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Elliptio nigella (I. Lea, 1852)
Winged Spike
Federal Protection: No US federal protection
State Protection: No Georgia state protection
Global Rank: G1
State Rank: S2
Element Locations Tracked in Biotics: Yes
SWAP 2015 Species of Greatest Conservation Need (SGCN): Yes
SWAP 2025 Species of Greatest Conservation Need (SGCN): Yes
2025 SGCN Priority Tier: High Conservation Concern
Element Occurrences (EOs) in Georgia: 9
Habitat Summary for element in Georgia: Large rivers in swift and shallow shoals. Often times associated with large crevices and cavities in and around limestone boulders.
Oval or axe-head shaped shell to 61 mm. Anterior margin rounded, posterior margin squared. Posterior ridge rounded forming a long, low wing from umbo to anterior margin. Periostracum dark brown to black. Teeth are compressed, small, and moderately thick. Nacre white, blue-white, or peach (Williams et al., 2008; Williams et al., 2014).
E. nigella can closely resemble several species in the Apalachicola River basin. The most similar species are other members of the genus Elliptio including Elliptio pullata, Elliptio fumata, and Elliptio chipolaensis. E pullata and E. fumata are both more compressed and lack the conspicuous indent anterior to the umbo found in E. nigella. E. chipolaensis is almost identical in shape to E. nigella; however, this species is consistently a dark orange to chestnut brown color with dark annuli on the shell easily differentiating it. Additionally, E. chipolaensis is restricted to the Chipola River in Florida and Alabama and is not known to occur in the state of Georgia. Male members of the species Villosa lienosa or Villosa vibex could potentially be confused with E. nigella; however, both of these species also lack the indent anterior to the umbo and are generally more inflated with umbos raised slightly above the hinge line.
E. nigella had been found in large rivers with moderate to swift currents and sandy or gravely substrates often around boulders and bedrock (Williams et al., 2014).
The diets of unionids are poorly understood but are believed to consist of algae and/or bacteria. Some studies suggest that diets may change throughout the life of a unionid with juveniles collecting organic materials from the substrate through pedal feeding and then developing the ability to filter feed during adulthood (Vaughn and Hakenkamp, 2001).
Like all unionids, E. nigella has an obligate parasitic life cycle. Larval mussels, known as glochidia, develop in water tube present in the gills of the female mussel and are released when disturbed by an appropriate host fish. E. nigella is believed to be a short-term brooder, like other members of its genus, and is presumed to be gravid in spring and early summer. Glochidia attach to the gills, fins, and skin of the host fish and encyst as an ectoparasite. Typical glochidia infestation appears to be minimally invasive to the host fish and is not fatal. Glochidia remain attached to the host for several weeks to several months before dropping off as juvenile mussels. These juveniles hopefully land in a new, suitable location and burrow into the sand where they feed on detritus using their foot and eventually grow into filter-feeding, adult mussels. Specific host species for E. nigella are unknown and the variety of host fish utilized by different species within this genus make host fish speculation based on congeners tenuous.
Surveyors should consider sampling during periods when female individuals are spawning or brooding as this species may have higher detection rates during this period. However, since basic life history information for many of Georgia’s unionids is lacking, sampling during periods when closely related species are spawning or brooding may increase probability of detection. Investigators should conduct tactile searches of stabilized sand and fine gravel substrates (i.e. sand around an embedded boulder or bedrock) away from the immediate margins of the river in high to moderate flows. The low number of individuals of this species that have been collected make specific survey recommendations difficult.
E. nigella is endemic to the Apalachicola River basin in Georgia.
Like most native freshwater mussels in Georgia, E. nigella is threatened by habitat destruction in the form of flow alteration, excessive water withdrawal, watershed land use changes, impoundments, and pollution. While the host fish species for E. fraterna are not known, freshwater mussels are dependent on them to complete their life cycle and threats to host fish or barriers to host fish movement are also serious threats. Flow and connectivity restoration through the removal of low head dams may allow this species to recolonize portions of its previous home range in the Chattahoochee River.
| Threat 1 | Threat 2 | Threat 3 | |
|---|---|---|---|
| General Threat | Natural system modifications | Agriculture & aquaculture | Natural system modifications |
| Specific Threat | Dams & water management/use | None | Dams & water management/use |
Conservation of E. nigella will primarily rely on habitat protection in the Apalachicola River basin in the form of drainage area, riparian, and water resource management. Due to a lack of basic knowledge about this species, specific management recommendations are difficult. Further research into the specific distribution, life history, host fish, and habitat requirements of this species are still needed to guide management efforts. Special attention should be paid to protecting the remaining known populations of E. nigella in the Lower Flint River main stem.
Vaughn C.C. and C.C. Hakenkamp. 2001. The functional role of burrowing bivalves in freshwater ecosystems. Freshwater Biology 46:1431-1446.
Williams, J.D., A.E. Bogan, and J.T. Garner. 2008. Freshwater mussels of Alabama and the Mobile Basin in Georgia, Mississippi, and Tennessee. The University of Alabama Press, Tuscaloosa.
Williams, J.D., R.S. Butler, G.L. Warren, and N.A. Johnson. 2014. Freshwater Mussels of Florida. University of Alabama Press, Tuscaloosa, Alabama.
Matthew Rowe
1/9/2020