Cryptic diversity of M elanochlamys sea slugs ( G astropoda, A glajidae) in the N orth P acific

North Pacific specimens of M elanochlamys sea slugs were examined morphologically (including the male reproductive morphology, shell and external coloration) and were sequenced for three genes (mitochondrial COI and 16S and nuclear H3). Phylogenetic and species delimitation analyses were used to determine the evolutionary relationships and species diversity among the specimens examined. Both molecular and morphological data revealed an unexpected high level of cryptic diversity. At least four distinct species occur on the Northwestern Pacific. M elanochlamys ezoensis occurs in Russia and temperate and cold areas in Japan. Three additional undescribed species occur in Japan and/or South Korea. One of the undescribed species occurs both in South Korea and in Japan, but only in Tokyo Bay, suggesting that it could be non‐native in Japan. Two distinct species occur on the Northeastern Pacific coast; M elanochlamys diomedea is widespread from Southern California to Alaska, whereas M . ezoensis was found only in San Francisco Bay, suggesting a human‐mediated introduction. This is further supported by the absence of records of M . ezoensis in San Francisco prior to 2001. The species diversity of M elanochlamys in the Northwestern Pacific is much greater than in the Northeastern Pacific; it is hypothesized that differences in geographic and ocean current system complexity might account for different responses to glacial extinction and postglacial expansion.