Distribution, Period of Gravidity, and Host Identification for the Narrow Pigtoe Mussel

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.

Morphological responses of a stream fish to water impoundment

Water impoundment imposes fundamental changes on natural landscapes by transforming rivers into reservoirs. The dramatic shift in physical conditions accompanying the loss of flow creates novel ecological and evolutionary challenges for native species. In this study, we compared the body shape of Cyprinella venusta collected from eight pairs of river and reservoir sites across the Mobile River Basin (USA). Geometric morphometric analysis of the body shape showed that river populations differ from reservoir populations. Individuals inhabiting reservoirs are deep-bodied and have a smaller head, a more anterior dorsal fin, a shorter dorsal fin base and a more ventral position of the eye than C. venusta in streams. The direction of shape divergence within reservoir–river pairs was consistent among pairs of sites, and the shape of C. venusta in reservoirs is strongly correlated with reservoir size. These findings show that physical characteristics of reservoirs drive changes in the morphological attributes of native fish populations, indicating that water impoundment may be an important, yet largely unrecognized, evolutionary driver acting on aquatic biodiversity.