Giant clam

Appearance

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.

"Tridacna 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.

Status

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.

Reproduction

"Tridacna gigas" reproduce sexually and are hermaphrodites. 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 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" 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.

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. Egg release initiates the reproductive process. An adult "T. gigas" can release more than 500 million eggs at a time.

Spawning seems to coincide with incoming tides near the second, third, and fourth quarters of the moon phase. Spawning contractions occurred 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.

Food

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 one meter 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 dry tissue weight—filter feeding provides about 65% of total carbon needed for respiration and growth; large clams acquire only 34% of carbon from this source. A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building corals.

Cultural

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau. 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. Recent developments in aquaculture, specifically at Harbor Branch Oceanographic Institute in Fort 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.

Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea. A programme to propagate endangered giant clams for release into the wild has been ongoing since 2007. Undertaken by the Marine Ecology Research Centre based in Gaya Island just west of Sabah’s capital, Kota Kinabalu, the programme successfully nurtured all seven species of the giant clams found in Malaysian waters to sufficient maturity for them to be placed in an ocean nursery for the first time during an awareness month from 22 March until 22 April 2012 in Maloham Bay. This marine awareness month had been planned to highlight and celebrate MERC's success in raising the giant clam larvae to juvenile stage, to highlight the importance of the giant clams and to raise awareness and support with the general public on the threats that are faced by the giant clams within the sea. During this marine awareness month, the coral restoration program entered its final stage and attachment of 1000 one-year-old coral fragments grown at MERC's ocean nursery onto the coral reef were done throughout the month. The coral restoration program is aimed to provide the giant clams with a suitable home surroundings when they are big enough in the future to be placed onto the reef.