Giant Clams

A Highlight On Any Trip To The Great Barrier Reef

The Giant Clam (Tridacna gigas) is the largest living bivalve mollusc. It’s also one of the most endangered clam species. They are targeted for their meat, which is considered an aphrodisiac in some countries, and for the aquarium trade. Luckily for us, there are still many to be found in the Great Barrier Reef Marine Park.

Giant Clams can weigh more than 200 kilograms, be as wide as 120 cm, and have an average lifespan in the wild of over 100 years. Adult T. gigas are the only clam unable to close their shell completely, with part of the mantle tissue always visible. Within this mantle tissue lives an algae called zooxanthellae (the same algae that live within coral). The adult clam receives most of their nutrition from this algae, and in return, the algae receives a home. The clam opens its shell and extends its mantle tissue so the algae also receives the sunlight it needs to photosynthesize.

Despite being hermaphrodites (producing both eggs and sperm), Giant Clams are unable to self-fertilize. Coinciding with incoming tides around specific moon phases, Giant Clams will release sperm and eggs into the water. This allows them to reproduce with other family members, who detect the spawning event in the water and release their own eggs and sperm. A sexually mature clam can release more than 500 million eggs at a time! Once fertilised, the larvae will begin to develop their limestone shell within a day or two. Once on the seafloor, they have a leg that they use to crawl around the reef with. They have one week to find the patch of reef that will be home for the next 100 years. Luckily for us, many have chosen the reefs ABC Snorkel Charters Port Douglas regularly visit to live, giving us plenty of opportunities to see this endangered species.