Giant clams can be as big as a large suitcase or just a foot wide, depending upon the species. They settle, like long-lived anchors, into rocks or coral reefs. Indeed some clams can be more than 50 years old.
When the clam is open, the rim of each shell is curvy, like the shape of ocean waves hitting a sandy shore. Whenever the curve goes out on one side, it goes in on the other, so that the shell is able to move together (although in the largest giant clams, the shell does not completely close). How does the creature develop such symmetry with such a complex shape?
Giant clams, despite their giant size, partner with a tiny plant, called dinoflagellate algae, to get nutrition. These algae have only one cell. They live in the mantle, an organ of the giant clam that covers the clam's body. When it is sunny, most giant clams spread the mantle outside their shells. Then the algae can catch the sunlight and produce food. Just as the clam receives benefits from the algae, it in turn often provides a home for a pair of pea crabs.
Clams are part of the mollusk phylum. Within the mollusk phylum are several orders, one of which is the bivalve order. Bivalves refer to mollusks with 2 more or less symmetrical shells. The giant clam is the largest bivalve in the world.
All clams are part of the bivalve order. A clam simply refers to a two-shelled, somewhat oval-shaped mollusk that is not an oyster, mussel, or scallop.
Giant clams encompass a number of species, classified by unique characteristics and habitat. The small giant clam (Tridacna maxima: small, at least for giant clams) lives in the waters off 3 continents: Asia, Africa, and Australia. It attaches to reefs, rocks or even sand by means of fibrous threads. It lives in well-lit areas. The true giant clam (Tridacna gigas) is the largest of all the giant clams. It lives in reefs and lagoons in Asia.
Coral reefs are the best places to see giant clams in their natural habitat. In the reefs of the Philippines, the small giant clam can be seen. However, during my limited snorkeling in the Philippines, I was not fortunate enough to see any. The true giant clam is almost extinct in the Philippines due to overfishing by humans. It persists only in isolated reefs, but conservation efforts are underway.
I have seen giant clams in the Monterey Bay Aquarium in Monterey, CA, and in the Aquarium of the Pacific in Long Beach, CA.
In a beautiful Polynesian myth, an old spider traveled over the sea and found a giant clam in which to live. Once inside, she found a snail and asked the snail to be the moon. The top shell became the sky, and the bottom, the earth. Some light came in as the shell opened, and then animals and plants were created. It started to get crowded and almost unlivable until the god of the forests helped to open the shell completely. Now every living thing in the world has room to grow. (More details on this myth.)