Phylogenetic relationships among mytilidae (Bivalvia): 18S rRNA data suggest convergence in mytilid body plans

Nearly complete sequences were determined for small-subunit (18S) rRNA genes from seven species representative of four subfamilies of Mytilidae: Modiolus modiolus and M. auriculatus (Modiolinae); Lithophaga lithophaga and L. nigra (Lithophaginae); Musculus senhousie and M. discors (Crenellinae); and Hormomya domingensis (Mytilinae). Small-subunit rRNA gene sequences were also determined for Solemya reidi (Subclass Protobranchia), Mya arenaria (Subclass Heterodonta), and Elliptio complanata (Subclass Paleoheterodonta) as outgroup taxa. Phylogenetic analyses including these and other nearly complete bivalve small-subunit rRNA sequences demonstrate support for the monophyly of the family Mytilidae and the subfamilies Crenellinae and Lithophaginae. However, the subfamilies Mytilinae and Modiolinae appear polyphyletic. Likelihood, parsimony, and distance analyses support the placement of H. domingensis (Mytilinae) in a clade with G. demissa (Modiolinae). This clade is distinct from those containing other species traditionally assigned to these two subfamilies. Kishino-Hasegawa tests support these nontraditional relationships, suggesting that the mytiliform and/or modioliform body plans have evolved independently in at least two mytilid lineages.