Petricola carditoides (Conrad 1837)

Biology/Natural History: Young individuals attach to the rock by byssal threads, but the adults do not. This clam secretes a fair amount of mucus.

Nestles in cavities in rock, especially those bored by piddock clams

Geographical Range: Sitka, Alaska to Baja California

Depth Range: Low intertidal to 50 m

This small nestling clam has two valves of similar size and shape, though the valves are usually deformed from growing into the contours of the hole the clam is nestling in. The shell is thick, the outside of the shell is usually a chalky white, and it may have patches of brown periostracum still adhering to it in places. Fine radial lines may or may not be visible on the outside of the shell, and concentric lines are usually present. The shell is at least 1/4 as wide as long., elongated, and usually slightly narrower posteriorly than anteriorly. Two adductor muscle scars are present and the umbo is near the middle of the dorsal margin of the shell. The shell has no radial ribs. The hinge has no chondrophore but has 3 cardinal teeth in one valve and 2 in the other. The ligament is external. The interior of the shell has a continuous pallial line and a pallial sinus. The siphons of living individuals are partly fused and have bright purple tips. Length to 5 cm.

How to Distinguish from Similar Species: Family Pholadidae, the piddock clams, are also found in holes bored into rock but their valves are more elongated and are very different on the anterior end than on the posterior end. Petricola pholadiformis is an Atlantic species that was introduced to quiet bays along with oysters. It bores in clay in a few places. It actually bores into the clay or into waterlogged wood rather than nestling in already-created holes, and its shell resembles that of a piddock clam. Its shell is more elongated and it has heavy radial sculpture. Petricola californiensis is found in southern California. Its shell is thinner and more elongate, with fine radial lines and purple-brown stains near the hinge. Hiatella arctica, also a nestler, has a thinner shell, more persistent periostracum, and bright red siphon tips.