Tridacna gigas

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Geographic Range

Giant clams are found throughout the Tropical Indo-Pacific oceanic region, from the south China seas in the north to the northern coasts of Australia and from the Nicobar Islands in the west to Fiji in the east.

Habitat

Giant clams occupy coral reef habitats, typically within 20 meters of the surface. They are most common found in shallow lagoons and reef flats, and are typically embedded in sandy substrates or those composed of coral rubble.

Physical Description

This is the largest living bivalve mollusk. The shell may reach up to 1.5 meters in length. They are characterized by having 4 to 5 large, inward facing triangular projections of the shell aperture, thick, heavy shells without scutes (juveniles may have some scutes), and an inhalent siphon with no tentacles. The mantle is usually golden brown, yellow, or green, with many irridescent blue, purple, or green spots, especially around the mantle edges. Larger individuals may have so many of these spots that the mantle appears solid blue or purple. Giant clams also have many pale or clear spots on the mantle, referred to as 'windows'. Giant clams cannot completely close their shell once fully grown.

  • Range mass
    0 to 0 kg
    0.00 to 0.00 lb
  • Average mass
    200 kg
    440.53 lb

Reproduction

Giant clams reproduce sexually via broadcast spawning. They expel sperm and eggs into the sea. Fertilization takes place in open water and is followed by a planktonic larval stage. The larvae (veligers) must swim and feed in the water column until they are sufficiently developed to settle on a suitable substrate, usually sand or coral rubble, and begin their adult life as a sessile clam.

Behavior

Adult tridacnids, including Tridacna gigas, are permanently sessile. If disturbed, the brightly colored mantle tissue (containing zooxanthellae) is retracted and the shell valves are closed.

Food Habits

Like the majority of other bivalve mollusks, Tridacna gigas can filter particulate food, including microscopic marine plants (phytoplankton) and animals (zooplankton), from seawater using its ctenidia ("gills"). However, it obtains the bulk of its nutrition from photosymbionts living within its tissues. These are unicellular algae (often called zooxanthellae) that are farmed by the mollusk host in much the same way that corals do. In some Tridacna gigas, the zooxanthellae have been shown to provide 90% of the carbon chains metabolized. This is an obligate association for the clam and it will die in the absence of the zooxanthellae, or if kept in the dark. The presence of 'windows' in the mantle may function to allow more light into mantle tissues to fuel zooxanthellae photosynthesis.

Economic Importance for Humans: Positive

Tridacnids are integral and colorful members of the Indo-Pacific coral reef ecosystems. All eight species of giant clams are currently being cultured. Tridacnid aquaculture ventures have diverse aims that include conservation and restocking programs. Farmed giant clams are also sold for food (the adductor muscle is considered a delicacy) and for the aquarium trade.

Economic Importance for Humans: Negative

Despite their classic movie depictions as "killer clams," there are no authentic cases of people being trapped and drowned by giant clams. Tridacnids are actually quite lethargic and slow about closing. Tridacnid-associated injuries are quite common however. They typically involve hernias, back injuries, and smashed toes induced when people lift adult clams out of the water unaware of their formidable weight in air.

Conservation Status

Giant clams are listed as vulnerable by the IUCN because of extensive collecting for food, aquaculture, and the aquarium trade. Numbers in the wild have been greatly reduced.

Other Comments

Giant clams (Cardiidae: Tridacninae) are among the most familiar marine invertebrates. Less well known is the remarkable fact that they are highly derived cardiids (cockles) whose adult morphology has been profoundly restructured by their long evolutionary association with photosymbionts. They have been severly over-harvested throughout much of their collective range and illegal fishing (poaching) remains a serious problem.

Contributors

Kari Tervo (author), University of Michigan-Ann Arbor, Rebecca Ann Csomos (author), University of Michigan-Ann Arbor, Cynthia Sims Parr (editor), University of Michigan-Ann Arbor.

Glossary

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

References

Accessed (Date Unknown) at http://www.exotictropicals.com/encyclo/reef/clams/tridacna.htm.

Carlos, A., B. Baillie, T. Maruyama. 2000. Diversity of dinoflagellate symbionts (zooxanthellae) in a host individual.. Marine Ecology Press Series, 195: 93-100.

Schneider, J., D. O Foighil. 1999. Phylogeny of giant clams (Cardiidae: Tridacninae) based on partial mitochondrial 16S rDNA gene sequences. Molecular Phylogenetics and Evolution, 13: 59-66.