Life History Characterization and Host Fish Identification for Federally Listed and Imperiled Freshwater Mussel Species in the Suwannee River Basin in Georgia and Florida

Science Center Objects

Freshwater mussels are considered the most imperiled group of animals in the United States. These animals provide valuable ecological services by filtering water, sequestering nutrients, and providing forage for migratory birds, small mammals, and turtles. They also have a unique and complex life cycle that makes them especially vulnerable to human disturbances. It includes a parasitic larval state that must attach to the fins or gills of a fish host before they become free-living juvenile mussels. USGS explores the reproductive biology and host fish requirements for imperiled freshwater mussels. 

Map of habitat for Imperiled Freshwater Mussel Species in the S

Suwannee River Basin

The Science Issue and Relevance: Freshwater mussels (Bivalvia: Unionidae) are considered the most imperiled group of animals in the United States. These animals provide valuable ecological services by filtering water, sequestering nutrients, and providing forage for migratory birds, small mammals, and turtles. The Suwannee River Basin (SRB) of Georgia and Florida (Figure 1) is home to 16 freshwater mussel species. Within the SRB the U.S. Fish and Wildlife Service (FWS) has several trust resources, including three National Wildlife Refuges (Okeefenokee, Lower Suwannee, and Cedar Key), one federally endangered mussel (Oval Pigtoe, Pleurobema pyriforme), and two other mussel species considered candidates for listing (Suwannee Lance, Elliptio ahenea and Suwannee Moccasinshell, Medionidus walkeri). The Suwannee Moccasinshell, a SRB endemic, was thought to be extinct until two individuals were located in the Suwannee River near Branford during the summer of 2012. In addition to the mussels with federal status, four other species (Florida Sandshell, Lampsilis floridensis, Suwannee Pigtoe, Quadrula kleiniana, Peninsular Floater, Utterbackia peninsularis, and Downy Rainbow, Villosa villosa) are listed as Species of Greatest Conservation Need (SGCN) by the State of Florida.

Freshwater mussels have a unique and complex life cycle that makes them especially vulnerable to anthropogenic perturbation. This life cycle includes a parasitic larval stage (glochidia) that requires a fish or suite of fishes as hosts. These parasitic glochidia must attach to the fins or gills of their host, where they remain attached for several weeks while completing metamorphosis into free-living juvenile mussels. Host specificity (i.e., the ability of the glochidia to successfully metamorphose on a specific host or set of hosts) and host attraction strategy (i.e., mechanism for getting larvae on gills or fins of host) is unknown for most freshwater mussel species in the SRB, and no data exists for the federally listed or candidate species in the basin. This information is required to understand which fish species make suitable hosts so that FWS biologists and refuge managers can develop conservation strategies to conserve populations of host fish species.

The primary goal of this project is to characterize the reproductive biology and host fish requirements for imperiled mussels in the SRB. For each species we aim to (1) identify the period of gravidity, (2) determine strategies for infecting host fishes, (3) determine the host fish requirements, and (4) develop propagation and culture techniques to aid the recovery and prevent extinction.

Methodology for Addressing the Issue: Gravid female mussels will be collected from the SRB and transported back to laboratory facilities at the USGS Wetland and Aquatic Research Center (WARC). Host infection strategies (e.g., mantle displays, conglutinates, broadcasting, etc.) will be documented using digital video and photography. Each mussel will be encouraged to actively release glochidia by manipulating water temperature in an environmental chamber before non-lethally extracting the glochidia. The total number of glochidia per female will be estimated volumetrically to provide measures of fecundity. A subset of the glochidia will be vouchered to determine morphology and size of glochidia. Fishes native to the SRB will be used for host suitability trials. When possible, potential host fishes will be cultured at WARC or collected from streams without mussels to avoid using fish with acquired immunity to glochidia. Potential host fishes will be inoculated by immersion in water containing approximately 4000 glochidia/L. Fish will be placed in the inoculation bath for ≥15 minutes while glochidia are kept in suspension by aeration. Each fish will be removed from the bath and placed in a separate tank. Tanks will be flushed daily into 100um filters. The number of glochidia and juveniles from each tank will be counted daily using a dissecting microscope. Once all the juveniles have recovered, the % metamorphosis will be calculated for each individual fish and the mean % metamorphosis will be calculated based on a minimum of 5 replicates per fish species. Relative transformation success will be determined if multiple hosts are identified for any mussel species tested. All transformed juvenile mussels will be stocked into a variety of mussel culturing systems to assess variation in mortality and growth between systems. Juveniles will be fed commercial algae diet and monitored weekly for growth and survival in these systems. Juveniles reaching total lengths >1000 μm will be transferred to upwelling buckets in ponds at WARC and converted to a natural food supply. 

Future Steps: This study will provide critical information for understanding fish host relationships of several federally listed and candidate mussel species. Information produced by this study can be used to develop a Conservation and Recovery Plan for freshwater mussels in the Suwannee River Basin and help direct future SGCN listings by the State of Florida. In addition, this study will help the FWS understand how to adaptively manage for freshwater mussels.