Giant clam

Tridacna gigas, thegiant clam, is the best-known species of the giant clamgenusTridacna. Giantclamsare the largestlivingbivalvemolluscs. Several other species of "giant clam" in the genusTridacnaare often misidentified asTridacna gigas.
These clams were known to indigenous peoples of East Asia for thousands of years and the Venetian scholar and explorerAntonio Pigafettadocumented them in a journal as early as 1521. One of a number of large clam species native to the shallowcoral reefsof the SouthPacificandIndianoceans, they may 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 more than 100 years.They also are found off the shores of thePhilippinesand in theSouth China Seain the coral reefs ofMalaysia.
The giant clam lives in flatcoralsand or broken coral and may be found at depths of as great as 20 m (66 ft).Its range covers theIndo-Pacific, but populations are diminishing quickly and the giant clam has become extinct in many areas where it was once common.Themaxima clamhas the largest geographical distribution among giant clam species; it may be found off high- or low-elevation islands, inlagoonsorfringing reefs.Its rapid growth rate is likely due to its ability to cultivatealgaein its body tissue.
Althoughlarvalclams areplanktonic, they becomesessilein adulthood.The creature'smantletissuesact as ahabitatfor thesymbioticsingle-celleddinoflagellatealgae (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 tophotosynthesise. This method of algal farming is under study as a model for highly efficient bioreactors.
YoungT. gigasare difficult to distinguish from other species ofTridacninae. AdultT. gigasare the only giant clams unable to close their shells completely, allowing part of the brownish-yellowmantleto remain visible.Tridacna gigashas four or five vertical folds in its shell, which serves as the main characteristic differentiating it from the similarT. derasathat has six or seven vertical folds.Similar to coral matrices composed ofcalcium carbonate, giant clams grow their shells through the process ofbiomineralization, which is very sensitive toseasonal temperature.Theisotopicratio of oxygen incarbonateand the ratio betweenStrontiumandCalciumtogether may be used to determine historicalsea surface temperature.
The mantle border itself is covered in several hundred to several thousandpinholeeyespotsapproximately 0.5 mm (0.020 in) in diameter.Each one consists of a small cavity containing apupil-like aperture and a base of 100 or morephotoreceptorssensitive to three different ranges of light, includingUV, which may be unique amongmolluscs.These receptors allowT. gigasto partially close their shells in response to dimming of light, change in the direction of light, or the movement of an object.The optical system forms an image by sequential, local dimming of some eyes usingpigmentfrom theaperture.
The largest knownT. gigasspecimen measured 137 centimetres (4 ft 6 in), and it weighed 230 kg (510 lb) dead and was estimated to be 250 kg (550 lb) alive. It was discovered around 1817 on the north western coast ofSumatra,Indonesia, and its shells are now on display in a museum inNorthern Ireland.
A heavier giant clam was found in 1956 off the Japanese island ofIshigaki. The shell's length was 115 centimetres (3 ft 9 in), and it weighed 333 kilograms (734 lb) dead and estimated 340 kilograms (750 lb) alive.
Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are supplied byzooxanthellae.This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters.The clams cultivate algae in a specialcirculatory systemthat enables them to keep a substantially higher number of symbionts per unit of volume.The mantle's edges are packed withsymbioticzooxanthellae, which presumably usecarbon dioxide,phosphates, andnitratessupplied by the clam.
In very small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides approximately 65% of total carbon needed forrespirationand growth; comparatively larger clams (10 grams (0.35 oz)) 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.
Tridacna gigasreproduce sexually and arehermaphrodites(producing botheggsandspermby one clam). While self-fertilization is not possible, having both characteristics does allow them to reproduce with any other member of the species as well as hermaphrodically. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations be passed to further generations.This flexibility in reproduction reduces the burden of finding a compatible mate, while simultaneously doubling the number of offspring produced.
Since giant clams cannot move themselves, they adopt broadcast spawning, releasing sperm and eggs into the water. A transmitter substance calledspawning induced substance(SIS) helps synchronize the release of sperm and eggs to ensure fertilization. The substance is released through asyphonal outlet. Other clams can detect SIS immediately. Incoming water passeschemoreceptorssituated close to the incurrent syphon that transmit the information directly to thecerebralganglia, a simple form of brain.
Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract itsadductor 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 adultT. gigascan release more than 500 million eggs at a time.
Spawning seems to coincide with incoming tides near the second (full), third, and fourth (new) quarters of themoon 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.
The fertilized egg floats in the sea for approximately 12 hours until eventually alarva(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. Larvae also can 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 onplankton. Also, free-floating zooxanthellae are 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 ajuvenilewhen it reaches a length of 20 cm (8 in).It is difficult to observe the growth rate ofT. gigasin the wild, but laboratory-reared giant clams have been observed to grow 12 cm (4.7 in) a year.
The ability forTridacnato grow to such large sizes with fleshymantlesthat extend beyond the edges of theirshellsis considered to be the result oftotal reorganizationofbivalvedevelopment and morphology.Historically, two evolutionary explanations have been suggested for this process. Sir Yonge suggested and maintained for many years that thevisceral-pedal ganglia complexrotate 180 degrees relative to the shell, requiring that they develop and evolveindependently.Stasek proposed instead that the growth occurs primarily in aposteriordirection instead of the more typical direction ofventralin most bivalves, which is reflected in thetransitionalstages of alternative ways of growing thatjuvenilesundergo.
The main reason that giant clams are becoming endangered is likely to be intensive exploitation by bivalve fishers. Mainly large adults are killed because they are the most profitable.
The giant clam is considered a delicacy in Japan (known ashimejako), France,Southeast Asia, and many Pacific Islands. Some Asian foods include the meat from the muscles of clams. Large amounts of money are paid for the adductor muscle, whichChinesepeople believe to haveaphrodisiacpowers.
On theblack market, giant clam shells are sold as decorative accoutrements.
As is often the case historically with uncharacteristically large species, the giant clam has been misunderstood.
Even in countries where giant clams are easily seen, stories incorrectly depict giant clams as aggressive beings. For instance, although the clams are unable to close their shells completely, aPolynesianfolk tale relates that a monkey's hand was bitten off by one, and even though once past larval stage, the clams are sessile, aMaorilegend relates a supposed attack on a canoe by a giant clam.Starting from the eighteenth century, claims of danger had been related to the western world. In the 1920s, a reputable science magazinePopular Mechanicsonce claimed that the great mollusc had caused deaths. Versions of theU.S. Navy Diving Manualeven 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 thePearl of Lao Tzu, Wilburn Cobb said he was told that aDyakdiver was drowned when theTridacnaclosed its shell on his arm.In reality, the slow speed of their abductor muscle contraction and the need to force water out of their shells while closing, prevents them from trapping a human.
Other myths focus on the huge size of giant clams being associated with long age.While giant clams do live a long time and may serve as a bio-metric for historic climatic conditions, their large size is more likely associated with rapid growth.
Mass culture of giant clams began at the MicronesianMaricultureDemonstration Center inPalau(Belau).A largeAustraliangovernment-funded project from 1985 to 1992 mass-cultured giant clams, particularlyT. gigasatJames Cook University'sOrpheus Island Research Station, and supported the development of hatcheries in the Pacific Islands and the Philippines.Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.
There is concern amongconservationistsabout 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 theConvention on International Trade in Endangered Species(CITES) meaning international trade (including in parts and derivatives) is regulated.
T. gigashas been reported aslocally extinctin peninsular Malaysia, whileT. derasaandHippopus porcellanusare restricted to Eastern Malaysia.These recent local extinctions have motivated theintroductionof giant clams toHawaiiandMicronesiafollowing maricultural advancements.Restocked individuals in thePhilippineshave successfully dispersed their ownspawnedlarvae to at least several hundred meters away after only ten years.