Pholadidae are the
piddock clams, which bore into shale, clay, or firm mud. Much
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
end of the shell, though narrower than the anterior,
does not taper to a point like a bird's beak. In small
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
posterior end has a flaring
which is heavy and chitinous or leathery but not lined by calcareous
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
end has a thick, shieldlike plate (protoplax)
dorsal to the anterior rasping portion (photo).
(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,
at the anterior end). shell white, with brown
Shell length to 7.6 cm.
Penitella penita (Conrad, 1837)
Common name(s): Common piddock,
Flat-top piddock, Flat-tipped
piddock, Piddock clam, Rock clam
|A Penitella penita shell from
Toleak Point, WA. No animal
is present. The right valve is shown. Anterior is
to the right.
plates flare on the left. The protoplax
top right of the shell.
|(Photo by: Dave Cowles,
How to Distinguish from
turnerae has no
Penitella richardsoni is more irregular in
pilsbryi has spinelike teeth on the shell.
Sound, Alaska to Baja California, Mexico
Mid intertidal to 22
Usually boring into shale,
almost entirely on the open coast (photo).
Can bore into concrete.
History: This is the
most common species of piddock clam here in the Northwest.
and length of the shell varies with the hardness of the rock it is
into. It is a significant agent in the erosion of coastal
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
The ridges on the shell seem to be produced between bouts of rapid
Up to 22 ridges per year may be laid down. The animal grows
digs deeper, so the deep portions of the hole are of greater diameter
the surface portions and the animal cannot back out of the
Since many clams may bore into the same rock, the clams often become
The clams seem to be able to sense when their boring is approaching the
burrow of another clam. When they approach another burrow
often leaving only a millimiter or so of rock between the
If the rock becomes so crowded that there is nowhere to turn the clam
growing and remains stunted and sexually immature. The empty
of piddock clams may contain small porcelain crabs, the flatworm Notoplana
inquieta, as well as other crabs, worms, and
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.
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
years unless the rock is broken away. They can live for
rock that has been buried by sand. In Oregon, spawning occurs
and Fairbanks, 1966
and Carlton, 1975
and Snook, 1955 (As Pholadidea penita)
and McConnaughey, 1985
et al, 1980
and O'Clair, 1998
et al., 1985
General Notes and
Observations: Locations, abundances,
Piddock clams are usually found embedded in their burrows in rock,
as is this individual. Photo by Dave Cowles
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.
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
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
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
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 Beach on the Washington open coast. The sphonoplax
piddocks can be seen in the lower photo. Photos July 2008 by
Authors and Editors of Page:
Dave Cowles (2006): Created original page