Giant clams host zooxanthellae, single-celled dinoflagellate algae, in a branching tubular system within the mantle. The clam supplies inorganic nutrients and a protected position in bright light; the algae fix carbon by photosynthesis and pass a large share of it to the host. This subsidy is what allows the animal to reach its size in nutrient-poor reef water.
The mantle's blues, greens and golds come partly from iridocytes, pigment-bearing cells that scatter and redirect light. Rather than simply reflecting sunlight away, they distribute it into the deeper algal columns, so photosynthesis is not confined to the surface layer. The clam continues to filter feed as well, but photosymbiosis is the dominant energy source.
The persistent legend is the man-trap: divers supposedly held underwater by a clamping giant clam. There is no substantiated case. A large Tridacna closes slowly and, in big individuals, the mantle is too bulky for the valves to shut completely. The real threat runs the other way, since the species is heavily harvested and listed as Vulnerable.