ZOOGEOGRAPHIC DISTRIBUTIONS OF THE SIBLING SPECIES MYTILUS-GALLOPROVINCIALIS AND M-TROSSULUS (BIVALVIA, MYTILIDAE) AND THEIR HYBRIDS IN THENORTH PACIFIC

Diagnostic length differences in a PCR amplified fragment of the gene for byssal adhesive protein were used to study the zoogeographic distr ibution of Mytilus galloprovincialis and M. trossulus along the west c oast of North America and in Japan. The distributions of M. galloprovi ncialis and M. trossulus are patchy, although an overall geographic pa ttern emerges. M. galloprovincialis was the only species found on eith er Kyushu or Honshu, and it was the most abundant mussel from Tomales Bay to San Diego, California. M. trossulus was the only bay mussel fou nd on Hokkaido and in Alaska, and it was by far the most abundant muss el along the coasts of Washington and Oregon. Mytilus galloprovinciali s and M. trossulus are sympatric and hybridize near Whidbey Island, Wa shington, in San Francisco Bay, and in San Diego Bay. A second diagnos tic anonymous nuclear PCR marker was used to examine the extent of hyb ridization at Palo Alto, California. At this site, genotypes appeared to be a mixture of M. galloprovincialis, F-1 hybrids between M. gallop rovincialis and M. trossulus, and backcrosses between the F-1's and M. galloprovincialis. The discontinuity between the zoogeographic distri butions of these two species at about 40 degrees-41 degrees 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.