval clams are planktonic, they become sessile in adulthood. The creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which it gets nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesize.

Anatomy
Young Tridacna gigas are difficult to distinguish from other species of Tridacnidae. Adult Tridacna gigas are the only giant clams unable to close their shells completely. Even when closed, part of the mantle is visible, unlike the very similar Tridacna derasa. However, this can only be recognized with increasing age and growth. Small gaps always remain between shells through which retracted brownish-yellow mantle can be seen. Tridacna gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the very similar shell of Tridacna derasa, which has six or seven vertical folds. As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells. The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.

Largest specimens
The largest known Tridacna gigas specimen measured 137 centimetres (4.49 ft). It was discovered around 1817 on the north western coast of Sumatra. The weight of the two shells was 230 kilograms (510 lb). This suggests that the live weight of the animal would have been roughly 250 kilograms (550 lb). Today these shells are on display in a museum in Northern Ireland. 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 (3.77 ft) and the weight of the shells and soft parts was 333 kilograms (730 lb). Scientists estimated the live weight to be around 340 kilograms (750 lb).

ECOLOGY
Feeding
Algae provide giant clams with a supplementary source of nutrition. These plants consist of unicellular algae, whose metabolic products add to the clam's filter food. As a result, they are able to grow as large as 100 cm length even in nutrient-poor coral-reef waters. The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume. In small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides about 65% of total carbon needed for respiration and growth; large clams (10 g) 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.

Reproduction
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 are passed to further generations. Since giant clams cannot move themselves, they adopt broadcast spawning. They release 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 inccurent syphon, which transmit the information directly to the cerebral 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 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 Tridacna gigas can release more than 500 million eggs at a time. Richard D. Braley of the University of New South Wales School of Zoology observed that spawning seems to coincide with incoming tides near the second (full), third, and fourth (new) quarters of the moon phase. Spawning contractions occurred every 2–3 minutes, with intense spawning ranging from thirty minutes to two and a half hours. Braley also hypothesized that clams that do not respond to the spawning of neighbor clams may be reproductively inactive.

Development
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. 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 . It is difficult to observe the growth rate of Tridacna gigas in the wild, but laboratory-reared giant clams have been observed to grow 12 cm a year.

Human relevance
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. The giant clam is considered a delicacy in Japan (known as Himejako), France, South East Asia and many Pacific Islands. Some Asian foods include the meat from the muscles of clam. On the black market, giant clam shells are sold as decorative accoutrements. At times large amounts of money were paid for the adductor muscle, which Chinese people believed have aphrodisiac powers. A team of American and Italian researchers analyzed bivalves and found they were rich in amino acids that trigger increased levels of sex hormones. Their high zinc content aids the production of testosterone.

Legend
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 or 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. 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. 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.

Aquaculture
Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (belau). A large Australian government-funded project from 1985–1992 mass cultured giant clams, particularly Tridacna gigas at James Cook University's Orpheus Island Research Station, and supported the development of hatcheries in the Pacific Islands and the Philippines. Recent developments in aquaculture, specifically at Harbor Branch Oceanographic Institute in Ft. Pierce, Florida, and in the Marshall Islands, have succeeded in tank-raising Tridacna 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 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 happening from 22 March till 22 April 2012 in Maloham Bay. This Marine Awareness month March 2012 – April 2012 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 program will enter its final stage and attachment of 1000 one year old coral fragments grown at MERC's ocean nursery onto the coral reef would be done throughout the month. The coral restoration program 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.

Conservation status
The IUCN lists the giant clams as vulnerable. 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.