Southern giant clam (Tridacna derasa)

Also known as: Derasa clam, smooth giant clam
GenusTridacna (1)
SizeShell length: up to 60 cm (2)

Classified as Vulnerable (VU) on the IUCN red List 2006 (1), and listed on Appendix II of CITES (3).

The southern giant clam is one of the largest of the ‘giant clams’, reaching up to an incredible 60 cm in length (2) (4). The species is also known as the smooth giant clam because of the relative lack of ribbing and scales on its thick shell. The smoothness of the southern giant clam’s shell, and its six to seven vertical folds, help to distinguish it from its larger relative, T. gigas, which has four to five folds and a rougher texture (4). The mantle usually has a pattern of wavy stripes or spots, and may be various mixtures of orange, yellow, black and white, often with brilliant blue or green lines (4) (5).

Native to waters around Australia, Cocos Islands, Fiji, Indonesia, New Caledonia, Palau, Papua New Guinea, Philippines, Solomon Islands, Tonga and Vietnam. Populations have also been introduced to American Samoa, Cook Islands, Marshall Islands and Samoa, and reintroduced after extinction in Guam, the Federated States of Micronesia and Northern Mariana Islands (2).

Found on the outer edges of the reef at depths of between 4 and 10 m (5).

Tridacna clams have muscles for opening and closing their shell and a foot for attaching themselves to rocks. They breathe through gills and feed through a mouth (6). Most clams fulfill their nutritional requirements by filter feeding and absorbing dissolved organic compounds from the water, but tridacnid clams have gone further than this by using zooxanthellae algae in their tissue to manufacture food for them (6) (7). The zooxanthellae transforms carbon dioxide and dissolved nitrogen, such as ammonium, into carbohydrates and other nutrients for their hosts (7).

When Tridacna clams first attain sexual maturity they are male, but about a year later become hermaphrodites, possessing both male and female reproductive organs. However, the release of sperm and eggs are separate in order to prevent self-fertilisation, although self-fertilisation can occur. The breeding season of the southern giant clam occurs in spring (7).

The southern giant clam is a popular food item and aquarium species, and has therefore been hunted extensively throughout its natural habitat (5).

Thankfully, the southern giant clam has been one of the first clams to be bred commercially, and the specimens traded today tend to be the result of aquaculture farms rather than wild-caught individuals (5).

For more information on the southern giant clam see:

Lukan, E.M. (1999) Critter Corner: Tridacna derasa. Fish ‘N’ Chips: A Monthly Marine Newsletter, 1999:0. Available at:

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  1. IUCN Red List (February, 2014)