Parapholas californica (Conrad 1837)

How to Distinguish from Similar Species: The distinctive, overlapping chitinous plates on the posterior end are one unique characteristic of this species.

Usually in shale, sometimes in hard mud or hard silicious chert. In the intertidal zone it is mainly found in large rocks at low tide level.

Geographical Range: Oregon to Baja California, Mexico; Common from Bodega Bay south.

Depth Range: Subtidal to low intertidal

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. This species is moderately thick-shelled, with an inflated anterior end and divided by an oblique furrow into three regions--an anterior rough, rasping region, a middle smooth region with concentric growth lines, and a posterior region with periostracum forming distinctive, overlapping chitinous plates. The anterior region has a protoplax, and the large anterior-ventral gape of the shell becomes overgrown with a calcareous callum in fully grown individuals. The posterior dorsal margin of the shell has a metaplax. There is no hinge ligament and the hinge, on which the shell rocks while boring, is rounded and has few teeth. There is a prominent myophore near the hinge of both valves. The ventral edge has a hypoplax. No siphonoplax is present but the animal constructs a "chimney" of tiny bits of rock cemented together with calcium carbonate. These chimneys line the outer walls of the burrow and may extend out 2-5 cm into the water column. The shell is dirty white on outside, white inside, shell length to 15 cm.

Biology/Natural History: To bore their hole, pholads extend the foot from the gape in the anterior end of the shell and apply it to the base of the burrow as a sucking disk. The anterior part of the anterior adductor, as well as the ventral adductor muscles contract, squeezing the valves into a narrow profile. The foot contracts, pulling the anterior end of the shell down to the bottom of the burrow. The posterior adductor and the posterior part of the anterior adductor muscles contract, pulling the anterior ends of the valves apart from one another and pressing them against the walls and base of the burrow. The shell rocks backward and upward, scraping the burrow with the rough anterior portion of the shell. The valves rock back and forth with the hinge as a fulcrum. Unlike most other clams, piddocks have no hinge ligament. After the clam is fully grown, the foot degenerates and the anterior-ventral gape between the valves, through which the foot formerly projected, is covered over by a callum, or calcareous plate. The united, flat-topped, white to reddish-brown siphons of this species are frequently seen by divers. The inhalant siphon is of larger diameter than the exhalant. They don't burrow more than about 30 cm into the rock but are important agents for rock erosion.