A variety of marine bivalve mollusks which penetrate solid substrata. They represent seven families and vary in the extent to which they are specialized, in the type of substrata they utilize, and in their method of boring.
Unlike other bivalves which use their shells as abrasive “tools,” date mussels (Lithophaga, Mytilidae) penetrate calcareous rocks, corals, and shells by chemical means, possibly a weak carbonic acid. Larvae of Botula (another small, elongate mytilid), which settle in crevices of soft rock, corals, or wood, abrade their way into the substrate by continued movement of their shells and siphons. Rupellaria and Petricola (Petricolidae) are nonspecialized borers in peat, firm mud, and soft rock. Platyodon (Myidae), closely related to the soft-shelled clam, press their valves against the walls of the burrow by engorging the mantle and then abrade the soft rock by continued movement of their unspecialized valves. Gastrochaena and Spengleria (Gastrochaenidae) are specialized for boring by having a closed mantle cavity, a large pedal gape, and a truncate foot, allowing them to press the foot and shell against the burrow wall. The Pholadacea (Pholadidae and Teredinidae) are worldwide in distribution. The family Pholadidae (common name: piddocks) is composed of 17 genera, of which 5 are restricted to wood. All species of Teredinidae are obligate wood, nut, or plant-stem borers. The major difference between these two families is the presence of accessory plates in the pholads and pallets in the teredinids. See also Mollusca; Shipworm.
