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Gregory R. Mann, Ph.D. {ret.}

Giant Clam

“Tridacna gigas”

The Giant Clam is the largest immobile mollusk in the world, with the occasional Giant Clam individual reaching nearly 6 feet in length although the average is 4 feet (1.2 meters) in length and up to 500 pounds (227 kilograms) in weight. Once the Giant Clam has settled somewhere, it remains there for the rest of its life. Giant Clams are founded anchored to the coral reefs in the warm, tropical waters of the Indian and South Pacific oceans where Giant Clams spend the majority of their time feeding on the abundant variety of food that a coral reef has to offer. Due to the large size of the Giant Clam, there have been reports of Giant Clams eating human beings. Despite this though, no reports of man-eating Giant Clams have ever been verified as it is thought that the Giant Clam would simply hide in its shell rather than attack an approaching human. Giant Clams are thought to grow to such enormous sizes due to the fact that Giant Clams consume a great deal of sugars and proteins that are produced by the algae that live on the Giant Clams. Although Giant Clams are omnivorous animals and therefore eat a mixture of both plant and animal matter, the nutrients produced by the algae provide the main source of food. Giant Clams are also known to eat small food particles and animals that are present in the surrounding water.

Despite the vast size of the Giant Clam, they are preyed upon by a number of marine predators, many of which are actually much smaller in size than the Giant Clam itself. Eels, snails, fish and sea stars are all known to snack on small parts of the Giant Clam. They are also preyed upon by humans, who catch the Giant Clams in order to feed on a particular muscle which is seen as a culinary delicacy in many countries. The excessive harvesting of Giant Clams by humans has led to rapid declines in the population. Giant Clams have both male and female reproductive organs but are not thought to self-fertilize. They release eggs and sperm into the water, where the eggs will generally be fertilized by the sperm from another Giant Clam. They are able to release more than 500,000,000 eggs at a time. Once fertilized, the eggs of the Giant Clam float around in the water for around 12 hours when the larvae hatches out. The Giant Clam larvae than begins to produce a shell and with just a few days, is big enough to find somewhere to settle on the sea floor. Once a Giant Clam has found somewhere in the reef that it likes, it anchors itself onto the reef where it remains for the rest of its life. Giant Clams are so successful within their environment that it is not uncommon for these giant mollusks to live for more than 100 years.

Interesting Facts

* All Giant Clams have clear spots on their mantle to let sunlight into their cavity.

* Some Giant Clams look iridescent because of their blue and purple spots, while others look gold or green.

* The Giant Clam is the heaviest and largest living bivalve mollusk.

* The Giant Clam cannot completely close its shell once fully grown.

* It starts life as a male but later becomes hermaphroditic, able to produce both sexes.

 *  The Giant Clam does not fertilize itself. It produces sperm and eggs one at a time and releases each into the water to be fertilized by another Giant Clam.

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