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 adults on rocky shores of western North America. We present morphological criteria, Visible under a dissecting microscope, that distinguish sea mussels, Mytilus californianus, from sympatric bay mussels, M. trossulus and M. galloprovincialis. The shells of early juvenile sea mussels from southern British Columbia and from southern California have (1) a more posterior dorsal apex and (2) a posterior adductor muscle (PA) attachment larger than do shells of bay mussels from similar locations (M. trossulus and M. galloprovincialis, respectively). Accuracy of these criteria was confirmed by comparing PCR products of genomic DNA of juveniles with those of adults. Additional criteria 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 constitute a fast, economical means of distinguishing early juveniles of those mussels and may benefit ecological research on Mytilus.