Nuttalia obscurata (Reeve, 1857)
Common name(s): Purple mahogany clam, Purple varnish clam, Dark mahogany clam, Varnish clam, Savory clam
Family Psammobiidae (Garidae)
|Nuttalia obscurata shell found at Sucia Island, San Juan Islands,, WA|
|(Photo by: Dave Cowles, August 2006)|
How to Distinguish from Similar Species:Nuttallia nuttallii lives farther south, in California and Baja California. The oval shape with extensive shiny periostracum and purple interior are unique to this clam in our area. Venerupis philippinarum, another introduced clam, may have a purple stain inside but the outer shell is sculptured with radial ridges and concentric growth lines, without shiny brown periostracum.
Geographical Range: Japan, British Columbia, San Juan Islands Area (Introduced from Japan)
Depth Range: High to mid intertidal
Habitat: In sand or sand/gravel, often in areas of freshwater seepage.
Biology/Natural History: Clams of
this genus live deeply buried (to 20 cm) in the sediment, resting on their
right side with their long siphons extending to the surface. They
are non-selective suspension feeders.
This species was recently (1980's or 1990's) introduced from Japan and is rapidly spreading through the area. This species is often imported for food and sold live in grocery stores (photo), often as "savory clams". It is said to be becoming quite common in some of the San Juan islands, such as Sucia island where this one was found. Pea crabs are often found inside.
Dudas et al. (2005) found that the common local cancer crabs Metacarcinus magister (Dungeness crab) and Cancer productus (red rock crab) preferred the thin-shelled introduced varnish clam Nuttallia obscurata to the thicker-shelled clams Leukoma staminea and Venerupis philippinarum if access to all was equally easy. However, Nuttallia obscurata typically lives deeper in the sediment than do Leukoma staminea or Venerupis philippinarum. If they had to dig for them, Metacarcinus magister still ate more Nuttallia obscurata than it did of the other clam species, but C. productus' preference switched to Leukoma staminea and Venerupis philippinarum.
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Morris et al, 1980 (on N. nuttallii)
Dudas, Sarah E., Iain J. McGaw, and John F. Dower, 2005. Selective crab predation on native and introduced bivalves in British Columbia. Journal of Experimental Marine Biology and Ecology 325:1 pp 8-17
Claudia Mills' web site on Nuttallia obscurata: Claudia Mills is based at Friday Harbor Labs and has been tracking the spread of this species in our area.
August 2006: A shell from this species was found on the beach in Bowman's Bay, within 1 km of the Rosario Marine Station.
2006-2007: A study by Lindsey Eggers around Whidbey Island revealed that this species is abundant at Long Point (east of Coupeville) and is present at Freeland beach and at Double Bluff.
Summer 2007: Another study by Lindsey Eggers found that very few of the Nuttallia obscurata at Long Point or Freeland contained any Pinnotherid crabs (0 crabs/48 clams at Long Point, 2 crabs/57 clams at Freeland). At Double Bluff, however, 51 Pinnotherid (pea) crabs were found in 51 Nuttallia obscurata clams. This is the highest rate of infestation by pea crabs reported for any species except for the pea crabs' definitive host, Tresus capax. The Pinnotherid crabs were not identified to species but most were Pinnixia, likely P. faba and P. littoralis. At least one crab appeared to be Fabia subquadrata.
The hinge appears to have only one cardinal tooth per side but keys say other members of the family such as Gari californica have 2 cardinal teeth per side.
The interior of the shell is purple. Anterior is up in this photo. The left valve is to the right and the right valve is to the left.
Because the clam has long siphons there is a deep pallial sinus, as can be seen at the bottom right of the photo.
These clams were purchased fall 2006 from the local Walla Walla Safeway store. The upper clam is still alive.
(the lower one was eaten by a red octopus in the lab)
Note the prominent external hinge ligament on the live individual.