- History and Culture
Tridacna is the genus of marine bivalve molluscs known as giant clams. These permanently attached or sessile organisms possess hinged shells that are fluted, or fan shaped with four to six folds with a brightly coloured mantle that is greatly expanded compared to other types of clams. They possess small lens structures called ocelli through which light penetrates. They are host to symbionts, microalgae called zooxanthellae which have the ability to photosynthesiseand provide the clam with nutrients. The zooxanthellae are dinoflagellates of the genus Symbiodinium that grow in the mantle tissue. Clams retreat and close their shells slowly when they are disturbed. The length of an adult individual can range from 15 cm in T. crocea to greater than 150 cm in T. gigas the largest species of the genus.
 The Paleobiology Database.
Giant clams are resident of coral reefs of shallow warm waters in the Indo-Pacific region. The giant clams obtain most of their food from the photosynthesising microalgae living in their tissues. During the day, the clams spread their shells so that the microalgae receive adequate light to photosynthesise. They are also filter-feeders.
The giant clams are hermaphrodites, possessing both testes and ovaries. The testes usually develop first, during the first two to six years of life. Afterwards, at around three to ten years, the ovaries develop. One organism will usually release sperm first and much later will release its eggs to minimise self-fertilisation. These organisms exhibit high fecundity, which increases with size. Larvae go through a planktonic phase before settling down on a substrate and developing into clams.
Giant clams are under the protection by CITES in 172 countries. Most of the species are also listed in the IUCN Red List. Tridacna rosewateri is listed as ‘Vulnerable’. They are consumed by humans and exploited commercially. The giant clams have been overexploited in most of their range. They are also sensitive to water pH and temperature, thus are directly affected by rising sea water temperature and ocean acidification.
 ‘Ion-Regulatory and Developmental Physiology of Giant Clams (Genus Tridacna) and Their Conservation Status on the Island of Mo’orea, French Polynesia.’
The endemic Tridacna rosewateri can be located close to the Saya de Malha Bank.
These giant clams can produce white pearls that have a porcelain like surface. These do not have the iridescent quality of usual gemstones but are large in size.
They have a lifespan of 100years.