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. It is listed as threatened under the U.S. Endangered Species Act. 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). Period of gravidity for the narrow pigtoe was determined by examining the gills of mussels in the field, and peak gravidity was considered to be the month in which the greatest number of gravid females was encountered. Fish hosts were determined 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 too few surveys have been conducted in Alabama subbasins for us to fully assess its status throughout its range. Peak months of gravidity were May-July, with the greatest percent of gravid females observed in May, although they were observed as early as 9 March and as late as 25 October. Nine fish species from five genera were identified 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.