Giant clams

Although they reach a diameter of 1.5 metres and a weight of up to one ton, and although they got the common name "killer mussel", giant clams (Tridacna gigas) aren't really frightening animals. At least, as long as they are left in peace. These giant bivalves are a common sight to divers in coral reefs or on other substrates of the warmer climate zones, particularly in the Indopacific region. They are thought to live up to 200 years.

Giant clams are sedentary, as adults being placed at the ground by their own massive body weight, while the opened shell halves are directed upwards. The shell halves are pronouncedly wavy. If the mussel is relaxed, they are always slightly open, showing part of the mantle tissue. This mantle tissue is often very colorful, mostly of a deep bluish color. It also contains symbiontic algae. These organisms live within the mussel and use light to perform photosynthesis, just like plants do. The nutrients that these algae produce are partly consumed by the mussel, which is how the mussel profits from its little guests. Giant clams have been called killer clams, because sea-farers tales say that they would close the shell, if a diver's leg accidentally gets in-between the shell halves, and won't let go again for a while, so that the diver risks to drown. However, if you are a sports diver, you should easily notice that such a case is rather improbable. But it is definitely not advisable to try anything out like that. The enormous power which the mussel has when closing its shell is indeed dangerous, and without help, no diver would be able to free himself from the mussel's grip.

Such an accident happened among pearl fishers in a village of the Philippines, in the early 1930's. One day, the local pearl fishers noticed that their best diver did not come back. They found him dead on the sea floor, where his hand was captured by a giant clam. The natives took him and the 160kg mussel out of the water. Later, it was found that this mussel contained the largest pearl which was ever found: a bizarre structure of 23 centimetres length and 15 centimetres height.

Today, giant clams in the Philippines area are threatened through over-exploitation by mussel catchers, who sell them as food on local markets. Also their massive shell is sometimes used. In the past, the giant shells of this clam often have been used as a vessel for holy water in churches. They can still be seen in some old churches.

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