World’s Largest Seashells

By: Lydia Weltmann

It’s a sunny day in the south Pacific, light bouncing off the water and making the waves shine. A diver leans overboard, sinking gracefully beneath the surface. Colors meet the diver’s eyes, bright oranges and pinks and yellows next to iridescent blues and greens. Coral reefs surround the diver, but those aren’t what he is searching for. Finally he spots his prize, a zig-zag of brown and yellow. The Gigas clam.

clam-brown and yellow

He paddles closer, slowing as he draws near. The shells are slightly parted, allowing a glimpse of the treasure inside: A pearl, the size of his fist. A gem this large would sell for thousands; he’d be set for life. He reaches inside, fingers wrapping around the white pearl.

The shells shift slightly, and just as the diver is pulling his arm back with his prize the giant clam shuts around his forearm. Bone breaks immediately. He loses his grip on his mouthpiece as he screams underwater. His other hand tries to pry the shell open again, but to no avail. His hand is now the clam’s dinner.

Just kidding.

Giant clams, also called Gigas clams, may have gained the reputation for being viscous man-eaters in the south giant-clam_541_600x450pacific, but it is highly unlikely anyone has ever met their death by these mollusks. The muscles that control the opening and closing of the shells move far too slowly to ever trap anyone. An unsuspecting diver would have plenty of time to move their arm out of the way once the bivalve starts moving.

A whale of a mollusk

Despite their bloody reputation being fictitious, there are still plenty of cool things about these mollusks. For one, their size isn’t made-up in the slightest. Giant clams can reach over four feet in length, and some can weigh more than five hundred pounds. Talk about giant!

The reason these clams grow to such incredible sizes is all thanks to the algae that grows inside them. While the clams provide a safe place for the algae to live, the algae releases sugars and proteins that the clams feed off of. The clams also filter feed plankton and other animal matter from the water, but the sugars and proteins contribute the most to a giant clam’s size.

Some facts


The shiny pearls that can be found in smaller, normal size clams, don’t grow in Giant Clams.  Instead the pearls that do grow are a bright white, almost like freshly fallen snow. And, just like the creatures that make them, the pearls can get to be pretty giant too.

Gigas clams come in just as many colors as the reefs they live on. From electric green to neon yellow to dark brown, these clams have it all. Most of the color is displayed on the zig-zag edge of the shells, and these bright colors actually reflect light to the inside of the shell where the algae lives, providing the light necessary for photosynthesis.

So where can you find these underwater rainbows? They favor the warmer temperatures that the Indian and south rainbow clamPacific oceans offer, and they make their homes among the equally colorful coral reefs. These mollusks like coral so much, they never leave. Ever. Initially they’re born as plankton floating through the water, but after a few days they find a nice bunch of coral to settle down on and they attach themselves to it until the end of their days. Assuming they get to live a long natural life, giant clams can live anywhere from twenty to a hundred years. That’s a long time to live with the same neighbors!

Not even these clams are exempt from natural predators and overfishing though. Eels, snails, and starfish like to make giant meals out of these mollusks, and their adductor muscles, the ones that open and close the shell, are considered a delicacy. Since these clams never move, they are easy targets for their predators. They aren’t endangered, but they are considered threatened. If efforts are not made to help conserve them, these giant clams could become giant myths.