marine invertebrates capable of boring passages or pits in wood, rocks, coral reefs, and, even, iron pilings (the sea urchin Strongylocentrotus purpuratus). Marine wood borers include bivalve mollusks of the family Teredinidae (for example, the shipworm), xylophaga of the family Pholadidae, limnorias and sphaeromas of the order Isopoda, Chelura of the order Amphipoda, and Sclerolinum of the pogonophora group. Stone borers include bivalve mollusks of the genera Lithodomus and Pholas, the sponge Cliona, certain polychaetes of the family Spionidae, the cirripede Lithotria, and certain sea urchins. The gastropod mollusks Nassa and Natica bore openings in the shells of mollusks on which they feed. Many borers cause extensive damage to submerged parts of wooden vessels and to pilings and other underwater structures.