Phylogenetic analysis based on the morphology of viviparous freshwater clams of the family Sphaeriidae (Mollusca, Bivalvia, Veneroida)

Abstract

The cosmopolitan viviparous bivalves of the family Sphaeriidae (fingernail clams) comprise not only some of the smallest freshwater molluscs, but also an abundant and ecologically important group that often dominates benthic communities of springs, ponds and swamps. The origin from marine ancestors and their subsequent evolution in limnic environments, in particular the development of distinct reproductive modes including complex parental care repertoires, are largely unresolved. This paper presents the results of a phylogenetic analysis of 69 morphological characters from 57 taxa (54 representing sphaeriids throughout the distribution area plus three outgroups). The heuristic search resulted in 753 most‐parsimonious trees of length 154 [consistency index (CI) = 0.682, retention index (RI) = 0.906, rescaled consistency index (RC) = 0.618]. Only Pisidium of the three traditionally regarded constituent genera is supported as a monophyletic group; Sphaerium and Musculium in their traditional deliminations are found to be paraphyletic. Instead, the analysis supports a clade comprising all species of Musculium plus Pisidium as suggested by previous morphology‐based classifications. However, the shortest tree alternatively supporting a Sphaerium + Musculium clade suggested by molecular data is only one step longer. Some clades that correspond to the subgeneric divisions of the three traditionally recognized sphaeriid genera are supported. Exclusion is suggested of the taxon Euperidae (Eupera and Byssodonta, both with ovo‐viviparous species) from the true Sphaeriidae that then comprises all species with eu‐viviparous reproductive mode, i.e. brooding and nourishing their embryos in specialized brood pouches in the gills. The implications for the reconstruction of the evolutionary history of Sphaeriidae and the availability of existing taxonomic names for the systematization suggested by the present analysis are discussed.