To the FWGNA group,
The message below was posted to the MOLLUSCA listserver late last
month. In it, our colleague Jim Carlton of Williams College
reports the discovery of an exotic Assiminea
newly invading brackish waters around Coos Bay, Oregon. I'm not
entirely sure that this critter qualifies as an element of the
freshwater fauna. Jim tells me that he does not have any good
data on the range of salinity tolerated. Nevertheless, I
thought I'd pass his message along, in the spirit of the "broad tent."
We'll keep in touch,
Rob
FROM: Dr. James T. Carlton
DATE: August 31, 2007
RE: Major new estuarine snail invasion in the Pacific northwest
Last month a major new snail invasion was detected in the Pacific
Northwest. Dr. Robert Hershler of the Smithsonian Institution is
working on the identification of this snail; at the moment we are
calling it "Assiminea" sp.
(The genus name is uncertain, and thus the quotation marks, and the
species remains unidentified at this time, but Dr. Hershler is pursuing
the identification). It very likely hails from Asia (Japan or
China, perhaps) where very similar looking species occur. It was
likely introduced with shipping.
This is an estuarine, brackish-water snail found in Coos Bay, Oregon.
It may be more widespread in bays and estuaries along the coast. It
should not be confused with the New Zealand Mud Snail, Potamopyrgus antipodarum.
In an upper estuary slough of Coos Bay, in waters of about 10 ppt, it
was found on July 5, 2007, where I found it to occur by the thousands per square meter on emergent mud banks; lower down on the same shore, on submerged stones, it co-occurs with 
Potamopyrgus.
In the middle Coos Bay estuary, in higher salinity waters (we do not
yet have a full range of salinities over which this species can exist,
but these middle bay sites are in the high 20's at the least), it is
found both on supralittoral emergent shores, and in salt marshes, where
it co-occurs with the native snails Assiminea californica and Littorina subrotundata, and the non-native snail Myosotella myosotis ( = Ovatella myosotis; = Phytia setifer).
On July 11, John Chapman and I revisited the upper estuary site, and
obtained more specimens that have been sent to Dr. Hershler, and to Dr.
James McLean of the LACM.
This is a small snail, about 5 mm in height, with bulbous whorls, a
broad inner aperture lip, light yellow bands at the top and bottom of
the body whorl in younger shells, and with a highly eroded shell as the
animal matures. The photo at left is provided through the
courtesy of Jim McLean.
The native Assiminea californica
is smaller, narrower (less bulbous whorls), has a narrow inner lip,
does not erode significantly as an adult, does not typically penetrate
into very low salinity sloughs (such as where one finds Potamopyrgus), and does not occur in large, dense concentrations on supralittoral mud banks.
At these densities, we should expect some of the impacts associated with the NZMS. We do not know the exact diet of "Assiminea"
sp., but it likely is a deposit-detrital feeder and
microherbivore. It should be expected to compete with other snail
populations at these densities, and perhaps displace or replace some
native and introduced species.
Please feel free to distribute this email. I suggest a widespread alert along the Pacific coat.
-- Jim Carlton
_____________________________________________________________
Dr. James T. Carlton
Professor of Marine Sciences, Williams College, Williamstown, Massachusetts
E: jcarlton@williams.edu
T: 860-572-5359
F: 860-572-5329
http://www.williams.edu/williamsmystic/
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