Evidence of selective mortality in favour of the Mytilus galloprovincialis Lmk phenotype in British mussel populations

Samples of mussels (Mytilus) were collected from 17 localities within hybrid zones of Mytilus edulis and Mytilus galloprovincialis in south-west and north-east England. The study of two polymorphic allozyme loci (esterase-D and octopine dehydrogenase), which are partially diagnostic for the two forms of mussel, reveal the existence of widespread length-dependent allele frequency variation. Larger mussels tend to have a higher frequency of alleles characteristically at high frequency in Mytilus galloprovincialis. Also at a given shell length galloprovincialis alleles have a higher frequency higher up the shore. Computer simulation is used to demonstrate that length-dependent variation may be generated not only by differential mortality but also by differential growth and in models including or excluding immigration. Evidence supports the hypothesis that selective mortality acting in favour of the galloprovincialis phenotype within hybrid populations in Britain is balanced by immigration of the more abundant Mytilus edulis.