Rough Piddock

Common in hard mud or clay, shale, or sandstone; in bays and estuaries; occasionally along the open coast. Occasionally bores into wood buried in the mud.

Geographical Range: Chukchi Sea, Siberia to Baja California, Mexico

Depth Range: Low intertidal to 126 m; mostly below -1.0 feet tide level.

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 rasping portion comprises about half the shelland is separated from the posterior nonrasping portion by a oblique groove. There is no protoplax but there is a mesoplax. Both valves have a myophore. The posterior shell tapers a bit but not to such an extent that it resembles a bird's beak. The white valves gape at the posterior end for the thick, united siphons. The siphons cannot be withdrawn into the shell. The valves have coarse concentric ridges; on the anterior end these ridges often have filelike projecting spines or teeth which are used for digging. The anterior end of the valves also often has projecting spines or teeth on the valve margin, and also has a gape through which the foot protrudes. This gape is not closed by a callum even in older individuals. the periostracum is brown and may be seen extending onto the base of the siphons. The siphons have small chitinous spots on the surface. The interior of the valves is chalky with well-defined muscle scars. Length up to 15 cm. This is one of our largest boring clams.

How to Distinguish from Similar Species: Barnea subtruncata does not have the anterior and posterior portions of the valves separated by a groove. Netastoma rostrata has a calcified siphonoplax and lacks myophores, plus the posterior end of the shell tapers to a point like a bird's beak. The Penitella species such as P. turnerae have the anterior rasping portion comprising less than half the length of the valve, have a protoplax, and the anterior gape becomes sealed with a callum in mature individuals. These other species also do not have teeth along the edge of the shell and most of them can withdraw their siphons into the shell.

Biology/Natural History: Unlike most piddock clams, this species can live outside its burrow for long periods of time. Can burrow to 50 cm depth. Sometimes burrows in clay which is covered by a layer of sand, in which case it must extend its siphons a long way. It can extend its siphons up through as much as 48 cm of clay plus 30 cm of sand. When boring, it holds onto the substrate with its foot and rocks its shell up and down against the burrow walls by alternately contracting its anterior and posterior adductor muscles. After each stroke the animal rotates about 12 degrees. It takes about 30 rocking motions to turn in a complete circle, which takes about 70 minutes. After about one complete turn, the direction of rotation is reversed. Periodic body contractions create a current which shoots debris up and out the incurrent siphon. Lives about 8 years, and never completely ceases digging its burrow.