Giant Clam, (Tridacna gigas)

Fish Facts

stewart whitfield - Saturday, September 12, 2009

The giant clam, Tridacna gigas, or traditionally, pa’ua, is the largest living bivalve mollusk. One of a number of large clam species native to the shallow coralreefs 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. They are also found off the shores of the Philippines.

Although larval clams are planktonic, they become sessile in adulthood. The creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellatealgae (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 orman-eating clam, and reputable scientific and technical manuals once claimed that the great mollusk had caused deaths; versions of the U.S. Navy Diving Manualeven gave detailed instructions for releasing oneself from its grasp by severing the adductor muscles used to close its shell.

In a colorful account^[2] 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.

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

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 fromKomodo 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