Giant Clams Can Weigh as Much as a Grizzly Bear

Imagine a clam so massive it could crush your foot if it snapped shut. The giant clam (Tridacna gigas) isn't just big for a shellfish—it's the largest living bivalve mollusk on the planet, reaching lengths of over four feet and tipping the scales at more than 500 pounds.

That's roughly the weight of a grizzly bear. In a shell.

Living Fortresses on Coral Reefs

Giant clams make their homes on coral reefs in the warm, shallow waters of the Indo-Pacific region, from the Red Sea to the South Pacific islands. Unlike the clams you might dig up at the beach, these behemoths anchor themselves to the reef and stay put for their entire lives—which can span over 100 years.

Their shells alone can weigh 200-300 pounds, with thick, fluted edges that look like something from a fantasy novel. The mantle—the fleshy, colorful tissue visible when the clam is open—can be electric blue, green, purple, or gold, thanks to symbiotic algae called zooxanthellae living in its tissues.

They're Solar-Powered (Sort Of)

Here's where it gets wild: giant clams are part-plant, part-animal. Those colorful mantles aren't just for show—they're packed with photosynthetic algae that produce nutrients through sunlight. The clam provides the algae with a safe home and raw materials, while the algae supply up to 90% of the clam's food needs.

The rest of their diet? They filter-feed like regular clams, sucking in plankton and organic particles from the water.

Can They Really Trap Divers?

You've probably heard the myth about giant clams trapping unsuspecting divers. It's mostly Hollywood fiction. While the shells can close with considerable force, they do it slowly—giving anything caught inside plenty of time to escape. There are no verified reports of human deaths caused by giant clams.

That said, you definitely don't want to stick your hand in one. The pressure could cause injury, and extracting yourself from a 500-pound vise grip wouldn't be pleasant.

Endangered Giants

Despite their intimidating size, giant clams face serious threats:

• Overharvesting for meat, shells, and the aquarium trade
• Habitat destruction from coral reef degradation
• Climate change causing coral bleaching and ocean acidification

Several species of giant clams are now listed as vulnerable or endangered. Conservation efforts include clam farming operations and marine protected areas, but wild populations continue to decline.

These ancient ocean giants have been filtering reef waters since before humans walked the Earth. The fact that a single organism can grow to half a ton just by sitting still and soaking up sunlight is a reminder that the ocean still holds wonders that defy our sense of scale.