Distribution, genetic structure, and morphometry of Mytilus edulis and M-trossulus within a mixed species zone

During 1994 to 1996, we sampled 25 wild mussel sites and 15 mussel farm sit es distributed widely throughout coastal Newfoundland, Canada. Allele frequ encies at four loci, Mpi, Lap, Pgm, and Gpi were determined, and several sh ell morphometric parameters were measured. Allelic variation at the Mpi loc us was used as the discriminating criterion to distinguish between the two species, Mytilus edulis and M. trossulus. Both species are widely distribut ed throughout Newfoundland. Sites typically have mixtures of both species d ominated by M. edulis but with M. trossulus ranging from a low of 0% to a h igh of 84% at individual sites. Commercial stocks on mussel farms tended to have significantly higher frequencies of M. trossulus as compared to wild mussel beds. Analysis of population structure using Wright's F statistics r evealed mussel populations throughout Newfoundland are genetically highly d ifferentiated. Sites in close proximity to each other were as genetically d iverse and varied as much in species proportions as sites large distances a part without any apparent macrogeographic distributional pattern. Morphomet rically, cultured M. edulis and M. trossulus differ significantly in shell width, depth, cavity volume, and shell weight (M. edulis > M. trossulus for all variables). Intraspecific variation in morphometric phenotype in both species was significantly related to variation in multilocus genotype. The impact of this morphometric variation on commercial mussel farm production in mixed M.edulis/trossulus areas is discussed.