Identification of settling and early postlarval stages of mussels (Mytilusspp.) from the pacific coast of North America, using prodissoconch morphology and genomic DNA

Detailed inventories of the benthos and field studies of the settlement and recruitment processes of marine benthic invertebrates require accurate ide ntification of newly settled larvae and early juvenile stages. We provide m orphological criteria, visible under a good quality dissecting stereomicros cope, by which to discriminate between species of the settling larval and e arly postlarval stages (similar to 250 to 700 mum shell length) of mussels of the genus Mytilus on the west coast of Vancouver Island and Southern Cal ifornia. Compared to the bay mussel (M. trossulus), the sea mussel (M. cali fornianus) has: (i) a shallower and flatter umbo, the latter corresponding to a significantly less pronounced prodissoconch I (PI) curvature and (ii) a greater PI length as well as (iii) a wider separation between the provinc ular lateral teeth (PLT). The PLT distance is a new term denoting the separ ation between the midpoint of two reddish pigment spots of the provinculum (larval hinge apparatus) region of settling larvae and early postlarvae of Mytilus spp. from the East Pacific Coast. These spots mark the larger provi ncular lateral teeth, situated at either end of the provinculum. We confirm ed the validity of morphological criteria by comparing PCR products of geno mic DNA of provisionally identified postlarvae. Furthermore, measurements o f PI lengths and PLT distance from well-preserved postlarvae of sea mussels (M. californianus) and of bay mussels (hl. galloprovincialis) from Souther n California indicate that the PI morphology and morphometry, and PLT dista nce criterion apply for that region as well. The criteria presented here ca n also apply to the advanced (competent) veliger stages, as the latter may settle (i.e. become the "settling" stage) upon encountering a suitable subs trate. Our present and previously published work provide economical and eff ective identification methods that can be used to discriminate among early life history stages (similar to 250 mum to 5.0 mm shell length) of Mytilus spp. along the west coast of North America.