The Carolina Heelsplitter - Lasmigona decorata
By Jennifer M. Koches, USFWS
A native freshwater mussel species first described as Unio decoratus in 1852 by Isaac Lea, A. H. C larke did not classify the Carolina heelsplitter as the distinct species Lasmigona decorata until 1985. This medium-sized mussel has an ovate, trapezoidal-shaped shell with the largest ever-recorded specimen measuring 4.6 inches in length, 1.56 inches in width, and 2.7 inches in height (Keferl 1991). The outer surface of the shell varies from greenish brown to dark brown in color. Younger shells have faint greenish brown or black rays. The inside surface (nacre) is often pearly white to bluish white, grading to orange in the area of the umbo (Keferl 1991). In older specimens, however, the entire nacre may be a mottled pale orange.
The species currently has a very fragmented distribution but historically was known from several locations within the Catawba and Pee Dee River systems in North Carolina and Catawba, Pee Dee and Savannah River systems in South Carolina. Historic records also exist for the Carolina heelsplitter in the “Abbeville District” which were originally interpreted as having been records from the Saluda River system. However, a population of the heelsplitter discovered in the Savannah River system in the spring of 1995 opened up the possibility that the “Abbeville District” records may have referenced either the Savannah, Saluda, or both river systems.
Recent collection efforts indicate that the Carolina heelsplitter has been eliminated from the majority of its historic range and only six small populations are known to exist. Of those six remaining populations, two are in North Carolina and four are in South Carolina. North Carolina’s two populations occur in Union county; one population in Waxhaw Creek (Catawba River system) and the other population in Goose Creek (Pee Dee River system). One of South Carolina’s four populations occurs in a relatively short reach of the Lynches River (Pee Dee River system) in Chesterfield, Lancaster, and Kershaw counties. This population also extends into Flat Creek, a tributary to the Lynches River in Lancaster County. In the Catawba River system, another population of the species is found in a short reach of Gill Creek in Lancaster County.
The Savannah River system has the two remaining populations. One of these populations is in Turkey Creek in Edgefield and McCormick counties and two of its tributaries, Mountain Creek and Beaverdam Creek in Edgefield County. Another population exists in Cuffytown Creek in Greenwood and McCormick counties. Despite the extensive surveys in recent years, no evidence has been found to support the existence of a population in the Saluda River system.
The Carolina heelsplitter has been recorded in a variety of substrates, including mud, clay, sand, gravel, and cobble/bolder/bedrock. A majority of these areas are without significant silt accumulations and are along stable, well-shaded stream banks. Stability of stream banks and stream bottom appear to be the habitat elements essential to the species.
Like other freshwater mussels, the Carolina heelsplitter feeds by filtering detritus, diatoms, phytoplankton, and zooplankton from the water column. Many aspects of the heelsplitter’s life history are unknown, such as the life span and fish host species. An obligate parasite during the larval stage of its life cycle, the Carolina heelsplitter larvae (glochidia) are released into the water from the adult female’s brood pouch (marsupium) where they must attach to the gills of their fish host to undergo the metamorphosis to free living mussels. Finding this fish host remains one of the greatest mysteries of the Carolina heelsplitter’s survival.
Reduced to only a narrow fraction of its historic range, the U.S. Fish and Wildlife Service listed the Carolina heelsplitter as endangered in 1993, and critical habitat was designated in 2002. Critical habitat identifies specific areas that are essential to the conservation of a listed species and areas that may require special management consideration or protection. All that aside, the remaining populations are more vulnerable than ever before. A single catastrophic event or alteration to habitat could cause irrevocable harm to the Carolina heelsplitter, possibly ushering it into extinction.
Pollutants from wastewater discharges are just one of the many factors affecting this species. Like canaries in a coalmine, freshwater mussels are one of the most reliable indicators of a stream’s health. Early life stages of the mussel are extremely sensitive to compounds such as chlorine and ammonia, heavy metals, and high concentrations of nutrients, all things commonly found in municipal and industrial wastewater effluents. Three of the four populations of Carolina heelsplitter in South Carolina exist in “impaired” water systems that do not currently meet State water quality standards.
Habitat loss and alteration associated with impoundments or dams, channelization, and dredging operations alter the quality and stability of remaining stream reaches by affecting flow regimes, water velocities, and water temperature and chemistry. Certain types of agricultural and forestry practices, highway and road construction projects, and residential and industrial developments can cause channel and streambank scouring associated with increased storm-water runoff, and add to the runoff of silt, fertilizers, pesticides, and other pollutants if inadequate or poorly maintained erosion and stormwater control measures are in place.
Compounded by the fact that the remaining populations of the Carolina heelsplitter are disjointed, it is easy to see that this is clearly a species in peril. The U.S. Fish and Wildlife Service, the South Carolina Aquarium, and The Nature Conservancy are co-hosting a Mussel Symposium in June to bring together malacologists (mussel biologists), resource agency representatives, and non-profit natural resource groups from across the region to discuss the need for statewide inventories of our native freshwater mussels, the status of the Carolina heelsplitter, and statewide water quality issues. Being successful at keeping the Carolina heelsplitter from slipping off into extinction will take much time, work, and cooperation.