Morphology and phylogenetic relationships of genera of North American Sphaeriidae (Bivalvia, Veneroida)

TitleMorphology and phylogenetic relationships of genera of North American Sphaeriidae (Bivalvia, Veneroida)
Publication TypeJournal Article
Year of Publication2004
AuthorsLee T
JournalAmerican Malacological Bulletin

Recent phylogenetic studies of the Sphaeriidae have produced conflicting results between morphological and molecular data sets for two major topological elements, i.e., monophyly of Pisidium and of asynchronous brooders (Sphaerium and Musculium taxa). Gene trees indicate a paraphyletic Pisidium while morphological trees suggest it is a derived monophyletic clade. Molecular analyses recover a derived monophyletic clade of asynchronous brooders suggesting the evolutionary elaboration of brooding character complexity from synchronous to sequential brooding. Conversely, morphological analyses indicate a sister relationship among Musculium and Pisidium taxa and propose that reduced Pisidium characters are derived from larger plesiomorphic sphaerium taxa. To test these competing hypotheses of sphaeriid relationships, the morphology of North American sphaeriid taxa was studied, and major anatomical and developmental features that have been considered fundamental by previous workers were coded. Parsimony analyses with and without outgroup rooting showed that Sphaerium and Musculium taxa form a monophyletic group, congruent with previous gene trees but not with morphological studies. The conflict between molecular and morphological trees for the monophyly of Pisidium, however, remains unsettled. At present, infomrative morphological characters appear insufficient to flesh out phylogenetic relationships among infra-generic sphaeriid taxa.