THE STRUCTURE AND DYNAMICS OF NATURALLY-OCCURRING HYBRID MYTILUS-EDULIS LINNAEUS, 1758 AND MYTILUS-GALLOPROVINCIALIS LAMARCK, 1819 (BIVALVIA, MOLLUSCA) POPULATIONS - REVIEW AND INTERPRETATION

This paper reviews the structure and dynamics of the naturally occurri ng hybrid zone between the marine bivalve molluscs, Mytilus edulis and Mytilus galloprovincialis. Biogeographical evidence indicates that, a s previously suggested by other authorities, the hybrid zone arose as a result of secondary contact during the Pleistocene. Concordant chang es in shell morphology, mantle edge colouration, allozyme variation an d mitochondrial DNA variation agree with this interpretation. The loca tion and structure of the hybrid zone are both strongly influenced by environmental factors (primarily temperature, secondarily salinity and wave exposure). It is suggested that the hybrid zone therefore confor ms to a mosaic hybrid zone type which was established by secondary con tact. Unlike the majority of other hybrid zones which are characterize d by maladapted hybrid recombinants of lower fitness than the parental types, all available data from a variety of authorities and geographi c locations indicate that hybrid mussels do not have decreased fitness (growth rate, fecundity, resistance to parasites, and viability) comp ared with parental types. The very high percentage of dilocus allozyme backcross genotypes within the hybrid zone in S.W. England support th is interpretation. The hybrid zone appears to be maintained by directi onal selection in favour of M. galloprovincialis which is counterbalan ced by massive immigration of M. edulis spat.