Zoogeographic Distributions of the Sibling Species Mytilus galloprovincialis and M. trossulus (Bivalvia: Mytilidae) and Their Hybrids in the North Pacific.

Diagnostic length differences in a PCR amplified fragment of the gene for byssal adhesive protein were used to study the zoogeographic distribution of Mytilus galloprovincialis and M. trossulus along the west coast of North America and in Japan. The distributions of M. galloprovincialis and M. trossulus are patchy, although an overall geographic pattern emerges. M. galloprovincialis was the only species found on either Kyushu or Honshu, and it was the most abundant mussel from Tomales Bay to San Diego, California. M. trossulus was the only bay mussel found on Hokkaido and in Alaska, and it was by far the most abundant mussel along the coasts of Washington and Oregon. Mytilus galloprovincialis and M. trossulus are sympatric and hybridize near Whidbey Island, Washington, in San Francisco Bay, and in San Diego Bay. A second diagnostic anonymous nuclear PCR marker was used to examine the extent of hybridization at Palo Alto, California. At this site, genotypes appeared to be a mixture of M. galloprovincialis, F1 hybrids between M. galloprovincialis and M. trossulus, and backcrosses between the F1's and M. galloprovincialis. The discontinuity between the zoogeographic distributions of these two species at about 40{deg}-41{deg}N latitude in both the eastern and western Pacific suggests that temperature is a factor in determining their present distribution and limiting their dispersal to other regions.