Phylogeny of the most species-rich freshwater bivalve family (Bivalvia: Unionida: Unionidae): defining modern subfamilies and tribes

Freshwater mussels of the order Unionida are key elements of freshwater habitats and are responsible for important ecological functions and services. Unfortunately, these bivalves are among the most threatened freshwater taxa in the world. However, conservation planning and management are hindered by taxonomic problems and a lack of detailed ecological data. This highlights the urgent need for advances in the areas of systematics and evolutionary relationships within the Unionida. This study presents the most comprehensive phylogeny to date of the larger Unionida family, i.e., the Unionidae. The phylogeny is based on a combined dataset of 1032 bp (COI + 28S) of 70 species in 46 genera, with 7 of this genera being sequenced for the first time. The resulting phylogeny divided the Unionidae into 6 supported subfamilies and 18 tribes, three of which are here named for the first time (i.e., Chamberlainiini nomen novum, Cristariini nomen novum and Lanceolariini nomen novum). Molecular analyses were complemented by investigations of selected morphological, anatomical and behavioral characters used in traditional phylogenetic studies. No single morphological, anatomical or behavioral character was diagnostic at the subfamily level and few were useful at the tribe level. However, within subfamilies, many tribes can be recognized based on a subset of these characters. The geographical distribution of each of the subfamilies and tribes is also presented. The present study provides important advances in the systematics of these extraordinary taxa with implications for future ecological and conservation studies.Financial support was provided by: the Portuguese Foundation for Science and Technology (FCT) Grant to EF (SFRH/BPD/108445/2015); the Vietnam Academy of Science and Technology under grant number IEBR.CBT.TS07/2015 for the Vietnamese field research; the Central Michigan University Poyang Lake Research Investment Fund and the Key Laboratory of Poyang Lake Wetland and Watershed Research Director’s Open Fund of Jiangxi Normal University for procurement of and analysis of some Chinese specimens. The authors wish to thank: Jamie Smith, Collection manager of Mollusks, North Carolina Museum of Natural Sciences, Raleigh, North Carolina, for all of her efforts to ship tissue samples to be used in this project; the Confederated Tribes of the Umatilla Indian Reservation for providing access to specimens of A. nuttalliana.