Pholadoidea

Pholadoidea is a superfamily of bivalve molluscs, commonly known as piddocks or rock borers. These marine clams are characterized by their elongated, thin, and often gaping shells. The shells are typically white or pale, and can be smooth or have concentric ridges. Unlike many other bivalves that secrete a nacreous layer, pholadoideans have calcareous shells that are less ornamented. Their foot is small and reduced, reflecting their sedentary lifestyle.

Pholadoideans are infaunal, meaning they live buried in the substrate. Their remarkable adaptation is the ability to bore into solid rock, wood, and even other hard materials. They achieve this by rotating their shells, which are equipped with abrasive ridges, against the surface. The boring process is facilitated by secretions that soften the material. Once a cavity is created, the piddock lodges itself within, using its siphons to extend to the surface for filter feeding. The siphons are often very long and can be fused together.

This superfamily is found in marine and estuarine environments worldwide, inhabiting a range of substrates from soft mudstone to hard granite. They play a role in the erosion of coastal formations and can be considered bioeroders. Reproduction is typically sexual, with external fertilization. The larval stages are planktonic before settling and commencing their boring activities. Some species are commercially harvested, though their association with hard substrates makes collection challenging.