The Underwater World of Giant Clams

2021 Pew marine fellow Mei Lin Neo shares her passion about the ocean’s largest bivalves

The Underwater World of Giant Clams

You might think an animal that can weigh up to 500 pounds, with vibrantly colored flesh peeking out from a massive shell, would be hard to miss. But giant clams can tuck themselves so artfully into their habitat that predators—not to mention divers and snorkelers—often drift past without noticing them.


These massive bivalves generally occupy tropical coral reefs and coastal areas in the Indo-Pacific where they are ecosystem engineers, influencing everything from water flow to the physical structure of their habitat.

Mei Lin Neo, a 2021 Pew marine fellow and marine ecologist from Singapore, is an expert on giant clams and is engaged in efforts to raise awareness about their declining numbers and the need to conserve them.

Mei Lin Neo holding a giant clam
2021 Pew marine fellow Mei Lin Neo with a young giant clam.
Denise McIntyre

“Receiving the Pew marine fellowship has validated my years of research and outreach focused on the conservation status of giant clams,” Neo said. “With support from the fellowship program, I want to inform more effective giant clam conservation efforts in Southeast Asia, guided by science.”

Neo’s interest in the world’s largest living mollusks started when she first saw the animals at a local marine station. From there, her fascination grew.

“My curiosity and love for them grew much more after I delved deeper into their behavior, ecology, and conservation status. I realized how vulnerable they were based on what I had read initially and, later, when I saw firsthand how human impacts affect their survival. I decided to make it my life’s mission to become a champion for their conservation,” she said.

Here, we dive into the world of giant clams by sharing some of Neo’s photos and five facts she wants you to know about these fascinating creatures.  

Giant clams are solar powered.

These animals are filter feeders and also derive nutrients from a symbiotic relationship with microalgae (Symbiodiniaceae) that live within the tissue folds of their mantle, the outer fleshy region that extends from the shell. In exchange for providing the tiny algae shelter and protection against ultraviolet radiation, giant clams consume byproducts produced as the microalgae generate food through photosynthesis.  

At two months old, this lab-grown juvenile fluted giant clam (Tridacna squamosa) has established a network of tubules hosting Symbiodiniaceae microalgae.
At two months old, this lab-grown juvenile fluted giant clam (Tridacna squamosa) has established a network of tubules hosting Symbiodiniaceae microalgae. The microalgae will continue to multiply as the young clam grows.
Mei Lin Neo

Giant clams can “walk” throughout their life.

Giant clams may appear immobile, but during experiments in 2006, our team discovered that young giant clams have the tendency to “walk” by extending their foot from an opening under their shells and using it to move slowly along a surface. We also observed in further experiments that young giant clams often move to form groups for protection against potential predators. Older and larger individuals have fewer predators and remain in one place.

In the wild, some species of giant clams, such as these Tridacna maxima, can be found living in small groups.
In the wild, some species of giant clams, such as these Tridacna maxima, can be found living in small groups.
Mei Lin Neo

Giant clam feces are a food source for some animals that live on coral reefs.

The organic materials in these bivalves’ feces, including mucous and proteins, are a nourishing food source for other sea creatures, such as the black damselfish. In a recent study published in PLOS ONE, researchers documented that giant clam feces also contains undigested and photosynthetically functional symbiotic microalgae, which may be taken in by other host organisms—including young giant clams.

An adult Tridacna squamosa, shown here, releasing a string of nutritious feces.
An adult Tridacna squamosa releases a string of feces. A 2019 study found that symbiotic microalgae in giant clam feces remain viable after they are expelled. These microalgae could benefit other photosynthetic coral reef animals.
Mei Lin Neo

Giant clams face many threats.

The suite of challenges facing these creatures include overfishing, habitat degradation and destruction, overharvesting for the ornamental trade, and impacts from climate change. These threats have led to local extinctions of multiple species—especially the larger ones, such as Tridacna gigas and T. derasa—in many areas.

Marine heatwaves affect more than just corals, including other marine animals that form symbiotic relationships with microalgae, such as giant clams.
Marine heatwaves can cause giant clams to lose their symbiotic microalgae—a process known as bleaching that turns their tissues partially or completely white.
Mei Lin Neo

Conservation efforts can help giant clams recover.

In recent decades, multiple species of giant clams have been recognized as threatened by the Convention on International Trade in Endangered Species of Wild Fauna and Flora and the International Union for Conservation of Nature; and conservationists have launched restocking and monitoring efforts in some areas, including parts of the Philippines.

In 1985, Filipino biologist Edgardo Gomez, who later became a Pew marine fellow, launched a program to replenish locally extinct Tridacna gigas populations across the Philippines. That effort has continued despite Gomez’s death in 2019. A study published in 2017 in the journal Coral Reefs found that these restocked giant clams have begun to naturally spawn.

To safeguard the future of giant clams, research will need to identify the root cause of ongoing population declines.

Young Tridacna gigas are cultured at the Bolinao Marine Laboratory of the University of the Philippines.
Young Tridacna gigas are cultured at the University of the Philippines’ Bolinao Marine Laboratory. These juveniles will be distributed in the country’s waters as part of ongoing restocking efforts, the success of which depends on local community members managing restocking sites.
Mei Lin Neo

Mei Lin Neo, one of The Pew Charitable Trusts’ 2021 marine fellows, is a senior research fellow with the National University of Singapore’s Tropical Marine Science Institute. Polita Glynn is a project director with Pew’s marine fellows program.