G is for Giant Clam

Did you know? Giant clams are the largest living bivalves on Earth.

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True to their name, giant clams are colossal. In fact, they hold the record for largest living bivalve on the planet.

Illustration for G is for Giant Clam with a Single Clam on the Sea Floor

Found in reefs of the South Pacific and Indian Oceans, these clams begin life as little larvae but can grow up to 4 feet and weigh a massive 500 pounds. Getting to that size is a marathon not a race for giant clams that can live for an estimated 100 years.

Once rooted to a reef, giant clams remain there for life. Luckily, they do not have to travel to reproduce. These bivalves are simultaneous hermaphrodites, meaning they produce both sperm and eggs. Giant clam spawning is synchronized. So, when it's time to reproduce, the bivalves open their mantles concurrently and expel eggs and sperm into the water.

Like their neighbor, coral, giant clams have a symbiotic relationship with algae. Algae occupy the giant clam's mantle—the soft, fleshy mass sheltered within its tough outer shell. During the day, the giant clam opens its shell, exposing its mantle to the sun.

Given uninhibited access to sunlight and other nutrients, algae is able to photosynthesize. In exchange, the giant clam feeds on the sugar and protein byproducts of that chemical process.

Giant clams also supplement their diet by filter feeding, sucking in water laden with plankton and other small prey.

Giant Clam in Coral

And just like with corals, it is algae that gives giant clams their beautifully distinct colors—no two exactly alike. The soft tissues of some giant clams are golden yellow or deep green. Others have rusty swirls of orange or midnight blues that melt into iridescent pinks and purples.

Strangely enough, the giant clam's adductor muscle—the muscle used to open and close its shell—is considered a delicacy in some regions. In recent years, the clams have also been targeted for their shells, which are made into carvings, ornaments and jewelry. Overharvesting of the species has helped land it on the International Union for Conservation of Nature's red list, where it is now considered vulnerable to extinction.

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