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.
Family Pholadidae are the piddock clams, which bore into shale, clay, or firm mud. Much of the anterior portion of the shell is roughened so that the animal can rasp a hole in the rock or clay much like an augur bit. The anterior portion of the shell, while higher and more globose than the posterior portion, is not nearly globular. In this species, the anterior portion occupies less than half the length of the valve and is separated from the posterior in a well-defined manner by a groove which runs from the dorsal to the ventral side. It has a myophore (apophysis) in both valves (photo). The posterior end of the shell, though narrower than the anterior, does not taper to a point like a bird's beak. In small individuals there is a gape between the valves at the anterior end for the foot to protrude, but in a full-grown specimen such as the individual above the gape has been covered over by a calcareous callum (photo). The posterior end has a flaring siphonoplax, which is heavy and chitinous or leathery but not lined by calcareous granules (photo). The flaring siphonoplax is what gives the "flat-top piddock" name (photo). The siphons are white, smooth, and fused together. The mesoplax is pointed posteriorly, truncate anteriorly, and has lateral wings. The anterior end has a thick, shieldlike plate (protoplax) dorsal to the anterior rasping portion (photo). The umbonal reflections (the calcified plates between the protoplax and the rasping portion of each valve) is tightly applied to the shell (as opposed to being free from the shell for part of their length, especially at the anterior end). shell white, with brown periostracum. Shell length to 7.6 cm.
Prince William Sound, Alaska to Baja California, Mexico