Giant clams (Tridacna gigas) have four or five large, inward, vertical folds in their thick, heavy shell. The shell does not have scutes and consists of two valves. Once they are fully grown they cannot close their shell completely. The mantle is usually yellow, golden brown or green, with many iridescent blue, purple, or green spots. They also have a number of pale or clear spots on the mantle, known as ‘windows’. These ‘windows’ allow sunlight to filter in through the mantle which is necessary for photosynthesis. The mantle is fused except for the two siphons (holes), an inhalant siphon and an exhalant siphon. The inhalant siphon is used to draw in seawater that is then filtered for planktonic food. The exhalant siphon is used to expel water during spawning. No two Giant clams have the same coloration.
Key Characteristics:
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Size: This is the largest species of bivalve mollusk. The shell can grow up to 59 inches (1.5 meters) in length, and the clam can weigh over 500 pounds (249 kg).
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Shell: The shell is thick and heavy, with four or five large inward vertical folds. It consists of two valves and lacks scutes. Once fully grown, these clams cannot completely close their shell.
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Mantle: The mantle is usually yellow, golden brown, or green, adorned with many iridescent blue, purple, or green spots. It also has pale or clear spots known as “windows,” which allow sunlight to filter through for photosynthesis. The mantle is fused except for two siphons: an inhalant siphon for drawing in seawater and an exhalant siphon for expelling water during spawning.
Behavior: Giant clams are sessile; once they settle in a location, they remain there for life. They can live for over 100 years within their environment.
Diet: Giant clams filter particulate food such as phytoplankton and zooplankton using their siphon. Most of their nutrients come from a symbiotic relationship with algae (zooxanthellae) living in their tissues. The algae provide carbon by-products from photosynthesis, while the clam offers protection.
Reproduction: Giant clams reproduce through broadcast spawning, releasing eggs and sperm into the water where fertilization occurs. The fertilized eggs develop into planktonic larvae that swim and feed in the water column until they settle and become sessile adults.
Habitat/Range: Giant clams are found in coral reef habitats, typically within 20 meters of the surface. They are common in reef flats and shallow lagoons in the warm waters of the South Pacific and Indian Oceans.
Conservation Status: Tridacna gigas is listed as Vulnerable on the IUCN Red List due to threats like overharvesting and habitat destruction.
Giant clams are remarkable for their size and vibrant mantle colors. They play a crucial role in coral reef ecosystems and have a symbiotic relationship with algae that helps them obtain nutrients. Conservation efforts are essential to protect this vulnerable species from overharvesting and habitat loss.
