By Ethan Hatchett
Shells are often associated with the seashore, but if you have visited Georgia’s lakes, streams or rivers it’s likely that you have encountered shells that are from the sea. These shells belong to freshwater mussels, a fascinating group of bivalves.
Like their saltwater counterparts, freshwater mussels lie partially buried in sediment underwater and are filter feeders, acting as living water filters. Georgia is home to 129 species of freshwater mussels. That’s roughly 10 percent of the world’s known freshwater mussels. Geologic features resulting in an assortment of habitats have allowed these animals to thrive in Georgia. Eleven major river basins flow across the state, radiating out into the Atlantic Ocean, eastern Gulf of Mexico and the Tennessee River drainage. This habitat complexity, combined with a warm climate and other factors, has resulted in a rich diversity of mussels and other aquatic wildlife.
However, freshwater ecosystems have been degraded by poor water quality, dams, alteration in flows, sediment buildup, draining and the introduction of invasive species. Mussels are filter feeders and the first to reflect significant changes in water quality. As a result, 50 of the freshwater mussels species in Georgia are state-protected and 30 are federally protected.
Altamaha arcmussel in net (Matthew Rowe/DNR)
The Altamaha arcmussel is one of these listed species. This threatened mussel with a distinct, triangular shell inhabits the Altamaha River drainage system as well as parts of the Ogeechee and Savannah rivers. Altamaha arcmussels are associated with a wide range of substrates, from coarse sand to medium silt, but this species is mostly found in fine sand. In 2007, surveys of the Altamaha River found most individuals in areas with gently sloping banks, often with low-hanging willows and soft mud.
Another key aspect to mussel survival is their method reproduction. Instead of sending numerous planktonic larvae adrift like their marine cousins, freshwater mussels have evolved an ingenious way to disperse their young: They hitch a ride.
Mussel glochidia are tiny larvae that attach to a host fish’s gills, fins and skin. They are not usually harmful to the host and often only a few glochidia will successfully attach to a fish. The glochidia clamp onto the outside of the fish using their two valves and enclose themselves for protection. While on the fish, glochidia develop into juvenile mussels, resembling a miniature version of their parents in a few weeks to months. The juveniles then drop from their swimming host – hopefully in a new, suitable habitat – and burrow into the bottom.
The tiny mussels spend the first months of life using cilia (microscopic hair-like structures) on their foot to collect food particles from the surrounding substrate and direct it into their shell to be digested. Once a juvenile has developed its adult filter-feeding structures, it will move to the surface of the sand, mud or gravel where it has been hiding and begin collecting suspended particles from the water column.
Many mussels, like the Altamaha arcmussel, rely on specific species of fish to successfully develop. Research has indicated, though not confirmed, that the Altamaha arcmussel may complete the initial stage of its life cycle on the robust redhorse, another rare species, and the striped jumprock.
The Altamaha arcmussel is a high-priority species on Georgia’s State Wildlife Action Plan. This comprehensive conservation strategy, which is undergoing a periodic revision, lists 640 native animal and plant species as priorities for conserving statewide. The 150 conservation actions recommended in the plan focus restoration and protection efforts where they’re most needed and most effective.
DNR biologists geared to survey Altamaha arcmussels and others. (Matthew Rowe/DNR)
DNR has partnered with Georgia Power and the U.S. Fish and Wildlife Service to ensure the safety of Altamaha arcmussels through the Altamaha Candidate Conservation Agreement. Under this 30-year pact, Georgia Power and DNR conduct research to help understand the distribution and ecology of this species, plus three other mussels and a snail species endemic to the river basin. The hoped-for result is reducing the threats these animals face, which will preserve biodiversity and improve water quality.
As an example of that work, in fiscal year 2023 DNR staff surveyed parts of the Altamaha River and Lake Oconee for mussels at multiple depths using snorkeling and scuba. The more than 2,100 mussels collected – including Altamaha arcmussels on the main stem of the Altamaha – yielded data on species distribution and demographics, while also pinpointing sites for future monitoring.
Protection of the Altamaha arcmussel and all freshwater mussels is critical to maintaining the health of Georgia’s unique freshwater ecosystems. Because of the vulnerability of mussels to environmental changes, they are one of the first organisms to reveal signs of declining water quality. Without mussels, Georgia’s freshwater would be more empty – and a lot less clean.
Ethan Hatchett is a communications assistant in DNR’s Wildlife Conservation Section.
Top: Altamaha arcmussel (Matthew Rowe/DNR)