Distinguishing early juveniles of Eastern Pacific mussels (Mytilus spp.) using morphology and genomic DNA

A lack of efficient criteria by which to discriminate among Mytilus spp. in early juveniles (similar to 0.5-5.0 mm shell length) has hindered studies of recruitment, despite the conspicuous ecological roles played by the adul ts on rocky shores of western North America. We present morphological crite ria, Visible under a dissecting microscope, that distinguish sea mussels, M ytilus californianus, from sympatric bay mussels, M. trossulus and M. gallo provincialis. The shells of early juvenile sea mussels from southern Britis h Columbia and from southern California have (1) a more posterior dorsal ap ex and (2) a posterior adductor muscle (PA) attachment larger than do shell s of bay mussels from similar locations (M. trossulus and M. galloprovincia lis, respectively). Accuracy of these criteria was confirmed by comparing P CR products of genomic DNA of juveniles with those of adults. Additional cr iteria useful in identification include: (3) the proximity of the PA to the dorsal apex Line, (4) the dorsal angle of the dissoconch, and (5) lateral hinge-tooth demarcations. Morphological criteria identified by this study c onstitute a fast, economical means of distinguishing early juveniles of tho se mussels and may benefit ecological research on Mytilus.