GENETIC-VARIABILITY IN EXPOSED-SHORE MUSSELS, MYTILUS SPP, ALONG AN ENVIRONMENTAL GRADIENT

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, Mytilus edulis L. and the Mediterranean mussel M. galloprovincialis Lmk. This paper presents an in-depth analysis, carried out between October 1984 and December 1986, of genetic variability at two partially diagnostic loci, Odh and Est-D, in two exposed-shore populations of Mytilus spp. in the west of Ireland. Significant differences at the Odh locus were observed in the genetic composition of adult mussels from different tidal levels. These differences were repeatable whether one was analysing replicate samples at a single point in time, samples collected at different points in time, i.e., in different years, or samples collected from different shores. Mussels recruiting to artificial substrates set out for a period of one month at different tidal levels at one of these sites were also observed to be genetically different; mussels higher up the shore exhibited higher frequencies of those alleles characteristically at high frequency in M. galloprovincialis for both the Odh and Est-D loci. Hence, the genetic differences observed in adult mussels are much more exaggerated in juveniles and are already apparent within the first month of benthic life. Possible reasons for the observed microgeographic differentiation are discussed. It is concluded that the observed genetic differences between mussels at different tidal levels arise either in the pelagic/attachment stage or very shortly after settlement. © 1990 Springer-Verlag.