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Ptychobranchus fasciolaris (Rafinesque, 1820)
Kidneyshell
Federal Protection: No US federal protection
State Protection: No Georgia state protection
Global Rank: G4G5
State Rank: S1
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: Moderate Conservation Concern
Element Occurrences (EOs) in Georgia: 2
Habitat Summary for element in Georgia: Small creeks to large rivers with moderately strong current in substrate of coarse gravel and sand
Ptychobranchus fasciolaris has an oval to squashed triangle outline and often has a depression in the center of the shell tapering towards the ventral margin, as if the shell had been “pinched” in the middle. Combined with the curved dorsal margin, this gives the animal its distinctive kidney shape. P. fasciolaris can reach up to 150 mm in length, though smaller individuals are far more common. It is often more compressed than other elliptical mussels and the shell is very thick for the size giving the animal an unexpectedly heavy feel. Growth rests are often deep and coarse resulting in concentric ridges on the shell. The periostracum is yellowish or yellow-green darkening to light brown with age and often display broken green rays radiating out form the umbo in younger individuals. Larger animals are usually lacking these rays completely. Dentition is thick with two heavy, short, widely spaced lateral teeth and two triangular pseudocardinal teeth in the left valve and one of each in the right valve. Female shells display a shallow groove running from the ventral side of the posterior abductor scar to the umbo. The genus Ptychobranchus is unique among North American Unionids in that female individuals have a conspicuously folded or wavy marsupial gill which is immediately obvious upon inspection.
P. fasciolaris can superficially resemble Ptychobranchus subtentum, however the distinctive plications of the posterior slope of subtentum quickly distinguish it from the smooth posterior slope of fasciolaris. P. subtentum, while present in the Tennessee Basin, has not been recorded in Georgia.
P. fasciolaris can resemble Eurynaia dilatata as they are both elliptical and have exceedingly thick shells, however, E. dilatata appears to be very dark brown to black and have a more pointed posterior end. Shell material is usually easy to differentiate as E. dilatata usually has dark purple nacre (salmon and white are also possible) while P. fasciolaris always has white or cream-colored nacre. Beak sculpture, if present can also quickly separate the two as P. fasciolaris has indistinct, low wavy bars while E. dilatata displays several thick and prominent curved bars.
P. fasciolaris is found in large streams and rivers and prefers flowing water habitats, often in association with stabilizing aquatic vegetation. Substrate preferences are stable sandy gravel. Still, disturbed, or silt-heavy habitats do not support this species.
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 nearly all unionids, P. fasciolaris 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. P. fasciolaris, like other members of its genus, have a unique host attraction strategy. Glochidia are released into the water in conglutinates which strongly resemble aquatic insects or small fish, including prominent eye spots, lateral lines, and myomeres. These conglutinates are negatively buoyant and tumble along the bottom in flow resembling dislodged prey items until they adhere to a hard surface with their sticky “posterior” end. They can remain viable for days in this state awaiting a curious host fish to take a bite. Glochidia are hookless and elliptical with a broad, rounded ventral margin and a narrow, convex hinge margin. These glochidia are released from the anterior end of the conglutinate when disturbed and 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 substrate where they feed on bacteria and detritus using their foot and eventually grow into filter-feeding, adult mussels. (Haag 2012). Host fish for P. fasciolaris can differ across its wide geographic range but darter species in the genus Etheostoma are assumed to be primary hosts. This species is a long-term brooder and is gravid from August to the following summer, some as late as the following August.
Surveys conducted during the mature gravidity period (May through August) are most likely to detect the species while individuals are at the surface to spawn. Surveyors can also be sure to target firm, sandy gravel in association with rooted emergent vegetation to improve chanced of detection.
Ptychobranchus fasciolaris has a wide geographic distribution and is present in a number of major mussel faunal provinces including the St Lawrence/Great Lakes, Ohioan, and Tennessee/Cumberland. In the Mobile Basin, P. fasciolaris is only known from the Tennessee River. In Georgia, P. fasciolaris is very rare and is only known from Northwest Georgia in the Upper Tennessee River in the Chickamauga Creek system.
Threats to P. fasciolaris in Georgia are shared by many species in the Chickamauga Creek system and include pollution, eutrophication, and sedimentation due to stream access by cattle and proximity of poultry production facilities in the vicinity (SCCHMP 2012). Additional threats include impoundments such as Dallas Lake and Nickajack Lake and the potential for impacts from viral disease known to cause mass mortality in other freshwater mussel species elsewhere in the Tennessee Basin.
| Threat 1 | Threat 2 | Threat 3 | |
|---|---|---|---|
| General Threat | Residential & commercial development | Agriculture & aquaculture | Pollution |
| Specific Threat | None | None | None |
Conservation of P. fasciolaris in Georgia will primarily rely on habitat protection in the South Chickamauga Creek watershed in the form of drainage area, riparian, and water resource management. Flow modifications in the form of dam construction, drainage structures, and course alterations are a primary concern for mussel species in the Southeastern US. Further research is needed to assess the potential for impacts of disease which is causing mussel mortality elsewhere in the Tennessee River Basin. Further research into the specific life history, host fish, Genetic structure, habitat requirements, and distribution of this species are still needed to guide management efforts.
Haag, W. R. (2012). North American freshwater mussels: natural history, ecology, and conservation. Cambridge University Press.
Limestone Valley RC & D Council (2012) South Chickamauga Creek Headwaters Management Plan. Retrieved from https://epd.georgia.gov/document/document/south-chickamauga-creek-middle-tennessee-river/download
Metcalf-Smith, J., A. MacKenzie, I. Carmichael, and D. McGoldrick. (2005) Photo Field Guide to the Freshwater Mussels of Ontario. St. Thomas Field Naturalist Club Inc.
Parmalee, P. W., & Bogan, A. E. (1998). Freshwater mussels of Tennessee. University of Tennessee Press.
Shilling, D. E. (2015) Assessment of morphological and molecular genetic variation of freshwater mussel species belonging to the genera Fusconaia, Pleurobema, and Pleuronaia in the upper Tennessee River basin. [Unpublished master’s thesis]. Virginia Polytechnic Institute and State University.
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.
Matthew Rowe
1/12/22
