Giant Clam is the largest living mollusc.

About Giant Clams

  • Size 1.21 metres
  • Weight: Average 200 kg
  • Family: Cardiidae
  • Genus: Tridacna
  • Class: Bivalvia
  • Species: Tridacna gigas

Giant clams are the largest extant bivalve molluscs and belong to the clam genus Tridacna.  Numerous species of “giant clams” of the Tridacna are sometimes mistaken for Tridacna gigas, the most widely referred to as “the giant clam.”

They are marine animals and live along shallow coral reefs, requiring sunlight for the algae to photosynthesize and create food.

In the first two years of the giant clams’ life, they grow rapidly and take around 30 years to reach full maturity.

All mature first as males, which takes around two years.

Light-sensitive patches on the clam’s mantle detect danger from shadows and force the clam to close.

Tiny algae live inside the flap of skin along the clam’s mantle, making vibrant colours that look like velvet.

Giant clams live in flat coral sand and along coral reefs.  They can grow to over one metre long and weigh over 200 kilograms.


The first giant clam recorded was found in 1817 off the northeastern coast of Sumatra.  Weighing 231 kilograms and is 4.49 feet long.  Pacific islanders have used shells for bathing babies and ceremonial washing containers.


Tridacna gigas are hermaphrodites that reproduce sexually (producing eggs and sperm).  An adult can release over 500 million eggs in one go.

Although self-fertilization is not feasible, this trait allows them to reproduce with any other species member.


Giant clams are hermaphrodites processing both male and female organs.

Because giant clams can’t move, they use broadcast spawning, releasing sperm and eggs into the ocean,

The reproductive process begins with the discharge of eggs.  An adult Tridacna Gigas can discharge more than 500 million eggs.


Spawning coincides with incoming tides near the moon’s second (full), third (new), and fourth (new) quarters.

Each spawning contractions occur every two or three minutes.   An intensive spawning lasts anywhere from thirty minutes to two and a half hours.

What do clams eat?

While giant clams filter feed, symbiotic algae constitute an essential source of nutrients for the giant clams.

These plants contain unicellular algae, whose metabolic products filter the clam’s diet.

As a result, giant clams can grow one metre long, even in nutrient-poor coral-reef waters.

Are clams dangerous?

Once considered a dangerous organism capable of killing people, they were called ‘man-eating clams’ in military survival guides even in the 20th century.

All reported deaths were false.  Clams can close but have no suction, making it challenging for a human to get trapped unless it is intentional.

Closing the shell is a defensive mechanism triggered by light dimming.

Giant Clams are now declining in numbers.

Unfortunately, humans are a significant contributing factor to the decline of clam populations due to their delicacy status in many South Asian and Pacific cultures.

Many cultures use the shell, and in some religions, the shell is considered a treasure believed to bring good luck and prosperity.

Protecting endangered species is essential to the Great Barrier Reef Marine Park’s green zones.

While giant clams live in many places throughout the World, our photos were all taken on Australia’s Great Barrier Reef.

Next, explore our Great Barrier Reef locations where the Giant clams are safe.

Next, learn about  Parrotfish and Pygmy seahorses.