The United States contains the greatest diversity of freshwater mussels in the world — roughly 300 species. In spite of its world class richness, this family of mollusks is the most imperiled group of animals in the country. Our latest tally indicates that 69 species are federally endangered or threatened, and 35 are presumed extinct. Most of the endangerment and extinction is attributed to habitat destruction and loss due to impoundments, pollution, sedimentation, dredging, and other human-induced factors.

The Tennessee River Basin, historic home to 91 species of mussels, is a case in point. Of the original 91 species, 10 are extinct, 20 are extirpated, 25 are endangered or threatened, 10 are relic species (not reproducing), and only 26 species have stable populations. Without immediate efforts to conserve these dwindling species, another round of extinctions is inevitable.

Freshwater mussels have a unique life history, requiring the use of host fish to successfully reproduce. Eggs of female mussels are fertilized internally by sperm released into the water and taken in by females – during siphoning. The embryos then develop in the gills of the female until becoming mature larvae (called glochidia). Once mature, the female mussel spews them into the water, where they must attach to suitable host fish for transformation from larva to juvenile in two to three weeks. The juveniles then drop from the fish to begin their lives on the river bottom.

As a pro-active strategy, the Tennessee Wildlife Resources Agency, the U.S. Fish and Wildlife Service, and the USGS Biological Resource Division's Virginia Cooperative Fish and Wildlife Research Unit at Virginia Tech entered into a five-year cooperative program to propagate, culture, and release endangered juvenile mussels selected from the suite of species federally protected in Tennessee. The goal of this project is to augment natural reproduction of resident populations and to release juvenile mussels into those rivers to expand the range of existing populations. Eventually, this propagation program will reintroduce these rare species into rivers where the species were extirpated years ago. Recovery of many species will require the re-establishment of extirpated populations, in addition to augmentation of extant populations.

The techniques and technology needed for mussel propagation required roughly 10 years of research to develop and perfect. All of this research, funded cooperatively by the states of Tennessee and Virginia, and the U.S. Fish and Wildlife Service, was conducted by the USGS Biological Resources Divisions cooperative research unit at the Freshwater Mollusk Conservation Center at Virginia Tech. This facility is the only one of its kind, dedicated to the propagation and conservation of mollusks in the United States. The prototype facility consists of a holding room for adult mussels and experimentation, a room for production, and a greenhouse for culture of juvenile mussels.

Producing juvenile mussels begins by collecting suitable host fish from the appropriate river and holding them in captivity until gravid female mussels can be found. In the laboratory, the glochidia in the gills of a gravid female are flushed out using a hypodermic needle and syringe filled with water. This non-lethal method allows us to return female mussels to the river unharmed. Glochidia can number more than 200,000 per female. These glochidia are then introduced into a bucket holding the host fish, and aeration is used to keep the water agitated to allow attachment of glochidia to the gills of fish.

After one hour of exposure, the fish are moved to large aquaria where glochidia begin the transformation process. Once these young juveniles drop from the host fish, they are siphoned from the aquaria and placed in sediment trays for culture in recirculating artificial stream systems. Juveniles are fed algae cultured in large fiberglass tubes and reared to a size suitable for release into targeted rivers. Once released, these young mussels require three to five years to reproduce on their own.

The first release of endangered juvenile mussels in the United States from laboratory propagation occurred in fall 1997 in the Hiawassee River, Tennessee. At that time, 52 endangered tan riffleshells, two to three months of age, were released as surviving progeny from culture experiments. In 1998, more than 35,000 endangered juvenile mussels of six species were released: oystermussel, Cumberland combshell, snuffbox, fanshell, birdwing pearlymussel, and cracking pearlymussel. They were released at four sites on the Clinch River and one on the Powell River in northeastern Tennessee.

An evaluation of the success of these and all subsequent releases will occur two years after the release date, to allow mussels to grow and occupy habitats frequented by the resident mussel species. In subsequent years, 50,000 to 100,000 juveniles of these and other endangered mussels will be propagated and released each year to attempt to recover the suite of endangered mussel species in the upper Tennessee River drainage of northeastern Tennessee and Southwestern Virginia. At long last, the ability to carry out the priority actions in individual recovery plans has been realized. The techniques worked out for propagation of endangered mussels can then be used in other states and federal fish hatcheries so that conservation and recovery of finfish and shellfish will become well integrated into the mission of natural resource agencies.

Dr. Richard Neves, the chief of the Virginia Cooperative Fish and Wildlife Research Unit, can be reached at (540) 231-5927 (or 7580). The unit's e-mail address is mussel@vt.edu.

People, Land & Water, Vol. 6, No. 1. January/February 1999.