Parapholas californica Conrad, 1848

Common name(s): California Piddock, Scale-sided piddock

Synonyms: x
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Myoida
    Suborder Pholadina
     Family Pholadidae
Shell of an immature Parapholas californica found in broken shale near Santa Barbara, CA.  The anterior, boring end is downward in this view.  Note the large myophore inside the shell near the hinge.
(Photo by: Dave Cowles, January 1996)
Description:  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 (photo) and divided by an oblique furrow (which starts near the umbo) into three regions (photo)--an anterior rough, rasping region (photophoto), a middle smooth region with concentric growth lines, and a posterior region with periostracum forming distinctive, overlapping chitinous plates (photo).  The anterior region has a protoplax, and the large anterior-ventral gape of the shell (photo) becomes overgrown with a calcareous callum in fully grown individuals.  The posterior dorsal margin of the shell has a metaplax (photo).  There is no hinge ligament and the hinge, on which the shell rocks while boring, is rounded and has few teeth (photo).  There is a prominent myophore (apophysis) near the hinge of both valves (photo).  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 (photos).  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 (usually not greater than 10 cm).

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

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

Depth Range:  Subtidal to low intertidal

Habitat:  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.

Biology/Natural History:  To bore their hole, pholads extend the foot from the gape in the anterior end of the shell (see photo) 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 (rocking on the hinge) 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.

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Dichotomous Keys:
  Smith and Carlton, 1975

General References:
  Johnson and Snook, 1955
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Morris, 1966
  Morris et al., 1980
  Ricketts et al., 1985

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

The anterior end of the shell gapes widely to give room for the foot to extend.  This view shows the shell of an individual in place in a broken piece of shale.  I turned the broken rock over so the interior end of the burrow is exposed.  A burrow of a neighboring individual can be seen below, and the shell of another neighbor appears to the right.  Note the large myophores (apophyses), to which the foot anchors, inside the shell.  The burrow becomes more narrow toward the entrance, which cannot be seen here but is down and to the right.  If this individual were fully mature a calcareous callum would have grown over the large anterior gape which is visible here.

Here is another view of the inside of the shell, showing the rounded hinge, the large myophore (apophysis), and the protoplax anterior of the hinge.

In this view, the rock has broken off at the very interior end of the hole so the position of the shell while boring can be seen.  The anterior teeth are used for boring.  The sinuous flap on the dorsal side of the shell is the protoplax.

Here is another view of the anterior end, with the protoplax downward.  Scale is millimeters (centimeters numbered).

This lateral and dorsal view of the right valve shows how the anterior (left in the photo) end is inflated relative to the posterior (right in the photo) end.  The protoplax can be seen along the dorsal (lower left) edge of the anterior end, and the metaplax can be seen along the dorsal (lower right) edge posterior to the umbo.

The posterior end of the shell has the periostracum formed into distinctive, overlapping chitinous plates.  Note that the posterior end is narrower than the anterior.

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This species lines its burrow with a "chimney" of rock particles cemented with calcium carbonate.  Here is a view of part of a chimney, taken from inside the burrow.  The burrow entrance is out of sight and downward. The inside of the chimney is rough.  Opening of the burrow is to the left. The outside of the chimney, where it attaches to the burrow, is very smooth.  Opening of the burrow is to the left.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page