The Giant Clam, Largest Shell on Earth

Photos © Jean Christophe Grignard

There are many horror stories about Tridacna gigas: tales of divers and swimmers who found themselves trapped by its powerful shells, their foot or the hand stuck as though caught in a stranglehold. Held underwater, they struggled and died slowly.

In other narratives of disappearances of trapped people and accidents created by giant clams, walkers in the shallows found themselves with a trapped limb, and they drowned during the rising tide. Myths or reality?

The reality is that although the adductor muscles used to close the shell are very powerful, they move far too slowly to take anyone by surprise, which makes the idea of a diver or swimmer being inadvertently caught by this animal pretty surreal. The accident stories that giant clams are being burdened with are more legend than reality.

Remarkable due to their impressive size, they are the largest sessile (attached to the ground) bivalve mollusks in the world. Generally, of a brown/green coloration, adult giant clams can measure up to 1.2 meters (4 feet) or more across and weigh more than 200 kilos (440 pounds). It is the best known among the several Tridacna species described in the scientific literature. Some of the more common species are Tridacna maxima, Tridacna squamosa, Tridacna derosa, Tridacna crocea and, of course, Tridacna gigas. The various species are difficult to positively identify in situ.

The two half-shells are articulated by an elastic ligament. They open and close their shells thanks to the action of the powerful muscles. Giant clams generally react to the variations of light. When a potential predator or a diver approaches, the shadow creates a movement of withdrawal or closure of the shell.

All species are tropical and display colors from florescent blue to dark brown via emerald green in their mantle. The mantle tissue contains symbiotic zooxanthellae (photosynthetic algae) that provide nourishment for the clam. The clams extend their mantle in the daytime to capture sunlight.

Being an integral part of the reef, these sedentary bivalve mollusks fasten themselves to a spot on a reef while they are juvenile with the hinge of the shell pointing downwards, and they sit there for the rest of their lives. As the animal develops, its environment is also evolving. This is the reason why these giants are often totally integrated into the coral reefs. The outside part of their shells is generally covered by sea sponges, bryozoans and seaweeds. They are interesting subjects for photographers, who are attracted by their magnificent, gaudy mantles and iridescent colors.

Photosensitive protuberances, called iridophores, are located on the edges of the mantle. They contain some kinds of lenses, which focus the light onto the tissue depths. The giant clam is also equipped with two siphons, small tubes that eject bodily fluids and ingest water containing small food particles and animals. The nutrients produced by the zooxanthellae provide the main source of food.

The giant clam is a protandrous hermaphrodite — species that change sex from male to female as they mature. Reproduction is based on the lunar cycle so that sperm and eggs are released at the same time. Each year, millions of eggs are produced, but just a few of them survive until adulthood. The fertilized eggs develop into freely moving larvae. Larvae will start producing a shell, and after a couple of days, they will become large enough to settle to the bottom.

Nowadays, certain species of giant clam are endangered due to overfishing, illegal fishing and pollution. Although protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, known by its initials CITIES, Tridacna gigas is an edible species that is regarded as a delicacy in many countries, and the abductor muscle has been considered an aphrodisiac. The shells have served as decorative objects, ashtrays or trinkets. Larger ones are well known for their traditional use in churches as containers for holy water or even baptismal fonts. Numbers of old churches still have the real shells of this enormous bivalve in their entrances, but more often these are only copies carved in more or less fine quality materials.

There are tales saying that the flesh of only one of these mollusks could feed a hundred people. Actually, some fish farms are considering cultivating this animal, which might eventually yield an important quantity of flesh. Giant clams are really outstanding animals due to their size, magnificent due to their bright colors and important members of the reef ecosystems.

Photos © Jean Christophe Grignard