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Ocean Life › Marine › Invertebrates › Mollusks › Bivalve › Clams › Giant Clams

Some Interesting Facts about Giant Clams

[Giant Clam Phylum: Mollusca] [Class: Bivalvia] [Order: Cardiida] [Genera: Hippopus and Tridacna]

The genus Tridacna contains the giant clam (Tridacna gigas) one of the most famous that often grows bigger than one metre across and weighs more than 200 kilograms.

Even though giant clams are members of a broader group of bivalve clams, there are some significant differences from the smaller edible shellfish you may see in some seafood soups.

Giant Clams Global Distribution and Habitat

The giant clam (genus Tridacna and Hippopus) is widely recognised as being the largest bivalve in the world.

Globally, the range of giant clams is concentrated in the warm tropical waters of the Indo-Pacific Region and the Pacific Ocean, particularly:

• Central Pacific islands (Fiji, Tonga, and French Polynesia)
• East coast of Africa (including Madagascar and the Red Sea)
• Southern Japan (Ryukyu Islands)
• Northern and eastern coasts of Australia (Great Barrier Reef)

Generally, the preferred habitat of giant clams are the shallow intertidal zones (neritic). They are found most often at depths around twenty metres (65 feet), though some species will tolerate deeper and darker environments.

They have a strong association with the four types of coral reefs, but especially sandy reef flats, outer reef slopes, and lagoons.

Interesting Fact: The Wildlife Trusts claim that a large burrowing bivalve (the ocean quahog clam) is the longest-lived animal - more than 500 years old.

Giant Clam Characteristics

So, where can you find giant clams while diving? The juveniles tend to anchor themselves to solid substrates, such as large hard corals or dead coral heads. Hence, they will be securely cemented into place by the time they reach adulthood.

Like most reef dwellers, they need clear water with plentiful supplies of sunlight, due to their symbiotic relationship with single-celled algae (zooxanthellae).

Giant Clam vs Other Clams

There are around 10,000 different clam species, but only ten (10) known species of giant clams. They are divided into two distinct genera, Hippopus and Tridacna.

The largest species of giant clam is Tridacna gigas - and the best known.

Their shells can grow to 140 centimetres long (nearly 5 feet) and weigh up to 260 kilograms (570 pounds).

However, the smallest species is Tridacna crocea, also known as the burrowing clam or crocus clam.

Still, it would be rare for the shell length to be more than 15 centimetres (6 inches).

What Do Giant Clams Eat?

Compared with most bivalve mollusks, the eating habits of giant clams follow two distinct methods, a combination of solar-power and filter-feeding.

Photosynthesis (via Symbiotic Algae)

Hosting millions of zooxanthellae (symbiotic algae) in their mantle tissues allows the algae to produce sugars and other nutrients by photosynthesizing sunlight.

In clear, sunlit, shallow water, clams are able to absorb a large part of this energy and fulfill around 70% of their nutritional needs.

Filter Feeding

Giant clams can also filter free-floating microscopic plankton and other tiny organic particles (such as zooplankton) from seawater.

In suspension feeders, water flows in through the 'incurrent siphon' and then it gets drawn over the gills. The food particles get trapped in mucus and the ciliated filaments (tiny hair-like extensions) move any food-laden mucus to the mouth and the stomach for digestion. Thus, clean water will exit through the 'excurrent siphon'.

Pro Tip: Even though the giant clam's main food source is carbohydrates and sugars produced by symbiotic algae, they can supplement their diet with plankton and detritus material that they filter from the water.

10 Types of Giant Clams

How Do Giant Clams Reproduce?

By and large, all marine invertebrates use environmental cues to trigger the release of gametes. In general, this will be lunar cycles or a rise in water temperature.

Tridacna and Hippopus are mostly simultaneous hermaphrodites. However, even though each individual has eggs and sperm, they do not actually self-fertilise.

Here's the thing:

The reproductive strategies of giant clams takes place through synchronised broadcast spawning and a process of external fertilisation, which often involves millions of eggs.

They also experience a planktonic larval stage (trochophore and veliger stages) before the young finally settle into suitable coral reef habitats.

Nonetheless, juvenile clams are slow to grow, and sexual maturity usually occurs after they are four (4) years old (determined by the species and their environment).

Giant Clam Predators and Threats

The vast majority of bivalves face multiple predators in the wild. Some of the creatures that naturally prey on most of the species (especially giant clams) include:

• Fish (e.g., pufferfish, triggerfish, and wrasses)
• Marine invertebrates (crabs, octopuses, sea snails, and sea stars, especially the crown-of-thorns starfish)
• Humans also harvest aquatic bivalves for food (especially clams, mussels, oysters, and scallops)

Are Giant Clams Endangered?

Some environmental threats that also threaten the long term survival of some giant clam species in vulnerable locations, include:

• Destruction of their natural habitat (mostly due to dredging, coastal development, and trawling)
• Sea pollution (chemicals, heavy metals, sedimentation, and sewage)
• Ocean acidification (climate change and warming seas weaken bivalve shells due to a reduction in the availability of calcium carbonate)
• Parasites and disease (particularly in farmed mussels and oysters)
• Invasive species (e.g., the accidental introduction of zebra mussels (Dreissena polymorpha) from the lakes of southern Russia are threatening native freshwater bivalves)

Important: In 2024, the IUCN Red List of Threatened Species assessed the conservation status of the giant clam (Tridacna gigas) and recorded them as being "Critically Endangered" (CR).

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