This small nestling clam
has two valves
similar size and shape, though the valves
are usually deformed (see photo above) from growing into the contours
the hole the clam is nestling in (photo).
The shell is thick, the outside of the shell is usually a chalky white,
and it may have patches of brown periostracum
still adhering to it in places. Fine radial lines may or may
be visible on the outside of the shell (mainly seen in juveniles), and
concentric lines are usually present. The shell is at least
wide as long., elongated (sometimes only slightly--note the differences
between the two shells above), and usually slightly narrower posteriorly
muscle scars are present and the umbo
is near the middle of the dorsal
margin of the shell (photo).
has no radial
The hinge has no chondrophore
but has 3 cardinal
teeth in one valve and 2 in the other (photo).
is external. The interior of the shell has a continuous pallial
line and a pallial
of living individuals are partly fused and have bright purple
Length to 5 cm.
Petricola carditoides (Conrad, 1837)
Common name(s): Rock dweller, Heart
rock dweller, Hearty
petricolid, Nestling clam
|Petricola carditoides shells found
nestling in piddock clam
holes at beach #4, Olympic Peninsula
|(Photo by: Dave Cowles,
How to Distinguish from
the piddock clams, are also found in holes bored into rock but their
are more elongated and are very different on the anterior end than on
posterior end. Petricola pholadiformis is
an Atlantic species
that was introduced to quiet bays (especially Willipa Bay) along with
It bores in clay in a few places. It actually bores into the
or into waterlogged wood rather than nestling in already-created holes,
and its shell resembles that of a piddock clam. Its shell is
elongated and it has heavy radial sculpture. Petricola
is found in southern California. Its shell is thinner and
with fine radial lines and purple-brown stains near the
arctica, also a nestler, has a thinner shell, more persistent
and bright red siphon tips.
to Baja California
Low intertidal to 50
Nestles in cavities in rock,
especially those bored by piddock clams
History: Young individuals
attach to the rock by byssal threads, but the adults do not.
clam secretes a fair amount of mucus.
and Carlton, 1975
and Snook, 1955
et al., 1980
General Notes and
Observations: Locations, abundances,
This species nestles in the holes left in shale by piddock
Several individuals are visible in this photo.
In this inside view, the anterior
end of the shell is up and posterior
The well-defined pallial
line is visible in both shells, as well as the deep pallial
sinus near the posterior
Both the anterior and posterior adductor
muscle scars are visible near the hinge, with the posterior
A good deal of brown, adhering periostracum
can still be seen near the hinge of this individual.
The hinge has two cardinal hinge teeth on one side and three on the
This broken piece of shale shows the clam's nestling position in an
abandoned pholadid clam burrow.
Photo at Shi Shi beach by Dave Cowles, August 2007
An individual 1.9 cm long found at Hole in the Wall near Rialto Beach, July 2020. Photo by Dave Cowles
Authors and Editors of Page:
Dave Cowles (2006): Created original page