The evolution of brooding characters among the freshwater pearly mussels (Bivalvia: Unionoidea) of North America

Abstract

Brooding characters have figured prominently in the classification of North American freshwater pearly mussels (Bivalvia: Unionoidea). The purpose of our study was to evaluate phylogenetic hypotheses of brooding character evolution in order to test homology statements suggested by earlier taxonomic systems of the Unionoidea. Parsimony analysis of partial COI sequences from 29 species of freshwater mussels and 13 outgroups were used to derive a phylogeny. Thirteen brooding characters (e.g., brooding period, marsupium arrangement, structure of interlamellar septa, etc.) were traced onto this phylogeny. Results indicate that long-term brooding (bradytictia) is the derived state among North American freshwater mussels; short-term brooding (tachytictia) is plesiomorphic. Bradytictia evolved independently in the Anodontinae and Lampsilini, with unique morphological modifications derived in those clades to facilitate long-term brooding.