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Fish of the month: Giant Clam

Photo by Mika d’Eau

Like corals, the giant clam lives in partnership with tiny plant-like algae (called zooxanthellae) that reside inside the clams’ tissues. And as with corals, the arrangement helps both creatures. The algae gain protection from grazing animals; the clam grows large with the benefit of food produced by the algae. At home on reefs in the Maldives, the giant clam lives on shallow reef flats down to depths of around 66 feet (20 m). Below that, the algae they depend on to survive wouldn’t have enough sunlight to grow. The relationship between the giant clam and algae is considered a symbiotic, mutualistic relationship.

Quick Facts

• The giant clam is the world’s largest living marine bivalve (clam, mussel or oyster).
• The largest species can grow to 1,3m (4.25 feet) long and 250kg (550 pounds)
• Their soft bodies account for only about 10 percent of their total weight.
• Once a giant clam settles into a place and begins to grow, it stays permanently attached to that spot for life. A giant clam can live 100 years or more!
• The clam’s bright colors are the result of the algae living inside its body.
• This unique species is found to be a wide variety of colours, created by a contrast between algae inside the clam and the clam’s natural pigment. This vibrancy is an indicator of clam health—if the clam is unhealthy, the dying algae will bleach it to a bright white. Giant clams, when healthy, can live up to 100 years.
• Legends from the South Pacific tell of giant clams able to trap—and even devour—passing divers. Reputable marine manuals from centuries before warned against giant claims, detailing how a trapped diver could sever the clam and escape without drowning. In truth, giant clams are harmless to people. Their shells move far too slowly to trap any unsuspecting diver. Furthermore, there’s never been a recorded incidence of human death by clam!
• Giant clam spawning sessions are suggested to coordinate with phases of the moon

Behaviour and Reproduction

Giant clams are hermaphroditic, producing both eggs and sperm. A fully-grown giant clam can release over 500 million eggs in one go. As they cannot fertilise themselves, they reproduce through broadcast spawning. When an egg is released, this triggers nearby giant clams to swell their mantles and contract their adductor muscles. This causes the clams to fill with water containing broadcasted sperm and eggs.
Though giant clams are sedentary, they can open and close their shells. Some clams are unable to close their shells completely, meaning there is always a gap left open.


Due to the aquarium trade and the fact that the giant clam’s abductor muscle is considered a delicacy, the giant clam population has decreased substantially.

Photo by Mika d’Eau


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