This particular variety of clam, with its brilliant blue mantle, is a common sight on shallow Philippine reefs. It measures about six inches from end to end. In this photo the fleshy mantle is entirely exposed, but patience is required to see the clam in this state-- the clam will abruptly close its shell with only a minor change in light, such as a shadow. Specialized cells called ocelli along the outer edge of the mantle are responsible for light reception.  The large mantle covers the entire shell of the clam to capture the most possible light. Within the mantle tissue live microscopic zooanthellae that provide the clam with an accessory source of nutrition, supplementing filter-feeding as water is circulated over the internal gills and the internal surface of the mantle. The meat of these clams is considered a delicacy by many of the banceros ferrying me to different dive sites on Philippine reefs. I have frequently seen them dive into the water with only a knife and mask, emerging seconds later with a clam cut loose from the reef. Using the knife to pry open the shell, they would make a quick snack of the raw clam flesh. Identification: genus Tridacna, species unknown