Marine Biology (1994) 119:243-249 9 Springer-Verlag 1994 J. B. Geller 9 J. T. Carlton 9 D. A. Powers PCR.based detection of mtDNA haplotypes of native and invading mussels on the northeastern Pacific coast: latitudinal pattern of invasion Received: 10 December 1993 / Accepted: 17 December 1993 Abstract Frequencies of mitochondrial haplotypes char- acteristic of native Mytilus trossulus and introduced M. galloprovincialis were determined in populations along the west coast of North America from San Diego, California, to the Aleutian Islands, Alaska. We also identified the hap- lotypes of mussels cultured from larvae arriving in Coos Bay, Oregon, during 1988-1990 from sites in Japan in the seawater ballast of ocean-going ships. All mussels from ballast-water samples were M. galloprovincialis. We found that sampled populations north of San Francisco Bay, including Coos Bay, were entirely composed of mus- sels with the M. trossulus haplotype, while haplotypes of both species were present in all sites in and south of San Francisco Bay. The presence of M. trossulus in southern sites is contrary to evidence from allozyme studies, and we suggest that mtDNA introgression from M. trossulus to M. galloprovincialis may explain this discordance. This study demonstrates that, despite continued transport and release, M. galloprovincialis has not become established in north- ern sites. Failure to invade the north coast of North Amer- ica may reflect environmental unsuitability for M. gallop- rovincialis. However, invasion success may be probabilistic, and the continuing release of M. gallopro- vinciaIis larvae may foreshadow a future successful inva- sion. Communicated by M. G. Hadfield, Honolulu J. B. Geller (~) Department of Biological Sciences, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297, USA J. T. Carlton Maritime Studies Program, Williams College, Mystic Seaport, Mystic, Connecticut 06355, USA D. A. Powers Hopkins Marine Station, Stanford University, Pacific Grove, California 93950, USA Introduction Biological invasions, the arrival and establishment of non- indigenous species, are recognized as one of the greatest threats to the preservation of biodiversity: non-native spe- cies can modify habitats, monopolize resources, and prey upon or outcompete native species (see compilations in Groves and Burdon 1986; Kornberg and Williamson 1986; Mooney and Drake 1986; Drake et al. 1989; Hengeveld 1989; and di Castri et al. 1990). Compared to the volumi- nous literature on biological invasions in terrestrial habi- tats (e.g. the multiauthored volumes cited above), inva- sions in the marine environment have been little studied, and investigations have been recent and confined to few geographic regions and taxa (Race 1982; Brenchley and Carlton 1983; Bertness 1984; Yamada and Mansour 1987; Carlton et al. 1990; Nichols et al. 1990; Berman and Carl- ton 1991; Berman et al. 1992). This is regrettable, as the frequency of marine invasions worldwide appears to have increased in the past two decades (Carlton 1989; Carlton et al. 1990; Carlton and Geller 1993). This increase can be largely attributed to the use of seawater ballast in ocean- going ships (Carlton 1985, 1987, 1989; Carlton and Geller 1993). A typical ship routinely takes up and releases un- treated seawater as ballast as the cargo of the ship and weather conditions change. Thus, a ship may take up coastal water from a port on one continent, cross an ocean, and release this water into a similar environment on an- other continent. Any organisms in shipside waters, includ- ing holoplankton, meroplankton, and tychoplankton, can be entrained in ballast water. These organisms are then transported across natural barriers to dispersal. Illustrating the magnitude of this phenomenon, Carlton and Geller (1993) have documented 367 species of marine inverte- brates and plants that are routinely transported in ballast water from ports in Japan to the west coast of the United States of America. Although the transport of marine organisms in ballast water is now well documented (Carlton 1985; Williams et al. 1988; Carlton and Geller 1993), dispersal to a new en-