T.J. Hilbish ÆE.W. Carson Æ J.R. Plante L.A. Weaver M.R. Gilg Distribution of Mytilus edulis , M . galloprovincialis , and their hybrids in open-coast populations of mussels in southwestern England Received: 16 January 2001 / Accepted: 4 May 2001 / Published online: 15 November 2001 Springer-Verlag 2001 AbstractThe distribution ofMytilus edulisLinnaeus, M. galloprovincialis Lamarck,and their hybridswas examined in mussel populations in southwest England in 1996 and 1998. This is a region where both parental taxa and populations containing large numbers of hybrids co- occur yet a fine-scale mapping of the hybrid populations has notbeen conducted. In this study the geographic distribution ofhybrid populationswasdetermined in southwest England over 360 kmof coast fromTintagel Castle in north Cornwall to Beer in south Devon. Sample localities were spaced from 5 to 20 km apart. Genotypes for individual mussels were determined using PCR to amplify a size polymorphism at the Glu-5¢ locus that is completely differentiated between M. edulis and M. galloprovincialis. Hybrid populations, characterized by high frequencies of individualswith heterozygous genotypesand a pattern of decreasing frequency of M. edulis alleles with increasing shell length,were con- tinuously distributed along 180 kmof open coastin southwest England. This ‘‘hybrid patch’’ was bordered at one end by geographically extensive populations of pure M. edulisand at the otherend by nearly pure populationsof M. galloprovincialis. Strong natural selection in hybrid populations results in a decline in the frequency of M. edulis alleles with increasing size. Wave- exposure has previously been implicated as the agent producing this pattern of selection, but in the present study the relationship between allele frequency and body size was notcorrelated with variation in wave shock intensity among localities within the hybrid zone. The transition between hybrid populations and those con- taining pure populations of M. edulis or M.gallopro- vincialis is abrupt which suggests that coastal circulation patterns may provide strong barriers to larval dispersal which accounts for the position and maintenance of the hybrid zone. Introduction Blue mussels in the genus Mytilus consist of a complex of well-differentiated taxa (Skibinski et al. 1983; Koehn et al.1984;Varvio et al.1988;McDonald et al.1991). Two of these species, M. edulis and M. galloprovincialis, co-occur in western Europe and, despite being differen- tiated morphologically (Beaumont et al. 1989; McDon- ald et al.1991;Gosling 1992), physiologically (Hilbish et al. 1994), and genetically (Skibinski et al. 1983; Koehn 1991;McDonald et al. 1991),readily interbreed and formhybrid swarms (reviewed by Gardner 1994). Hy- brid populations contain mussels with parental geno- types and high frequencies of F 1 and F 2 hybrids,and mussels of mixed genetic ancestry (Skibinski et al. 1983; Gardner 1994; Hilbish et al. 1994; Wilhelmand Hilbish 1998).Within hybrid populations the frequency of tax- on-specific alleles at allozyme locis strongly age de- pendent; alleles specific to M. galloprovincialis increase in frequency with age and size (Skibinski 1983; Gardner and Skibinski 1988; Gosling and McGrath 1990; Wilhelmand Hilbish 1998). This relationship between allele frequency and size is produced by strong viability selection, which occurs as mussels grow from juveniles to adults (Gardner and Skibinski 1988;Skibinskiand Roderick 1991;Gardner et al. 1993;Wilhelmand Hilbish 1998). Skibinskiet al.(1983) showed that in Great Britain mussels form a mosaic of populations containing either species or their hybrids. They sampled 68 sites in Britain and Ireland at approximate 50-km intervals, providing the firstdescription ofthe location ofhybrid mussel populations in this region.Finer spatial resolution is Marine Biology (2002) 140: 137–142 DOI 10.1007/s002270100631 Communicated by J.P. Grassle, New Brunswick T.J. Hilbish (&) ÆE.W. Carson Æ J.R. Plante Æ L.A. Weaver M.R. Gilg Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA E-mail: hilbish@biol.sc.edu Fax: +1-803-7774002