Larval fish can be a freshwater mussel host

North American freshwater mussels (Order Unionida, Families Margaritiferidae and Unionidae) are a diverse group of invertebrates with many species threatened by extirpation or extinction. A deeper understanding of freshwater mussel ecology will enhance the effectiveness of conservation strategies. While older life stages of fishes have been studied as hosts for the parasitic life stage of unionids, larval fishes have received little attention. The aims of this study were to: (1) determine if larval Yellow Perch (Perca flavescens) support metamorphosis of Fatmucket (Lampsilis siliquoidea) larvae (glochidia) in the laboratory, and (2) measure the natural glochidia infection frequency on wild-caught stream and river larval fishes. Using standard methods (laboratory infection in a bath of suspended glochidia), we determined that metalarval Perca flavescens support Lampsilis siliquoidea glochidia metamorphosis, and describe low natural glochidia infection frequency among larval fishes living in Rice Creek and the St. Croix River. This study and others across three continents have demonstrated that larval fish can be naturally parasitized by glochidia; however, this study documents glochidial metamorphosis. These results suggest that fish larvae may be an important resource for unionid recruitment because larval fishes are much more abundant than older life stages in some habitats during the spring. Our observation of glochidia metamorphosis on metalarval fish shows that early life stages of fishes are not necessarily a ‘parasite sink’ or ‘ecological trap’ for glochidia. Larval fishes may be an unsuspected—but possibly not underutilized—host. As unionids are distributed globally, interactions between the young life stages of fishes and mussels may occur on a worldwide scale. Although the scope of our study is limited, the importance of larval fishes to unionid recruitment is unclear and should be studied further.