Petricola carditoides (Conrad, 1837)

Common name(s): Rock dweller, Heart rock dweller, Hearty petricolid, Nestling clam

Synonyms:  Rupellaria carditoides
Phylum Mollusca
 Class Bivalvia
  Subclass Heterodonta
   Order Veneroida
    Family Petricolidae
Petricola carditoides shells found nestling in piddock clam holes at beach #4, Olympic Peninsula
(Photo by: Dave Cowles, July 2006)
Description:  This small nestling clam has two valves of similar size and shape, though the valves are usually deformed (see photo above) from growing into the contours of 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 not be visible on the outside of the shell (mainly seen in juveniles), and concentric lines are usually present.  The shell is at least 1/4 as wide as long., elongated (sometimes only slightly--note the differences between the two shells above), and usually slightly narrower posteriorly than anteriorly.   Two adductor muscle scars are present and the umbo is near the middle of the dorsal margin of the shell (photo).   The shell has no radial ribs.  The hinge has no chondrophore but has 3 cardinal teeth in one valve and 2 in the other (photo).  The ligament is external.  The interior of the shell has a continuous pallial line and a pallial sinus (photo).  The siphons of living individuals are partly fused and have bright purple tips.  Length to 5 cm.

How to Distinguish from Similar Species: Family Pholadidae, the piddock clams, are also found in holes bored into rock but their valves are more elongated and are very different on the anterior end than on the posterior end.  Petricola pholadiformis is an Atlantic species that was introduced to quiet bays (especially Willipa Bay) along with oysters.  It bores in clay in a few places.  It actually bores into the clay or into waterlogged wood rather than nestling in already-created holes, and its shell resembles that of a piddock clam.  Its shell is more elongated and it has heavy radial sculpture.  Petricola californiensis is found in southern California.  Its shell is thinner and more elongate, with fine radial lines and purple-brown stains near the hinge.  Hiatella arctica, also a nestler, has a thinner shell, more persistent periostracum, and bright red siphon tips.

Geographical Range:   Sitka, Alaska to Baja California

Depth Range:  Low intertidal to 50 m

Habitat:  Nestles in cavities in rock, especially those bored by piddock clams

Biology/Natural History:  Young individuals attach to the rock by byssal threads, but the adults do not.  This clam secretes a fair amount of mucus.

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Dichotomous Keys:
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Harbo, 1997
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris, 1966
  Morris et al., 1980

Scientific Articles:

Web sites:

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

This species nestles in the holes left in shale by piddock clams.  Several individuals are visible in this photo.

In this inside view, the anterior end of the shell is up and posterior is down.
The well-defined pallial line is visible in both shells, as well as the deep pallial sinus near the posterior end.
Both the anterior and posterior adductor muscle scars are visible near the hinge, with the posterior scar most evident.
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 other.

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
 P carditoides
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