Giant Clam

Quick Take

• A giant clam that never moves still manages to eat, and its feeding strategy is stranger than you'd expect. See how it feeds →
• There's a persistent legend that giant clams kill humans, and the truth behind it is far more interesting than a simple debunking. Explore the clam myth →
• This clam can release 500 million eggs at once, and yet it is still Critically Endangered. See the spawning facts →
• Giant clams may have played a pivotal role in one of humanity's most consequential journeys, and this was long before anyone thought to study them. Discover its human history →

Giant clams can grow to over a meter in length, reaching up to about 4.5 feet. They live in the Indian and South Pacific Oceans. The exposed mantle, or soft tissue, of a giant clam features a combination of colors and patterns that can include yellow, green, iridescent blue, and purple. This clam is not a traditional omnivore; instead, it derives most of its nutrition from symbiotic algae called zooxanthellae living in its mantle tissue, while also obtaining food by filter feeding on tiny particles suspended in the water. This mollusk can live for 100 years or more.

4 Incredible Facts

• After attaching itself to a coral reef, this mollusk stays there for the rest of its life.
• The bright colors in this clam’s mantle are caused by the algae in its system.
• There is a legend claiming that these clams eat humans, but this is not true.
• Many clams of this species live on the Great Barrier Reef off the coast of Australia.

Classification and Scientific Name

The scientific name of this clam is Tridacna gigas. The word Tridacna comes from the Greek word tridaknos, meaning ‘oyster,’ and the Latin word gigas means ‘giant.’

It belongs to the phylum Mollusca, the class Bivalvia, the order Cardiida, and the family Cardiidae, subfamily Tridacninae.

Bivalve relatives include mussels, scallops, and oysters. Brachiopods and bivalves are distinct groups that superficially resemble one another. Around 252 million years ago, brachiopods largely died out following the Permian-Triassic extinction, after which bivalves became the dominant clam-like species. They evolved into clams in what was once the Tethys Sea back when Europe was underwater around 40 million years ago. Those long-ago, shallow, and warm waters are now the Mediterranean, Black, and Caspian Seas. It is theorized that giant clams were a critical food source for prehistoric humans before they migrated out of Africa.

Appearance & Behavior

These clams have a thick, heavy shell with fluted edges that protects the mantle, or soft tissue, inside it. Its mantle, which accounts for only about 10 percent of its body weight, is colorful, displaying patterns of brown, iridescent blue, green, yellow, and purple. The algae stored in this clam’s body is what makes its mantle so colorful. An adult clam’s mantle has pale spots on it known as windows.

The size of these clams can exceed 1.4 meters (4.5 feet) in length. They can weigh more than 440 pounds, with the heaviest specimens on record exceeding 400 kilograms (about 900 pounds). Picture three bowling pins lined up end to end, and you have the length of a 4-foot giant clam. A 500-pound giant clam is equal in weight to half a horse.

The sheer size of an adult giant clam, as well as its thick shell, serves as excellent protection against most predators. Furthermore, this clam can close its shell most of the way to protect itself from threats, though an adult clam is not able to snap it fully shut.

Habitat

These clams live in warm water habitats. Specifically, they are found in the South Pacific and Indian Oceans. They live off the northern coast of Australia, as well as near the Nicobar Islands and Fiji. The Great Barrier Reef near Australia is home to many of these clams. They live in reef flats as well as in shallow, sandy areas of lagoons.

They are usually found at a depth of around 65 feet. In the ocean, a depth of 65 feet is not very deep. Living in shallow areas allows these clams access to filtered sunlight.

Diet

These clams are omnivores, obtaining nutrition both from filter feeding and from algae living in their tissue. However, since they are attached to a coral reef, they can’t hunt for food. These mollusks have a siphoning system that takes in tiny prey, such as phytoplankton and zooplankton, that float by. This siphoning system is similar to the filtering system in a backyard swimming pool.

Also, the clam gets nutrients, including sugar and protein, from the billions of particles of algae stored in its tissue. This is called a mutually beneficial relationship. The algae enjoy the protection of living in the clam’s body tissue while the clam receives essential nutrients in return. Further, these clams open their shell during the daylight hours to allow sunlight to filter in through the clear spots, or windows, in their mantle; thus, the sunlight plays a part in the photosynthesis that feeds the algae.

Predators and Threats

It may seem like the sheer size of this clam would keep all threats away. Nonetheless, this mollusk does have some predators. Reef fish, eels, and starfish eat the meat of the clam. When one of these predators takes a bite, it doesn’t kill the giant clam. However, it does cause injury.

Humans are another threat to this clam. People harvest them for their meat and shells. Specifically, the adductor muscle of a giant clam is considered one of the most delicious parts of this sea creature. They are especially popular in Japan, France, and throughout Southeast Asia.

The larvae of these clams are eaten by fish, octopuses, and crabs. These larvae are so small that thousands of them could be eaten by one large fish. This is the main reason why a giant clam releases so many eggs; it increases the chances that some of them will survive to adulthood.

Water pollution is another threat to these clams. Pollution can affect their rate of reproduction.

Reproduction, Babies and Lifespan

Each giant clam has both sperm and eggs; that is, it is hermaphroditic. It reproduces by releasing both sperm and eggs into the open water. One giant clam can release 500 million eggs at one time. Eggs are fertilized by the sperm released by another giant clam. This is called broadcast spawning, and it may be coordinated with the phases of the moon.

The fertilized eggs hatch into larvae. Giant clam larvae circulate in the water looking for plankton and other tiny particles of food until they grow large enough to settle into one place near or on a coral reef.

A developing giant clam goes through many stages. It starts as an egg and hatches into a larva. After that, it develops into a veliger, a pediveliger, a juvenile, and an adult. Although it takes years for a giant clam to achieve adult weight and size, it has a lifespan of up to 100 years. With such a long lifespan, it has plenty of time to grow into a 4-foot, 500-pound adult.

Population and Conservation Status

According to the IUCN Red List of Threatened Species, the exact population size of these clams is unspecified. The species is threatened by humans and water pollution. The conservation status of this clam is Critically Endangered.