Characterizing the early life history of an imperiled freshwater mussel (Ptychobranchus jonesi)

Conservation of imperiled species is frequently challenged by insufficient knowledge of life history and the environmental factors that affect various life stages. The larvae (glochidia) of most freshwater mussels in the family Unionidae are obligate ectoparasites of fishes. We describe the early life history of the federally endangered Southern Kidneyshell, Ptychobranchus jonesi, and compare methods for estimating fecundity and conducting host trials on conglutinate-producing mussel species. Both the glochidial inoculation baths and direct feeding of conglutinates to Percina nigrofasciata, Etheostoma edwini, and Etheostoma fusiforme resulted in successful metamorphosis to the juvenile life stage. Ptychobranchus jonesi glochidia did not metamorphose on the 25 other species of fishes tested representing 11 families. Three juveniles were recovered from Gambusia holbrooki resulting in a metamorphosis rate < 1%. We characterize P. jonesi as a host fish specialist that fractionally releases conglutinates from late January to early June. Intact P. jonesi conglutinate resemble a simuliid fly larva attached to an egg, but the majority of conglutinates were released as segments representing separate egg-- or larva--mimics. Viability of glochidia encased within a conglutinate was > 90% for at least 5 days. Directly feeding conglutinates to fishes allowed us to estimate natural infestation rates and calculate average numbers of juveniles produced per conglutinate, unlike the traditional approach of infesting fish hosts using an inoculation bath. Each method for measuring fecundity produced similar estimates but the regression, which estimated fecundity based on the physical dimensions of each conglutinate or conglutinate segment, was most practical. The distribution information, coupled with early life history description and methods developed for determining fecundity and conducting host trials, may assist in the conservation of P. jonesi, specifically during recovery options that include captive propagation, augmentation, and reestablishment.