giant clam

The name is also used for other gigantic clam species. See Tridacnidae. In Japanese cuisine, Geoduck (mirugai) is sometimes referred to as "giant clam".

Giant Clam
Conservation status
Vulnerable (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Family:	Tridacnidae
Genus:	Tridacna
Species:	T. gigas
Binomial name
Tridacna gigas Linnaeus, 1758

The giant clam, Tridacna gigas, or traditionally, pa’ua, is the largest living bivalve mollusk. Tridacna gigas is one of the most endangered species amongst the clams. It was mentioned as early as 1825 in scientific reports.^[1] 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 pounds), measure as much as 1.2 metres (4 feet) across, and have an average lifespan in the wild of 100 years or more.^[2] They are also found off the shores of the Philippines, where they are called taklobo. 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. maxima has the largest geographical distribution of the giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs.^[4] The giant clam's growth rate is likely due to its ability to cultivate plants 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 it gets its nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesize.

History and 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 mollusk 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 a colorful account^[6] 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 holding one fast in its grip, in reality the shell's closing action is a defensive response, not an aggressive one, and the process of closing the shell valves is slow enough not to pose serious threat. Furthermore, many large individuals are unable to close their shells completely. In other words, the clam is unlikely to suddenly snap shut on a person's arm or leg and drown them.

Anatomy

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.^[7]

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.^[8] As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells.^[9] The edges of the clam's mantle are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.^[10]

Diet

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.^[11] Algae provide giant clams with a supplementary source of nutrition.^[12]. These plants consist of unicellular algae, the metabolic products of which add to the filter food of the clam.^[13] As a result, they are able to grow as large as 100 centimeters in length even in nutrient-poor coral-reef waters.^[14] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per square unit.^[15]

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.^[16] 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.^[17]

Reproduction

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.^[18]

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.^[19]

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.^[20]

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.^[21]

Development

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.^[22]

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.^[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 8-12 centimeters a year.^[24]

Largest Clams

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.^[25]

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.^[26]

In the Human Diet

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.^[27] 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.^[28]

Conservation status

The IUCN lists the giant clams as vulnerable. There is concern among conservationists for the sustainability of practices among those who use the animal as a source of livelihood. The numbers in the wild have been greatly reduced by extensive overharvesting for food and the aquarium trade. On the black market, giant clam shells are sold as decorative accouterments, and the meat, called Himejako in Japan, is prized as a delicacy (both in Japan and in France).

Aquaculture

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Centre in Palau (belau) [Heslinga, G.A., Perron, F.E. and Orak. O. 1984. Mass culture of giant clams (F. Tridacnidae)in Palau. Aquaculture 39: 197-215.]. 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 [Copland, J.W. and J.S. lucas, eds,1988. Giant Clams in Asia and the Pacific. ACIAR Monograph No. 9, 274 p.; Braley, R.D. 1988. Farming the Giant Clam. World Aquaculture 20(1):7-17; FittW.K, ed., 1993. Biology and Mariculture of Giant Clams; a workshope held in conjunction with the 7th International Coral Reef Symposium, 21-26 June 1992, Guam, USA]. Recent developments in aquaculture, specifically at Harbor Branch Oceanographic Institute in Ft. Pierce, Florida, and in the Marshall Islands, have succeeded in allowing T. gigas to be tank-raised both for use in home aquariums and for release into the wild.

Gallery

Green and blue giant clam from East Timor	Camouflaged giant clam	Colorful giant clam from Komodo National Park	Giant clam with patterns on mantle
Fully opened giant clam from Komodo National Park	The largest of all clam species	Mantle detail 1	Giant clam siphon
Crocus giant clam
	A giant clam from East Timor of over 1 meter in length.

See also

• Platyceramus, the largest bivalve in the fossil record

Citations

References

• 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.

External links

Wikimedia Commons has media related to: Tridacna gigas

• Wells (1996). Tridacna gigas. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 06 May 2006. Listed as Vulnerable (VU A2cd v2.3)

• ARKive - images and movies of the giant clam (Tridacna gigas)
• Tridacna gigas entry on Animal Diversity Web
• Giant clam entry on the IUCN Red List of threatened species
• Giant clam conservation research project at Universiti sains Maylaysia
• Giant Clams of the Great Barrier Reef
• Microdocs: The solar powered clam & Growing a giant clam