Penitella penita (Conrad, 1837)

Common name(s): Common piddock, Flat-top piddock, Flat-tipped piddock, Piddock clam, Rock clam

Synonyms:  Pholadidea penita
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Myoida
    Suborder Pholadina
     Family Pholadidae
A Penitella penita shell from Toleak Point, WA.  No animal is present.  The right valve is shown.  Anterior is to the right.  The siphonoplax plates flare on the left.  The protoplax covers the top right of the shell.
(Photo by: Dave Cowles, Sept. 2006
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.  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.

How to Distinguish from Similar Species: Penitella turnerae has no flaring siphonoplax. Penitella richardsoni is more irregular in outline.  Zirfaea pilsbryi has spinelike teeth on the shell.

Geographical Range:  Prince William Sound, Alaska to Baja California, Mexico

Depth Range:  Mid intertidal to 22 m

Habitat:  Usually boring into shale, almost entirely on the open coast (photo).  Can bore into concrete.

Biology/Natural History:  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.

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Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Carefoot, 1977
  Harbo, 1997
  Johnson and Snook, 1955 (As Pholadidea penita)
  Kozloff, 1993
  McConaughey and McConnaughey, 1985
  Morris, 1966
  Morris et al, 1980
  Niesen, 1994
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:

Web sites:

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

Piddock clams are usually found embedded in their burrows in rock, as is this individual. Photo by Dave Cowles

In shale
This view of broken shale at Shi Shi beach shows the holes many Penitella penita (piddocks) have bored into it, plus the empty valves of one individual which died inside the rock. Note how the anterior end of the piddock is at the large, inner end of the hole, in drilling position, while the siphons extend toward the opening. This piddock is 5 cm long. Photo by Dave Cowles, June 2023.

This view shows the piddock above after opening it. The structure projecting from the hing area is the myophore, to which muscles attach.Length = 5 cm. Photo by Dave Cowles

The anterior, boring end of the piddock clam (to the right) is sculptured dramatically differently from the posterior end (left), from which it is separated by a modest groove. Photo by Dave Cowles

Anterior end
Although the two valves of this dead individual have now separated, in this view of the anterior, grinding end one can see that the gape between the valves at the anterior end  of this mature individual had been sealed by a calcareous callum. The thick, shieldlike plate at the anterodorsal end (to the left in this photo)  is called a protoplax. Photo by Dave Cowles

A view of the inside of the left valve.  Note the projecting myophore near the hinge.  There are no hinge teeth.  The siphonoplax, which would normally be attached to the posterior end of the shell at the left, is detached from this specimen.

The chitinous siphonoplax flares out from the posterior end.

The flaring siphonoplax (on the posterior end, at the left; the mesoplax, and the protoplax (right) can be clearly seen in this dorsal view.

This posterior view clearly shows the flaring siphonoplax.

This ventral view shows that the anterior gape of the shell (to the right) has been nearly completely sealed by the callum.

Piddocks can be very abundant and can cause substantial erosion to coastal rocks.  These piddock-infested rocks are at Shi Shi Beach on the Washington open coast.  The sphonoplax of several piddocks can be seen in the lower photo.  Photos July 2008 by Dave Cowles.


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