The giant clams are the members of the clam genus Tridacna that are the largest living bivalve mollusks. There are several species of "giant clams" in the genus Tridacna, which are often misidentified for Tridacna gigas, the most commonly intended species referred to as "the giant clam".
Tridacna gigas is one of the most endangered clam species. Antonio Pigafetta documented these in his journal as early as 1521. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they can weigh more than , measure as much as across and have an average lifespan in the wild of over 100 years. They are also found off the shores of the Philippines and in the South China Sea in the coral reefs of Sabah (Malaysian Borneo).
The giant clam lives in flat coral sand or broken coral and can be found at depths of as much as 20 m (66 ft). Its range covers the Indo-Pacific, but populations are diminishing quickly, and the giant clam has become extinct in many areas where it was once common. The maxima clam has the largest geographical distribution among giant clam species; it can be found off high- or low-elevation islands, in lagoons or fringing reefs. Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.
Although larval clams are planktonic, they become sessile in adulthood. The creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which the adult clams get most of their nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesise.
Anatomy
Young T. gigas are difficult to distinguish from other species of Tridacninae. Adult T. gigas are the only giant clams unable to close their shells completely. Even when closed, part of the mantle is visible, unlike the very similar T. derasa. However, this can only be recognized with increasing age and growth. Small gaps always remain between shells through which retracted brownish-yellow mantle can be seen.
Tridacna gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the similar shell of T. derasa, which has six or seven vertical folds. As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells. The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam.
The mantle border itself is covered in several hundred eyespots about .5mm in diameter. Each one consists of a small cavity containing a pupil-like aperture and a base of one hundred or more photoreceptors. These receptors allow T. gigas to respond to sudden dimming of light by withdrawing their mantles and partially closing their shells, presumably to protect from potential predators. They do not retract their mantles in response to increased illumination, but it has been observed that a change in the direction of light results in a shift in mantle orientation. In addition to a dimming response, T. gigas also responds to the movement of an object before a shadow has been cast. In order for this to happen, an image forming optical system is required as the response is based on the local dimming of one part of the generated image relative to the rest. This sequential dimming of receptors caused by the movement of a dark object allows enough time for the mantle to be retracted before a potential predator is directly overhead and casting a shadow.
Largest specimens
The largest known T. gigas specimen measured . It was discovered around 1817 on the north western coast of Sumatra, Indonesia, and its shells are now on display in a museum in Northern Ireland. The joint weight of the two shells is , which suggests that the live weight of the animal would have been roughly .
Another unusually large giant clam was found in 1956 off the Japanese island of Ishigaki. However, it was not examined scientifically before 1984. The shell's length was and the weight of the shells and soft parts was . Scientists estimated the live weight to be around .
Ecology
Feeding
Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are covered by symbiotic unicellular algae (zooxanthellae). This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters. The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume.