Biology/Natural History: This is the most common species of piddock clam here in the Northwest. The shape and length of the shell varies with the hardness of the rock it is boring into. It is a significant agent in the erosion of coastal shale. The clams bore 4-5 mm/year, depending on the hardness of the rock, and may burrow to depths of 15 cm. Drilling seems to be entirely mechanical. The ridges on the shell seem to be produced between bouts of rapid drilling. Up to 22 ridges per year may be laid down. The animal grows as it digs deeper, so the deep portions of the hole are of greater diameter than the surface portions and the animal cannot back out of the hole. Since many clams may bore into the same rock, the clams often become crowded. The clams seem to be able to sense when their boring is approaching the burrow of another clam. When they approach another burrow they turn, often leaving only a millimiter or so of rock between the burrows. If the rock becomes so crowded that there is nowhere to turn the clam stops growing and remains stunted and sexually immature. The empty holes of piddock clams may contain small porcelain crabs, the flatworm Notoplana inquieta, as well as other crabs, worms, and sipunculids. Predators include the leafy hornmouth Ceratostoma foliatum. Sexual maturity is reached when the animal stops drilling and a callum covers the anterior gape in the shell. In soft shale the animal may mature in 3 years, while in harder rock it may not mature until 20 years or later. Mature animals may live for many years unless the rock is broken away. They can live for months in rock that has been buried by sand. In Oregon, spawning occurs in July.