Family 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. This species is
thick-shelled, with an inflated anterior
end (photo) and divided
by an oblique furrow
(which starts near the umbo)
into three regions (photo)--an
rough, rasping region (photophoto),
a middle smooth region with concentric growth lines, and a posterior
region with periostracum
forming distinctive, overlapping chitinous plates (photo).
region has a protoplax,
and the large anterior-ventral
gape of the shell (photo)
with a calcareous callum
in fully grown individuals. The posterior
margin of the shell has a metaplax
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
near the hinge
both valves (photo).
edge has a hypoplax.
is present but the animal constructs a "chimney" of tiny bits of rock
together with calcium carbonate (photos).
These chimneys line the outer walls of the burrow and may extend out
cm into the water column. The shell is dirty white
white inside, shell length to 15 cm (usually not greater than 10 cm).
Parapholas californica Conrad, 1848
Common name(s): California Piddock,
|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,
How to Distinguish from
The distinctive, overlapping chitinous plates on the posterior
end are one unique characteristic of this species.
Oregon to Baja
California, Mexico; Common from Bodega Bay south.
Subtidal to low intertidal
Usually in shale, sometimes
in hard mud or hard silicious chert. In the intertidal zone
mainly found in large rocks at low tide level.
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
pulling the anterior
end of the shell down to the bottom of the burrow. The
adductor and the posterior part of the anterior adductor muscles
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.
rocks backward and upward, scraping the burrow with the rough anterior
portion of the shell. The valves rock back and forth with the
as a fulcrum. Unlike most other clams, piddocks have no hinge
ligament. After the clam is fully grown, the foot
and the anterior-ventral
gape between the valves, through which the foot formerly
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
about 30 cm into the rock but are important agents for rock erosion.
and Carlton, 1975
and Snook, 1955
and McConnaughey, 1985
et al., 1980
et al., 1985
General Notes and
Observations: Locations, abundances,
end of the shell gapes widely to give room for the foot to
This view shows the shell of an individual in place in a broken piece
shale. I turned the broken rock over so the interior end of
is exposed. A burrow of a neighboring individual can be seen
and the shell of another neighbor appears to the right. Note
to which the foot anchors, inside the shell. The burrow
narrow toward the entrance, which cannot be seen here but is down and
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
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.
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.
end of the shell has the periostracum
formed into distinctive, overlapping chitinous plates. Note
end is narrower than the anterior.
|This species lines its burrow with a "chimney" of rock
with calcium carbonate. Here is a view of part of a chimney,
from inside the burrow. The burrow entrance is out of sight
||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