Giant clam
T.gigas, Michaelmas Cay Great Barrier Reef, Queensland, Australia
Conservation status
Vulnerable (IUCN 2.3)[1]
CITES Appendix II (CITES)[2]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Cardiida
Family:	Cardiidae
Genus:	Tridacna
Species:	T. gigas
Binomial name
Tridacna gigas (Linnaeus, 1758)
Synonyms[3]
Chama gigantea Perry, 1811

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 200 kilograms (440 lb), measure as much as 120 cm (47 in) across and have an average lifespan in the wild of over 100 years.^[4] 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).^[5] 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 can be found off high- or low-elevation islands, in lagoons or fringing reefs.^[6] Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.^[5]

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 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 photosynthesise.

Anatomy

Young T. gigas are difficult to distinguish from other species of Tridacninae. Adult T. 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 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.^[7]

Tridacna gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the similar shell of T. derasa, which has six or seven vertical folds.^[7] As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells.^[8] The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.^[9]

The mantle border itself is covered in several hundred eyespots about .5mm in diameter. Each one consists of a small cavity containing a pupil-like aperture and a base of one hundred or more photoreceptors. These receptors allow T. gigas to respond to sudden dimming of light by withdrawing their mantles and partially closing their shells, presumably to protect from potential predators. They do not retract their mantles in response to increased illumination, but it has been observed that a change in the direction of light results in a shift in mantle orientation. In addition to a dimming response, T. gigas also responds to the movement of an object before a shadow has been cast.^[10] In order for this to happen, an image forming optical system is required as the response is based on the local dimming of one part of the generated image relative to the rest. This sequential dimming of receptors caused by the movement of a dark object allows enough time for the mantle to be retracted before a potential predator is directly overhead and casting a shadow.^[11]

Largest specimens

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]

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

Ecology

Feeding

Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are covered by symbiotic unicellular algae (zooxanthellae).^[14] 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.^[15][16]

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 grams (0.35 oz)) acquire only 34% of carbon from this source.^[17] A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building (hermatypic) corals.^[9]

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 be passed to further generations.^[18]

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.^[19]

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.^[20]

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.^[21]

Development

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 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.^[23]

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) .^[24] 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.^[25]

The ability for Tridacna to grow to such large sizes with fleshy mantles that extend beyond the edges of their shells is considered to be the result of total reorganization of bivalve development and morphology.^[26] 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.^[27] 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.^[28]

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.^[29]

The giant clam is considered a delicacy in Japan (known as himejako), France, Southeast Asia and many Pacific Islands. Some Asian foods include the meat from the muscles of clams. On the black market, giant clam shells are sold as decorative accoutrements. Large amounts of money are paid for the adductor muscle, which Chinese people believe to have aphrodisiac powers.^[30]

Legend

As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. 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.^[31]

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.^[31]

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.^[32]

In practice, their abductor muscle's slow speed leaves little room for unpleasant surprises.^[4]

Aquaculture

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau).^[33] 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.^[34][35][36] Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.^{[citation needed]}

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.^[2]