Ecological Genetics of the Mussels Mytilus edulis and M. galloprovincialis on Irish Coasts

Genetic polymorphism at the loci encoding the enzymes phosphoglucose isomerase, leucine aminopeptidase and phosphoglucomutase was investigated in Irish Mytilus populations. Allele frequencies and heterozygote proportions indicated that populations on exposed Atlantic coasts differed from those on sheltered Atlantic coasts, and from Irish Sea mussels. When compared with data on M. edulis collected in northern France and M. galloprovincialis from the Mediterranean these data indicate that in Ireland M, galloprovincialis occurs mainly at exposed sites, and hybridization with M. edulis occurs frequently. The systematic status of the two forms remains uncertain In the l ~ g h t of genetic evidence of hybridization and intergradation between them. INTRODUCTION galloprovincialis occurs only to a very limited extent and hybrids are consequently rarer. Mytilus edulis L. and Mytilus galloprovincialis Lmk. occur on the coasts of Ireland (Baird, in Hepper, 1957; Kitching et al., 1959; Ebling et al., 1960). They are not, MATERIALS AND METHODS however, evenly distributed on all coasts; Seed (1974) reported that M. edulis alone occurred on the Irish Sea Approximately 4100 individual mussels were analcoast, whereas both mussels were found on the South, ysed in the period since October 1974. These were West and North coasts. Until recently, the two mussel collected from 26 sites on the coasts of Ireland types were distinguished by physiological and mor(Table 1). Mussels were collected from two types of phological characters alone (Seed, 1971). These habitat: exposed sites, located on headlands lacking characters are very variable in British and Irish musprotection from wave action and prevailing winds and sels (Lewis and Seed, 1969), and accurate identificasheltered sites located at the inner end of inlets and tion of populations of the two types based on these bays in fully saline waters. At four sites (2,5, 17 and 28) characteristics is difficult, especially at exposed sites mussel populations were entirely sublittoral; these (Seed, 1974). Biochemical genetic studies have indiwere classified as sheltered sites. For purposes of comcated that they differ also at a small number of enzyme parison pure samples of Mytilus galloprovincialis were gene loci (Ahmad and Beardmore, 1976; Gosling and analysed from Venice and Cannes on the MediterraWilkins, 1977; Skibinski et al., 1978) and in addition, nean coast and a single sample of M. edulis was suggest that some hybridization may occur between analysed from San Vaast on the Channel coast of the two forms on the Atlantic coasts of Ireland and France. parts of Britain. The extent of hybridization varies from Technical details of sample preparation, elecsite to site, and it has been suggested (Skibinski and trophoretic procedure and visualization of Pgi and Pgm Beardmore, 1979) that in some environments hybrids have been described previously (Gosling and Wilkins, and individuals of mixed ancestry may have fitness 1977). Staining for Lap was performed using the superior to that of the edulis form. The results of the method of Murdock et al. (1975), but the Lap locus present study, takenin conjunction with those from our studied by us is not that studied by Murdock et al., nor preliminary report (Gosling and Wilkins, 1977), sugthe Lap-1 locus described in Skibinski and Beardmore gest that the M. galloprovincialis form favours the (1979). offshore habitat and hybridization with M, edulis is The terminology used for all allozymes is as follows: common there; in sheltered shore environments, M. a indicates the least anodal allozyme, and b, c, d etc.