Tridacna gigas is one of the most endangered species amongst the clams. It was mentioned as early as 1825 in scientific reports. The giant clam has been reported to exceed 1.2 m in length and 250 kg in weight, making it the largest living bivalve.^[1] The giant clam's growth rate is likely due to its ability to cultivate plants in its body tissue.^[2] T. gigas lives in flat coral sand or broken coral and can be found at depth of as much as 20 meters^[3]. Its range covers the entire Indo-Pacific, but populations are diminishing quickly and the giant clam has become extinct in many areas where it was once common. T. gigas has the largest geographical distribution of the giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs.^[4]

Young Tridacna gigas are difficult to distinguish from other species of Tridacnidae. Adult Tridacna gigas are the only giant clams unable to close the upper rim of their shells completely. Even when closed, part of the mantle is visible, unlike the very similar T. 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.^[5]

T. gigas has four or five vertical folds in its shell; this is the main criterion that separates it from the very similar shell of T. derasa, which has six or seven vertical folds.^[6] As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to massive calcium carbonate shells.^[7] The edges of the clam's mantle are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.^[8]

The clam's mantle is full of zooxanthellae, or brown symbiotic algae, which photosynthesize and provide food for the clam. These zooxanthellae are considered to be closely related to dinoflagellates.^[9] Algae provide giant clams with a supplementary source of nutrition.^[10]. These plants consist of unicellular algae, the metabolic products of which add to the filter food of the clam.^[11] As a result, they are able to grow as large as 100 centimeters in length even in nutrient-poor coral-reef waters.^[12] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per square unit.^[13]

In small clams (10 mg 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.^[14] Giant clams are associated with coral reefs where the same species of zooxenthellae they contain are also symbiotic with many massive reef building (hermatypic) corals.^[15]

According to Daniel Knop in his book Giant Clams, T. gigas reproduce sexually. They are hermaphrodites (producing both eggs and sperm), but self fertilization is not possible. However, since they are hermaphrodites, they do not have to search for a mate of the opposite gender. This guarantees that the gene pool is mixed. New combinations of genes are transferred to further generations.^[16]

Since giant clams can't move across the ocean floor, the solution is broadcast spawning. This entails the release of 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 during reproduction through a syphonal outlet. Other clams can detect SIS immediately. Incoming water passes chemoreceptors situated close to the inccurent syphon, which transmit the news directly to the cerebral ganglia, a simple form of the brain.^[17]

When detected, the giant clam's mantle swells in the central region and the adductor muscle contracts. Each clam then fills its water chambers and closed the incurrent syphon. The shell contracts vigorously with the help of the adductor muscle, so the contents of the excurrent chamber is pressed through the excurrent syphon. After a few contractions containing only water, eggs and sperm are released into the excurrent chamber and then pass through the excurrent syphon into the water. Female eggs have a diameter of 100 micrometers (1/10th millimeter). The release of eggs initiates the whole process of reproduction. An adult Tridacna gigas can release more than 500 million eggs at a time.^[18]

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 which do not respond to the spawning of neighbor clams may be reproductively inactive.^[19]

The fertilized egg floats in the sea for about 12 hours until eventually a larva (trocophore) hatches. It then starts to produce a chalk shell. Two days after fertilization it measures 160 micrometers. Soon it develops a “foot,” which is used to move on the ground; it can also swim around in search of appropriate habitat.^[20]

At roughly one week of age, the clam settles on the ground, although it changes its location frequently within the first couple weeks. The larva does not yet have symbiotic algae, so it depends completely on plankton. Free floating zooxanthellae are also taken in while filtering food. Eventually the front adductor muscle disappears and the rear muscle moves into the center of the clam. Many small clams die at this stage. The clam is considered a juvenile when it reaches a length of 20 centimeters.^[21] It is difficult to observe the growth rate of T. gigas in the wild, but laboratory-reared giant clams have been observed to grow 8-12 centimeters a year.^[22]

The largest known T. gigas specimen measured 137 centimeters. It was discovered around 1817 on north western coast of Sumatra. The weight of the two shells was 230 kilograms. This suggests that the live weight of the animal would have been roughly 250 kilograms. Today these shells are on display in a museum in Northern Ireland.^[23]

Another unusually large giant clam was found in 1956 off of the Japanese island of Ishigaki. However, it was not examined scientifically before 1984. The size of the shells was 115 centimeters and the weight of the shells and soft parts was 333 kilograms. Scientists estimated the live weight to be around 340 kilograms.^[24]

The main reason that giant clams are becoming endangered is likely to be intensive exploitation by mussel-catching vessels. Mainly large adult animals are killed since they are the most profitable.^[25] The giant clam is considered a delicacy in South East Asia and the Pacific Islands. Some Asian foods include the meat from the muscles of clam. The large shells are also used, and at times large amounts of money were paid for the adductor muscle, which Chinese people believed have aphrodisiac powers.^[26]

• Yonge, C.M. 1936. Mode of life, feeding, digestion and symbiosis with zooxanthellae in the Tridacnidae, Sci. Rep. Gr. Barrier Reef Exped. Br. Mus., 1, 283-321
• Knop, Daniel. Giant clams a comprehensive guide to the identification and care of Tridacnid clams. Ettlingen: Dähne Verlag, 1996. Print
• Munro, John L. "Giant Clams." Nearshore marine resources of the South Pacific information for fisheries development and management. Suva [Fiji]: Institute of Pacific Studies, Forum Fisheries Agency, International Centre for Ocean Development, 1993. Print.
• Dame, Richard F. Ecology of marine bivalves an ecosystem approach. Boca Raton: CRC, 1996. Print.
• Gosling, Elizabeth. Bivalve Molluscs Biology, Ecology and Culture. Grand Rapids: Blackwell Limited, 2003. Print.
• Jeffrey, S. W., and F. T. Haxo. "Photosynthetic Pigments of Symbiotic Dinoflagellates (Zooxanthellae) from Corals and Clams." Biological Bulletin 135.1 (1968): 149-65. JSTOR. Web. 23 Nov. 2009.
• Norton, J. H., M. A. Shepherd, H. M. Long, and W. K. Fitt. "The Zooxanthellal Tubular System in the Giant Clam." The Biological Bulletin 183.3 (1992). The Biological Bulletin. Web. 23 Nov. 2009.
• Klumpp, D.W., Bayne, B.L. & Hawkins, A.J.S. (1992) of the giant clam, Tridacna gigas (L). 1. Contribution of filter feeding and photosynthesis to respiration and growth." J. Exp. Mar. Biol. Ecol., 155, 105-22.
• Braley, Richard D. "Reproduction in the giant clams Tridacna gigas and T. derasa in situ on the north-central Great Barrier Reef, Australia, and Papua New Guinea." Coral Reefs 3.4 (1984). SpringerLink. Springer Berlin / Heidelberg, 30 Nov. 2004. Web. 23 Nov. 2009.
• Beckvar, N. "Cultivation, spawning, and growth of the giant clams Tridacna gigas, T. derasa, and T. squamosa in Palau, Caroline Islands." Aquaculture 24.1 (1981): 21-30. FAO of the United Nations. Web. 23 Nov. 2009.