Giant clams are large mollusks belonging to the Tridacna family. Like oysters and mussels, giant clams are bivalves. This means that they have no head, and a compressed body hidden in a shell that has two hinged parts. The largest species, known as the gigas giant clam, is the largest mollusk in the world. However, despite their reputations as man-eaters and sea monsters, giant clams are gentle giants capable of producing gorgeous, rare pearls.

Giant clam

1. The Largest Gigas Giant Clam weighed 250kg (551lbs)

The gigas giant clam is not only the largest bivalve in the world. It is also the largest mollusk. Giant clams usually weigh around 181kg (400lbs). They can also grow up to four feet. However, the largest giant clam ever discovered was a gigas giant clam that weighed 250kg (551lbs). This clam measured 137cm (4’5”).

2. Each giant clam shell is made of calcium carbonate

Giant clam shells are made of the chemical compound calcium carbonate. Some forms of calcium carbonate are fragile, like chalk. But the calcium carbonate of a giant clam shell is tough and durable to protect the soft body inside. Scientists recently discovered that the hard calcium carbonate crystals that form the shell have soft particles inside that strengthen them.

A giant clam’s soft body only accounts for 10% of its body weight

The actual body of a giant clam in its shell is very light. The weight of the mollusk is only 10% of its body weight, and the rest is its huge shell. Lean body is nearly all protein. The lean meat is considered a delicacy in many parts of the world. Giant clams are fished and sold to restaurants around the globe. But in impoverished coastal areas, the clams are considered an important source of protein.

the shell of a giant clam
Image by Andy Nelson

3. Adult giant clams are immobile once they settle into place

Giant clams settle on the ground at only one week old. However, clams will move around quite a lot in the next few weeks before finding the perfect spot. Once the clam is settled, it doesn’t leave its spot for the rest of its life. Many giant clams settle around coral reefs. Others settle in long, wide rows along the ocean floor. With their large, dark shells closed, endless rows of giant clams can look like an underwater city.

The giant clams breeding program
Image by Klaus Stiefel

4. Giant clams can live up to 100 years

Since giant clams settle at only a few weeks old, it's very important that they pick the perfect spot. They’re going to be there for a while! A healthy giant clam can live for 100 years. Many soft shell clams only live between 10-12 years. Oysters usually live 20 years in captivity. But giant clams in the wild outlive most humans.

the shells of a giant clam
Image by Rod Waddington

5. Giant Clams live as low as 20m (66ft) below the ocean surface

Giant clams live on ocean floors and coral reefs in shallow, tropical waters. Giant clams only live 20 meters (66 feet) below the ocean surface, or closer. Since the algae in the clam photosynthesize like plants, the clam needs to have plenty of access to sunlight.

6. Giant Clams and unicellular algae form symbiotic relationships

Giant clams are filter feeders. They eat by sucking in water and consuming plankton. But giant clams don’t eat all algae. They have symbiotic relationships with unicellular algae. The algae live inside the clam. When the algae photosynthesizes, it feeds the giant clam the excess energy. This extra feeding allows giant clams to grow so large even in habitats low in nutrients. The algae benefits from the protection it gets living inside the clam.

a giant clam
Image by Pierre Pouliquin

Sunlight and algae give the giant clam its vibrant colors

Giant clams are vibrant and colorful on the inside. No two giant clams are identical when they open their shells. The algae living inside clams produce stunning patterns. The giant clam’s “lips” have a shiny, iridescent sheen. When the clam opens up, the sheen directs sunlight into all the inside of the clam. This is so even algae in the dark, back parts of the clam can photosynthesize. The sunlit algae illuminate the clam.

7. Giant clams reproduce by shooting eggs and sperm into the water

Because mature giant clams are immobile, they cannot travel to reproduce. Instead, these claims mate via broadcast spawning. One giant clam will release eggs into the water, and this triggers nearby giant clams to release sperm to fertilize the eggs. One giant clam can release 500 million eggs at once!

Each giant clam produces both eggs and sperm

Giant clams are hermaphrodites. This means they have both male and female reproductive organs. Since these clams are stuck in place, they have to be able to mate with whoever is nearby. Humans who are born like this are called “intersex.” Before you ask, no, a giant clam cannot fertilize its own eggs.

8. A giant clam pearl is made of calcium carbonate, like its shell

Giant clams don’t form pearls from scratch. When bits of sand get inside the clam, it becomes irritated. It’s like when a human gets a splinter. Since clams can’t pull it out, they encapsulate the irritant in layers of calcium carbonate. This is also a protective measure. Parasites bore into the clam through its shell. So the clam patches up holes in its shell with calcium carbonate to protect it from parasites.

A giant clam shell is made of calcium carbonate
Image by Amy the Nurse

9. The largest giant clam pearl weighs over 34kg (75lbs)

In 2006, a fisherman in The Philippines discovered a 34kg pearl. The man was out fishing when his anchor accidentally snagged a giant clam. He reeled in the clam, and discovered a huge 75 pound pearl inside. Instead of putting the massive pearl on the market, the man kept it for nearly a decade as a good luck charm. Officials in The Philippines dubbed it the “Pearl of Puerto.”

The Pearl of Puerto is worth an estimated $100 million

The Pearl of Puerto is 175,000 carats. Carats are the unit of mass used to measure gemstones. Typically, larger gems with more carats are more expensive. At 175,000 carats, the Pearl of Puerto is worth an estimated $130 million. However, the fisherman didn’t sell it because when he rubbed the pearl before going to work, he caught a lot of fish. The pearl is now on display in Puerto Princesa.

10. Giant Clam Pearls have rings in the center, like trees

Like a tree trunk, a giant clam pearl has rings in the center. Pearls are incredibly durable and valuable, so scientists can’t cut them in half. Instead, they use x-rays to see inside. With x-rays, scientists can count the growth rings. Unlike trees, the rings don’t tell us about the age of the clam.  Instead, they indicate how long it took the clam to form the pearl.

South Pacific legends falsely labeled giant clams as man eaters

No, a giant clam cannot eat a human. No human deaths have ever been attributed to giant clams. However, these creatures have still haunted fishermen’s imaginations for centuries. One legend stated that the man who discovered the large Pearl of Lao Tzu drowned when the clam clamped down on his arm. However, clams close their shells too slowly to threaten a human. They also usually don’t close their shells all the way. Gotta get that algae fed!

a giant clam
Image by Marc Tarlock

How endangered is this animal?

  • Giant clams are classified as vulnerable
    Giant clams are classified as vulnerable. Although they are not common pearl producers, their meat is considered a delicacy. Decades of overfishing, poaching, and habitat destruction have decimated giant clam populations throughout the world. Giant clams are not endangered yet, but they are vulnerable. And this means they are still at risk of extinction.
  • Carbon Dioxide in the ocean threatens giant clams
    Climate change threatens millions of species. Giant clams are no different. Due to human emissions, more carbon dioxide is dissolving in the ocean each year. This process is called acidification. Scientists have discovered that high levels of carbon dioxide stunts a giant clam’s growth. However, clams exposed to high levels of light were the least affected. As ocean acidification worsens, giant clams will need more and more sunlight just to survive.
  • Scientists in the Philippines restored the country’s giant clam population
    In the 1970s, scientists discovered that the giant clam population around the Philippines was nearly extinct. So scientists at the University of Philippines began raising giant clams in a marine lab. They imported larvae from different islands and let them grow in the lab. They then released the mature clams into the ocean. After thirty years, the giant clam population of the Philippines is thriving!



Also Known As

Tridacna gigas, Southern giant clam


4.25 feet (1.3 m) long and 550 pounds (250 kg)


Indian and Pacific Oceans


Saltwater seafloors


Algae, Phytoplankton, Nutrients In Water


Up to 100 years

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