Distinguishing Early Juveniles of Eastern Pacific Mussels (Mytilus spp.) Using Morphology and Genomic DNA

Abstract

A lack of efficient criteria by which to discriminate among Mytilus spp. in early juveniles (-0.5-5.0 mm shell length) has hindered studies of recruitment, despite the conspicuous ecological roles played by the adults on rocky shores o n 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. Additional key words: dissoconch, shell morphology, recruitment Since the early 1980s, ecologists have increasingly supported the proposition that recruitment is an essential feature of the dynamics of benthic communities (Keough 1984; Underwood & Denley 1984; Gaines & Roughgarden 1985; Gaines et al. 1985). Variation in densities of juvenile invertebrates produces dominant age classes (Dayton 1971; Keough 1984), modulates the intensity of species interactions (Menge et al. 1994; Robles 1997), and influences movements and standing stocks of predators (Menge 1992; Robles et al. 1995). Unfortunately, detailed study of recruitment may be hindered by the lack of simple criteria by which to identify animals at the early post-metamorphic or juvenile stages. Such a problem is encountered studying mussels of the genus Mytilus. The adults of three species, the sea mussel, Mytilus californianus CONRAD 1837, and the bay mussels (also called blue mussels), M. trossulus GOULD 1850 and M. galloprovincialis LAMARK 1819, play varied and important ecological roles along the a Author for correspondence. Present address: Bamfield Marine Station, Bamfield, B.C., VOR iBO, Canada. E-mail: amartel@bms.bc.ca western coast of North America (Paine 1966, 1974, 1976; Suchanek 1981, 1985). However, the distinguishing morphological features of the adults are not differentiated in early juvenile stages. Indeed, while presenting molecular criteria for juvenile identification, Heath et al. (1996) repeat the view (Suchanek 1978) that the juveniles of bay mussels <10 mm long cannot be physically distinguished from those of sea