Giant clam  (Tridacna gigas)

This enormous shellfish is the largest species of bivalve mollusc in the fossil record, and the heaviest of all the living molluscs (4). Like all bivalve molluscs, the shell consists of two valves, although in the larger giant clams these cannot close completely (6). The shell is extremely thick and lacks bony plates; when viewed from above, each valve has four to five inward facing triangular projections (6). The mantle of the clam is visible between the two shells, and is a golden brown, yellow or green, although there may be such an abundance of small blue-green circles that the overwhelming impression is of a beautiful iridescent colour (6) (7). A number of pale or clear spots on the mantle, known as 'windows', allow sunlight to filter in through the mantle (6). The mantle is completely fused with the exception of two holes (or 'siphons'). The gills are visible through the inhalant siphon, while the exhalent siphon is tube-like and is capable of expelling a large volume of water during spawning or if the clam's shells close suddenly (5) (7).

Found in shallow waters of the Pacific Ocean, from Thailand and Japan to Australia and Micronesia (1). However, the giant clam's range has reduced since the 1970s due to over-harvesting (7).

The giant clam inhabits warm tropical waters on reef flats and shallow lagoons to a depth of up to 20 metres (6).

Adult giant clams are completely sessile, unable to move from their position on the coral reef. They reproduce by expelling sperm and eggs into the sea (6), where fertilization occurs. The fertilised eggs quickly enter a swimming stage, (where they are known as trochophores), before entering a planktonic stage (7). During this stage, the larvae, (known as 'veligers'), inhabit the open ocean for one week, before settling in the substrate. If a clam is disturbed it will close its shell valves (6).

Giant clams have an inhalant siphon, which they use to draw in seawater that is then filtered for planktonic food (6). The majority of the clam's nutrients however, are obtained by a mutually beneficial relationship with minute algae known as zooxanthellae (6). These plant-like algae exist in delicate tubules which are extensions of the stomach (8). The algae gain protection from predation by being associated with such a large organism, while the clam obtains the carbon by-products of photosynthesis (9). Giant clams also provide protection for a species of pea crab (Xanthasia murigera); a single pair will often be found living within the cavity of the clam (5).

Giant clams have been extensively harvested for their meat and to supply the aquarium trade with such exotic specimens (6). Unable to sustain this exploitation, populations are now showing signs of decline; Tridacna gigas have not been seen in Fiji for over 50 years, primarily as a result of past over-collection for food (2).

These clams are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which requires a permit to be granted before clams can be exported (3). There has been considerable success with farming (10), which may help to alleviate the pressure on wild populations in the long-term. Farmed clams may also be used in restocking programmes, where numbers have become severely restricted in the wild. Giant clams were reintroduced to Tongan waters in 1990 (2), from quarantined-reared stocks cultured in Australia under the Australian Centre for International Agricultural Research and James Cook University giant clam project (11). These enormous molluscs have inspired awe for centuries and effective protection measures are vital if they are to be adequately conserved for future generations.

Authenticated (25/02/08) by Dr Rick Braley, Aquasearch Lab, Aquarium and Consultancies, Queensland, Australia.
http://www.aquasearch.net.au