Picture credit : Simon Dowling
The Giant Clam, (Tridacna Gigas), are an endangered species due to its popularity as a food. They are also collected for souvenirs (since their shells are highly sought after).
They are a “Keystone species” in Thailand, which means that they are very valuable to sustaining a good ecosystem. They are bivalves who filter the water and remove excess nutrients that flow into the reef system from the land. This process makes the water much cleaner, clearer and higher quality helping coral reefs to grow.
Giant Clams are hermaphodites, and since they do not move in the ocean, they adopt a process called Broadcast Spawning. They release sperm and eggs into the water (at different intervals) and the release can contain up to 500 million eggs at a time. Once fertilised, the floats in the sea for about 12 hours until eventually a larva hatches. It then starts to produce a calcium carbonate shell. After a while it will develop a basic “foot,” which is used to move on the ground. At about 1 week old, it will settle on the ocean floor, moving around until it has found a suitable location for it to settle. Once its decided to settle, the “foot” will begin to disappear, and sticky filaments called Byssal threads extended from the opening at the bottom of the clam shell and begin to attach to the substrate.
Giant clams achieve their enormous proportions by consuming the sugars and proteins produced by the billions of algae that live in their tissues. In exchange, they offer the algae a safe home and regular access to sunlight for photosynthesis, with their shells open and their multi-colored mantles exposed. They also use a siphon to draw in water to filter and consume passing plankton.
When danger is sensed by using hundreds of basic eyespots (that can sense light and dark), they will close their shells to protect their soft mantle. Although, that may not be enough protection from predators. Besides humans, we have witnessed TriggerFish using rocks as tools to create an opening in the Giant Clam’s protective shell. Triggerfish also have sturdy teeth that can do damage to the toughest of surfaces. Once a crack appears then the clam has no chance against such an aggressive and strong predator. Other predators, like Octopus can use their slinky bodies to squeeze their way through the security of the Clam’s shell.
One of our ongoing projects for the last few years has been raising Giant Clams with our in-water nurseries. We are donated the clams as part of a program set up by the Queen of Thailand. We have built cages designed to prevent predators attacking and keep the location secret so that they are less at risk from poaching. The clams remain in the cages being monitored and measured regularly until they are large enough to be transported underwater to various parts of the natural reef.
As part of an Eco Internship, you would be actively involved in helping maintaining the nurseries and monitoring the clams health.