World's Largest Shell: Tridacna Gigas

The giant clam, Tridacna gigas (known as pā’ua in Cook Islands Māori), is the largest living bivalve mollusc. T. gigas is one of the most endangered clam species. It was mentioned as early as 1825 in scientific reports. 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 200 kilograms (441 lb) measure as much as 120 cm (47 in) across, and have an average lifespan in the wild of 100 years or more. They are also found off the shores of the Philippines, where they are called taklobo. T. gigas lives in flat coral sand or broken coral and can be found at depth 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. T. maxima has the largest geographical distribution among giant clam species; it can be found in high- or low-islands, lagoons, or fringing reefs. Its rapid growth rate is likely due to its ability to cultivate plants in its body tissue.

The largest known T. gigas specimen measured 137 cm. It was discovered around 1817 on north western coast of Sumatra. The weight of the two shells was 230 kilograms (507 lb). This suggests that the live weight of the animal would have been roughly 250 kilograms (551 lb). Today these shells are on display in a museum in Northern Ireland.

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 115 cm and the weight of the shells and soft parts was 333 kilograms (734 lb). Scientists estimated the live weight to be around 340 kilograms (750 lb).

Young T. gigas are difficult to distinguish from other species of Tridacnidae. 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.

T. gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the very 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.