| Summary: | Some North Pacific species of marine invertebrates have distinct populations that occur separately in the western and eastern Pacific. Several North Pacific species of opisthobranchs have unusually large geographic ranges, including California through Alaska, Siberia and northern Japan. The explanation for this wide range is unclear and there are two competing hypotheses: 1) Bidirectional expansion: the ranges of transpacific species during the previous interglacial period/s were split by costal ice formation in the Northern Hemisphere and diverged allopatrically resulting in sister species pairs - these sister species are similar morphologically and are currently classified as the same species, but because they have been separated for at least 100,000 years should be distinguishable using molecular tools. 2) Unidirectional expansion: postglacial expansion northward from refugia in North America allowed species to cross the North Pacific and become established in Siberia and northern Japan - because of the direction of ocean currents, expansion from Asia to North America is less likely. My preliminary data for Diaulula sandiegensis shows that it is not a single species as previously thought, but two distinct species. One species has a range that encompasses the entire north Pacific Ocean while the second is found from the tip of Baja California through British Columbia. Future research will focus on developing a larger genetic library for comparison of these two species and morphological comparison using SEM of the radula and penis. Furthermore, molecular clock dating will be used to determine the timing for the expansion and population history of this species.
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