Most freshwater mussels (Unionidae) require a specific host fish to advance their life cycle. Currently, hosts are known for only one-third of the mussel species endemic to the United States and Canada. Texas boasts the greatest diversity of freshwater mussels in the southwestern United States. However, information on mussel-host relationships for ∼52 species known to occur within the state is either lacking or incomplete, including two species, Cyclonaias necki (Guadalupe Orb) and Fusconaia mitchelli (False Spike), currently under review for listing under the U.S. Endangered Species Act. To address this deficiency, we conducted laboratory trials that tested 12 fish species (four families and 11 genera) for C. necki and eight species (four families and seven genera) for F. mitchelli. For C. necki, we identified four host species, Ictalurus punctatus (Channel Catfish), Pylodictis olivarus (Flathead Catfish), Noturus gyrinus (Tadpole Madtom), and Ameiurus natalis (Yellow Bullhead). The transformation period was 11 to 22 d for I. punctatus (peak metamorphosis at 15 d), 16 d for P. olivaris and A. natalis, and 10 d for N. gyrinus. For F. mitchelli, we identified two host species, Cyprinella lutrensis (Red Shiner) and Cyprinella venusta (Blacktail Shiner); for both, the transformation period was 18 d. Current information on the status of these six host species within the Guadalupe River suggests that imperilment of C. necki and F. mitchelli may be partly related to the status of their host fishes. Our results also provide critical information for informing recovery activities, such as translocation and captive propagation, if deemed necessary for one or both mussel species.
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