Distribution, period of gravidity, and host identification for the narrow pigtoe mussel

Abstract

The narrow pigtoe Fusconaia escambia is a freshwater mussel found only in the Escambia and Yellow river basins in northwest Florida and southern Alabama. The U.S. Endangered Species Act lists it as threatened. Like other freshwater mussels (Unionidae), its life cycle involves a larval stage (i.e., glochidial) in which most species are obligate parasites on the gills or fins of fishes. Knowledge of life history, population demographics, population genetics, and threats for the narrow pigtoe is lacking throughout its range, which impedes conservation of this species. Therefore, our objectives were to 1) compare historical and current distribution data using a conservation status assessment map, 2) determine period of gravidity, and 3) identify fish hosts. We used a conservation status assessment map to examine spatial and temporal changes in narrow pigtoe distribution and the possibility that the species has been extirpated from a subbasin (i.e., Hydrologic Unit Code level 10 watershed boundary; U.S. Geological Survey National Hydrography Dataset). We determined period of gravidity for the narrow pigtoe by examining the gills of mussels in the field, and considered peak gravidity to be the month in which we encountered the greatest number of gravid females. We determined fish hosts by infecting individuals of 18 fish species with glochidia in a laboratory setting. Overall, the narrow pigtoe appears to be maintaining stable populations in Florida, but researchers have conducted too few surveys in Alabama subbasins for us to fully assess its status throughout its range. Peak months of gravidity were May–July, with the greatest percentage of gravid females observed in May, although we observed them as early as 9 March and as late as 25 October. We identified nine fish species from five genera as hosts for narrow pigtoe, with Blacktail Shiner Cyprinella venusta and Weed Shiner Notropis texanus consistently producing the greatest number of viable juvenile mussels. Host and gravidity findings from this study will be useful if propagation efforts become necessary for conservation of the narrow pigtoe.