Tridacna gigas, Photo: Michael Lahanas
The giant clam, Tridacna gigas, or traditionally, pa’ua, is the largest living bivalve mollusk. T. gigas is one of the most endangered clam species. It was mentioned as early as 1825 in scientific reports. 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 (441 lb) measure as much as 1.2 meters (4 ft) 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. T. gigas lives in flat coral sand or broken coral and can be found at depth of as much as 20 meters (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. T. 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 plants 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.
Young T. gigas are difficult to distinguish from other species of Tridacnidae. 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.
T. gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the very similar shell of T. 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.
The largest known T. gigas specimen measured 137 centimeters (53.9 in). It was discovered around 1817 on north western coast of Sumatra. The weight of the two shells was 230 kilograms (507 lb). This suggests that the live weight of the animal would have been roughly 250 kilograms (551 lb). Today these shells are on display in a museum in Northern Ireland.
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 shells' length was 115 centimeters (45.3 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).
The clam's brown symbiotic algae, which photosynthesize . These zooxanthellae are considered to be closely related to dinoflagellates. 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 centimeters (39.4 in) in 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.
T. gigas reproduce sexually, and 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 sex. This ensures mixing of the gene pool. 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 conract 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.
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.
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 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 centimeters (7.9 in). 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 centimetres (3.1–4.7 in) a year.
Relation to Humans
The main reason that giant clams are becoming endangered is likely to be intensive exploitation by mussel-catching 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 accouterments. At times large amounts of money were paid for the adductor muscle, which Chinese people believed have aphrodisiac powers.
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 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.
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 T. 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 T. gigas both for use in home aquariums and for release into the wild.
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.
1. ^ "World Register of Marine Species". 2009. http://www.marinespecies.org/aphia.php?p=taxdetails&id=382197. Retrieved 15 March 2010.
* 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
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License