Mark is correct, ballast water is the culprit. There is no direct shipping
traffic from the Caspian Sea to the Great Lakes, but there is from the Black
Sea. Originally people argued for the Caspian Sea as the source of
introduction because a striped pattern found in Great Lakes populations of
Dreissena polymorpha is also found in Caspian Sea populations. However, the
same color form is also found in the estuaries where the Dnieper and Bug
Rivers enter the Black Sea, as I discovered during field work in 1993. This
is also the area of origin for Dreissena bugensis (it's named for the Bug
river, pronounced Boog), althought the two species were introduced separately.
---------------------------------------------------------------------
Gary Rosenberg, Ph.D.                     [log in to unmask]
Malacology & Invertebrate Paleontology    gopher://erato.acnatsci.org
Academy of Natural Sciences               http://www.acnatsci.org
1900 Benjamin Franklin Parkway            Phone 215-299-1033
Philadelphia, PA 19103-1195 USA           Fax   215-299-1170