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The Explorer's Guide contains a treasure trove of aquatic science resources developed for use in K-12 classrooms. Back to Explorer's Guide.

Giant Clam Scientific Name: Tridacna maxima
The giant clam houses billions of tiny algae under its skin. In exchange for a safe home, the algae uses the energy from the sun to consume the clam’s waste and turn it into food for its host.

Region: Philippines

Like a bright blue pair of lips, the fleshy part of the giant clam peeks out of the top of the shells during the day. There are not two giant clams with the exact same coloration.

Each side of the shell is made by outer edge of the mantle or the skin of the giant clam. The shell is fused together and encloses the mantle cavity which is the entire fleshy part of the giant clam. The giant clam has two siphons, or holes where water is sucked in or out, in the middle its mantle cavity, one that sucks water in and the other pushes water out. The giant clam can squirt a forceful jet of water out of one of the holes by quickly opening and closing its shell. The shell is the heaviest part of the animal. The clam can grow up to 12 inches (30 cm) long and weigh 572 pounds (260 kg). The giant clam can live for more than one hundred years.

Typically found in shallow water to help their zooxanthellae receive ample amounts of sunlight, the giant clam typically lives on the higher parts of the reef or in shallow sandy areas. It may be surrounded by corals, sponges or other immobile creatures. The shell of the giant clam is often home to many other creatures like corals, algae, sea anemones, and sponges. Once the giant clams settles onto an area on the reef, it remains in that same spot for the rest of its life.

Giant clams live in the warm parts of the Pacific Ocean and the Red Sea.

Filtering water through their gills, a giant clam will sift zooplankton or tiny animals and phytoplankton or tiny plants out of the water passing through.

The colorful skin of the giant clam hides a thick layer of zooxanthellae or algae. The tiny algae and the clam have a symbiotic relationship or partnership needed for survival. The algae make food for the clam using the energy from the sun. The algae feed off the clam's waste. The water the clam sucks through its mantle brings the tiny algae oxygen and flushes away any excess waste materials. Most of the giant clam's nutrients come from the zooxanthellae living under its skin. The giant clam actually farms its zooxanthellae. Each night, the clam uses amboecytes or motile feeding cells to eat some of its algae.

Since they can't move, giant clams have to spawn or release their gametes or sex cells into the water at the same time, in hopes that there will be enough gametes to fertilize each other. All the clams in one area spawn at the same time. When this happens the water looks milky. Each clam comes equipped with male and female sex organs, but they can only release one type of gamete at a time.

>A species or group of organisms that is in danger of extinction or disappearing from the face of the earth in the near future if its situation is not improved.

A species that can be found throughout its natural range but is declining in number and may become endangered in the absence of special protection efforts.

A species that is not declining in number but is of special concern because it is sensitive to pressure by particular human activities or natural events.

A species that is not declining in number and is not sensitive to pressures by human activities or natural events.

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Threats and Management
The giant clam is classified as stable, although it faces many population threats and is close to being qualified as vulnerable. These clams are hunted throughout their range by fishers for their meat and shells. Many people are beginning to farm giant clams to sell them for their meat and shells and also to replenish them on the reef.

Because this clam is so colorful, it is frequently collected for the pet trade. But these clams don't make good inhabitants of the home aquarium. They get so big, they outgrow most tanks.

Did You Know?
Using special motion-sensing tissue on its skin, the giant clam can detect water movement, retracting its lips and quickly closing its thick heavy shell. Also, it has small eyes that detect shadows. If the sunlight is suddenly blocked, the clam can snap its shell shut. Predators can crack open the shells of the smaller clams, but the shell of the large ones may be a few inches thick, making them almost impossible to break.

Many cartoons have portrayed giant clams eating people. However, this would be nearly impossible. It is unlikely that the shell valves could open wide enough to even allow a foot to enter. But they can give you a good pinch if you stick your finger in there.

There are seven species all called giant clams that belong to the Family Tridacnidae.

Web Sites:
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Print Publications:
Allen, G. R. & Stene, R. 1996. Indo-Pacific Coral Reef Field Guide. El Cajon, California: Odyssey Publishing Company. 378p. ISBN# 981-00-5687-7.

Allen, G. R. 1997. Tropical Marine Life. North Claredon, Vermont: Periplus Editions Ltd. 64p. ISBN# 962-593-157-0.

Banister, K. & Campbell, A. 1988. Encylopedia of Aquatic Life. New York, New York: Equinox (Oxford) Ltd. 349p. ISBN# 0-8160-1257-101-X.

Colin, P. L. & Arneson, C. 1995. Tropical Pacific Invertebrates. Beverly Hills, California: Coral Reef Press. 296p. ISBN# 0-9645625-0-2.

Coleman, N. 1999. Dangerous Sea Creatures Aquatic Survival Guide. Australia: Neville Coleman's Underwater Geographic Pty Ltd. 95p. ISBN# 0-947325-24-7.

Perrine, D. 1997. Mysteries of the Sea. Lincolnwood, Illinois: Publications International, Ltd. 312p. ISBN# 0-7853-2430-5.

Reader's Digest. 1984. Reader's Digest Book of the Great Barrier Reef. Sydney, Australia: Reader's Digest Services Pty Limited. 384p. ISBN# 0-949819-41-7.

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Vocabulary Words

Amboecytes - Motile feeding cells

Extinction - An organism that has not been present on the face of the earth for over 50 years.

Gametes - Sex cells; eggs and sperm.

Mantle - The fold of skin covering all or part of the body of a mollusk. The outer edge of the mantle secretes the animal's shell.

Organism - A living thing.

Overfishing - The excessive fishing or catching of aquatic (ocean or freshwater) animals to the point that the amount of animals being caught is greater than the amount of animals born. When more animals are caught than are being born, the aquatic environment is left depleted of the targeted animals.

Pet trade - An industry or business in which animals are taken from the wild and sold to pet stores, zoos, and aquariums.

Phytoplankton - Microscopic plants that float at the mercy of the ocean currents.

Siphon - A hole or tube through which water enters or leaves an animal's body cavity.

Spawn - The release of sex cells, eggs or sperm, into the water.

Species - A group of organisms capable of breeding and producing fertile offspring; organisms that share the same gene pool.

Symbiotic relationship - Symbiosis - In order to survive, certain creatures form partnerships with other kinds of animals or plants for feeding, housing or protecting one another.

Zooplankton - Microscopic animals that float at the mercy of the ocean currents and have weak swimming ability.

Zooxanthellae - Tiny single-celled algae called dinoflagellates that have a symbiotic relationship with an organism like the giant clam or corals. In exchange for a safe place to live under the organism's skin, the zooxanthellae provides its host with nutrients it makes using the sun.

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Giant clam
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