Introduction to Boring Bivalves
Families Teredinidae, Pholadidae and the veneroid Petricola pholadiformis Lamarck, 1818
Many bivalves have adapted to boring into hard substrates such as wood, peat, consolidated clay, limestone, etc. The highest number of boring bivalves are found in tropical waters where coral reefs make a suitable substrate as do mangrove trees. A few species occur in temperate regions, but wood-boring species are restricted to driftwood and man-made structures as vessels and ports. Rock-boring species also have restricted possibilities to find suitable habitats. In the boreal North Atlantic wood-boring species are found in the family Teredinidae and one or a few species in the family Pholadidae. Most of the rock-borers belong to the family Pholadidae, the only exception being the alien Petricola pholadiformis. In warmer waters there are also rock-boring bivalves in the family Mytilidae, e.g. the now rare Lithophaga lithophaga from Mediterranean waters.
The shells of boring bivalves are highly modified for boring into hard substrates. Under high magnification the outside often looks like a rasp, and this is also how they function. The shells may also be equipped with specialized structures for attaching muscles, and the hinge is usually reduced to permit rotating movements of the valves. In Teredinidae the valves are reduced in size, covering only the anterior part of the body, and even leaving a large anterior gap for the foot. In the Pholadidae the shell also gapes at both ends, though most of the soft parts can be withdrawn into the shell. Many Pholadidae have accessory calcareous plates to cover the exposed parts.
Teredinidae in Nordic waters are always associated with man-made structures, e.g. wooden vessels, piers, etc., and therefore all species should be considered cryptogenic or alien. Teredo navalis furthermore broods its larvae and has a very short pelagic stage, so it could not have dispersed far by its own means.
More information on boring bivalves included in this identification keys: