Entodesma navicula A. Adams and Reeve, 1850

Common name(s): Northwest ugly clam, Rock entodesma, Wrinkled rock clam, Rock-dwelling clam

Synonyms:  Agriodesma saxicola, Agriodesma saxicolum, Entodesma saxicola, Lyonsia saxicola
Phylum Mollusca
 Class Bivalvia
  Subclass Anomalodesmata
   Order Pholadomyoida
    Family Lyonsiidae
Entodesma navicula from Padilla Bay, collected near shore on March Point by Jon Mayberry
(Photo by: Dave Cowles, July 2005)
Description:  This clam has a deformed-appearing shell, longer than high (at least in adults), with no hinge teeth nor chondrophore. Johnson and Snook say that the hinge is covered internally by "an oblong white ossicle".   The shell has no radial ribs but is often covered with a golden-brown or greenish periostracum which, in this specimen, appears to be loose and baggy.  If the periostracum is worn off, the outside of the valves is chalky.  The valves tend to crack when dried.  The inside of the valves is at least slightly pearly, and is glossy and iridescent.  It has a continuous pallial line and pallial sinus.  The valves gape on the posterior end (photo), and the periostracum often extends past the shell at this point. Siphons are orange (photo, but see this photo).  Length may exceed 10 cm.

How to Distinguish from Similar Species:  Mytilimeria nuttalli has valves with a nearly circular outline or higher than long. Entodesma pictum has valves which do not gape, and is only up to 2.5 cm long.

Geographical Range:  Aleutian Islands, Alaska to Baja CA

Depth Range:  Low intertidal to 20 m

Habitat:  Holes and crevices in rocks; Holdfasts of kelp and giant barnacles at the lowest low tide

Biology/Natural History:  Simultaneous hermaphrodites, though fertilization is external.  Animals alternately release sperm and eggs.  Shell often grows to conform to the hole or crevice it is in; or if not constrained, the anterior end grows narrow and short and the posterior end is swollen but abruptly truncate and gaping.  Can attach by byssal threads to its crevice.  Predators include Pycnopodia helianthoides.  May contain a symbiotic pea crab Pinnixa faba.



 
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References:
Dichotomous Keys:

Flora and Fairbanks (1966) [as Entodesma saxicola]
Kozloff (1987, 1996) [as Agriodesma saxicola]
Smith and Carlton, 1975 [as Entodesma saxicola]
 
 

General References:
Harbo, 1997
Johnson and Snook, 1955 [as Entodesma saxicola]
Morris, 1966 [as Entodesma saxicola]
Morris et al., 1980 [as Entodesma saxicola]
Sept, 1999

Scientific Articles:



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

I have not often seen this clam.  It was dug up by a student, Jon Mayberry, near shore (approx. +1 tide level) at March Point in Padilla Bay, July 2005



The gape in the shell at the posterior end can be clearly seen here.



View of the right valve.  Note the loose periostracum.


The siphons are said to be orange, though in this view in our tank they look more brownish-orange or cream colored (compare below).  Photo by Dave Cowles, July 2005

Orange siphons

This individual has siphons which are more orange-colored.  Note also the foot projecting from the anteroventral side (top right), which appears to be specialized for holding onto rocks rather than the standard hatchet-shape seen in species which dig in soft substrates.  Photo by Dave Cowles, July 2012
 


Smoother
This individual, with a shell 4 cm long, is a bit smoother. Note that the periostracum hangs off the edges of the shell-as much as 1 cm on the posterior end (to left). Photo by Dave Cowles, August 2017


Periostracum
This close view of the same individual shows the abundant periostracum and the fact that it overhangs the margin of the shell by quite a lot.  Photo by Dave Cowles, August 2017


Inside
This view inside shows the pearly interior and the 'oblong white ossicle' that covers the hinge internally. Photo by Dave Cowles, August 2017



Authors and Editors of Page:
Dave Cowles (2005):  Created original page