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The giant clam is the world's largest bivalve. This smaller species of giant clam have exquisite iridescent blue and green mottling on the mantle. Bivalves are mollusks with two convex halves, or shells, that hinge along one edge with the animal in between. The shells are shut together and held closed by two large adductor muscles, this seals the animal shell, protecting it from predatory attack.
Photo: Brian Hall

November 1, 2000
The Giant Clam
Real Audio


Log Transcript

This is Genevieve Johnson talking to you from the Odyssey. While provisioning in Christmas Island in preparation for our next leg in search of sperm whales, we have had several opportunities to snorkel and dive on the coral reefs. There is one creature that we continue to see over and over again, it is a huge invertebrate of mythical stature, considered by many to be the embodiment of the tropical pacific, the giant clam. The giant clam is the world's largest bivalve. This smaller species of giant clam is one of several different giant clam species found in tropical waters, and has exquisite iridescent blue and green mottling on the mantle. Bivalves are mollusks with two convex halves or shells that hinge along one edge with the animal in between. The shells are shut together and held closed by two large adductor muscles, this seals the animal shell, protecting its vital organs from predatory attack.

The clam is a filter feeder, as seawater passes over the gills for gas exchange, food particles are also captured and passed into the mouth. The species of Giant clam we have consistently encountered here in Kiribati are slightly smaller than the enormous 2-meter long specimens, the creatures that have captured our imagination from early Polynesian times to present day Hollywood, where stories are told of a divers hand or foot being trapped in a closing shell. Of course this rarely happens since the clam has no interest in trapping a large foreign body inside its shell and will not fully shut when a strange object is inside.

As we dove down to take a closer look, the clams would immediately withdraw into their ridge shaped shells, the corrugated structure increasing the strength of the two halves upon closure. The large shell is built up over time as the animal secretes proteins and inorganic substances such as calcium, which it then adds like building blocks to the thick, rough outer layer of the shell, its size and form being a function of the rate of shell deposition. Sometimes foreign bodies such as grains of sand can become lodged inside the clam between the shell and the mantle, if this happens secretions can build up around the grain, perhaps to reduce irritation, a round smooth structure is formed that may eventually result in a pearl. Giant clams were once a regular component of most tropical pacific reefs, but have been exploited in many areas. The thick muscle inside the shell is highly prized on the Asian market. Illegal poaching of this species throughout its range in the pacific has led to its extinction in many areas, and where it is still present, it is extremely rare. The largest species of giant clam is now listed under the Convention on International Trade in Endangered Species, (CITES) as a threatened species. As we continue to explore the world's oceans we hope to encounter more of these animals, maybe even the rare giant clam of enormous proportions.

Log by Genevieve Johnson

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