Facts: The Giant Clam

Giant clams are the largest living marine bivalves. They typically inhabit tropical coral reefs in coastal regions throughout the Indo-Pacific. There are several species. The biggest can grow to over 4 ft across. The smallest species only averages 6 in. A giant clam has two thick shells joined with a flexible ligament hinge. Once a giant clam settles and begins to grow, it stays permanently attached to the same spot for life. They can live more than 100 years. They live in partnership with single-celled algae called dinoflagellates. They harbor these symbiotic algae, called zooxanthellae, within their tissues. The algae contribute to the nutrition of their hosts by photosynthesis. The algae get protection and the clams get to eat the sugars and proteins produced by the algae. Giant clams must live on shallow reefs so that there is enough sunlight for the algae to photosynthesize. They can be found down to 66 ft deep. Adults get most of their nutrients from the algae and the rest from filter feeding. They have two siphons, an inhale siphon and an exhale siphon. The inhale siphon pulls in seawater that is filtered for plankton. The shimmering patterns on a giant clam's mantle come from iridophores, which are cells that reflect light. The mantle is the fold of skin between the shells. Iridophores and pigments act as sunscreen, blocking UV rays that could damage the clam's flesh. Giant clams also have amino acids that serve as sunblock. Each individual has a different mantle coloration. They have small eyes that can only detect shadows. Giant clams are hermaphrodites. They are broadcast spawners, casting sperm or eggs into the water column. They are heavily fished for meat, as curios, and to supply the marine aquarium trade. Due to exploitation, worldwide populations of giant clams are on the decline. Their late maturity, sessile adult phase, and broadcast spawning strategy make them especially vulnerable to stock depletion. High end restaurants serve the adductor muscle as a delicacy. Their other tissues have a low demand due to the seaweed taste from the zooxanthellae. The majority of giant clam aquaculture targets aquarium markets. The clams offer great potential for aquaculture because the symbiotic algae render them almost self-sustaining. Sea stars, snails, and some fish like to consume giant clams or nibble on exposed parts. When disturbed, a giant clam will close its shell. For more marine facts, click the SUBSCRIBE button!!