DRESSED in scuba gear, Peter Itirau is tending a clam nursery 10 feet beneath the ocean's surface.Swimming along the ocean floor, Mr. Itirau and a colleague crush snails that have found a few of the 4,000 clams. If the snails are not removed, they will make a feast of the clams. Soon the clams (Tridacna gigas and Tridacna derasa) willgrow large enough to fend for themselves, and will be shipped to reefs in Yap's outer islands. Over-clamming by the islanders and poaching by foreign boats has depleted clam populations on the reefs. In response, the Yap government, as well as those in Kosrae, and Pohnpei, and the neighboring nations of Belau (Palau) and the Marshall Islands are trying to restock their reefs. The Yap effort began in 1987 with the purchase of 10,000 clams from a nursery in Belau. The derasa clams cost $1 each and must be ordered in lots of 1,000. The gigas are 40 cents each with a minimum order of 5,000. Yap has also tried cultivating ornamental shells, valued by the garment industry for buttons. The first crop of clams has already been distributed to the outer islands. "We give them to the chiefs so the villages look after them," says John Iou (pronounced "Yo"), division chief of the Marine Resources Management Division. Mr. Iou estimates about 10 percent of the clams will be poached. "It's always by people from other villages," he says with a laugh. Iou hopes the villagers will let the clams grow for five to seven years. The clams will then spawn, allowing the process to continue naturally. Later, the clams will be harvested for their meat and shells. An adult clam can provide up to a pound of meat, which many villagers eat raw.