The objective of this paper is to review the current state of our knowledge of freshwater bivalve diversity and evolution in order to identify some of the “Great Unanswered Questions” in the field. Twenty-one bivalve families have been attributed to fresh waters, but only 16 actually live and reproduce in inland waterways. Of 1209 species of freshwater bivalves, 1178 (97%) belong to eight primary freshwater families: Unionidae, Margaritiferidae, Hyriidae, Mycetopodidae, Iridinidae, and Etheriidae (all Unionoida), Sphaeriidae, and Cyrenidae (both Veneroida). The remaining 31 species represent secondary freshwater lineages of predominantly brackish/marine bivalve families. The global geographical patterns of freshwater bivalve richness are discussed, and an appendix detailing the families, genera, and species of freshwater bivalves is provided.
Although the primary freshwater bivalve families represent at least three independent radiations from marine into inland aquatic habitats, these lineages have converged on similar adaptations to life in a flowing hypoosmotic medium. For example, all have abandoned broadcast spawning and planktonic veligers. Phylogenetic studies of the Unionoida, Sphaeriidae, and Cyrenidae have also converged on a suite of common challenges: outgroup issues, biased ingroup taxon and character sampling, and atypical modes of genetic inheritance that uncouple gene trees from species trees. The recent phylogenetic literature on the three primary radiations is reviewed, emphasizing areas in need of research. Ample opportunities exist in freshwater bivalve research but progress is hindered by the limited numbers of researchers and students in the field.