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Pyganodon gibbosa (Say, 1824)
Inflated Floater
Federal Protection: No US federal protection
State Protection: No Georgia state protection
Global Rank: G3Q
State Rank: S3
Element Locations Tracked in Biotics: Yes
SWAP 2015 Species of Greatest Conservation Need (SGCN): No
SWAP 2025 Species of Greatest Conservation Need (SGCN): No
2025 SGCN Priority Tier:
Element Occurrences (EOs) in Georgia: 25
Habitat Summary for element in Georgia: Large rivers, oxbows, reservoirs
Large thin-shelled mussel to 180 mm. Shell surface smooth with umbo rising well above the hinge line. General shape is oval and inflated, but a small wing on the posterior dorsal area makes the profile somewhat angular. Shell color is variable; tan, green, gray, dark brown or black. Individual annuli or groups of annuli often vary in color. Irregular fine rays are sometimes present but do not extend all the way to the umbo. Nacre white to blue-white. Teeth are completely absent. Umbo cavity can be fairly deep on large individuals. Beak sculpture is multiple looped bars with irregular nodules.
P. gibbosa can be easily mistaken for Pyganodon cataracta, Utterbackia imbecillis, or Utterbackiana couperiana, but characters are usually present to differentiate them with close inspection. P. gibbosa most closely resembles P. cataracta; however, P. cataracta has a more elongate shape relative to its degree of inflation, is less inflated, and its umbos do not extend as far above the hinge line. P. cataracta tends more towards light brown coloration and often has a slightly more clothy periostracum. P. cataracta beak sculpture lacks the irregular nodules found in P. gibbosa. P. gibbosa superficially resembles U. imbecillis and U. couperiana due to similar coloration and shell thickness. However, the umbos on these two species are totally flat and never extend above the hinge line allowing them to be quickly eliminated as possibilities.
P. gibbosa can be found in sandy, silty, or muddy substrate in rivers and impoundments. This species prefers moderate to low flow areas and is often found along margins of rivers, in oxbows, backwaters, and other flow sheltered areas (Wisniewski et al., 2005; Meador et al., 2011). P. gibbosa, like most toothless, thin-shelled species, seems to prefer soft substrates and can tolerate impoundment conditions.
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, P. gibbosa 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. gibbosa’s host infection strategy is not known but it is likely that it is a broadcast spawner and releases its glochidia into the water in mucus strands. Like closely related Pyganodon grandis, P. gibbosa is believed to be a long-term brooder gravid from summer to the following spring (Williams et al., 2014). P. gibbosa has been observed gravid in late November. 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 substrate where they feed on detritus using their foot and eventually grow into filter-feeding, adult mussels. Specific host species for P. gibbosa are unknown; however, other members of the genus Pyganodon are known to be broad host generalists and use multiple genera of fish as hosts. Close relative P. grandis is known to successfully metamorphose on 43 different species of fish from 15 different genera.
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 (such as P. grandis or P. cataracta) are spawning or brooding may increase probability of detection. Investigators should conduct tactile searches of substrates such as sand, mud, or silt along stream margins, oxbow lakes, backwaters, and reservoirs away from high flow areas. P. gibbosa has been found at depths greater than 8 meters in reservoirs. Special attention should be paid to habitats where host fish species might be likely to spend time such as around submerged woody debris, rock outcrops, and man-made structures.
P. gibbosa is endemic to the Altamaha River basin in Georgia.
Like most native freshwater mussels in Georgia, P. gibbosa is threatened by habitat destruction in the form of flow alteration, excessive water withdrawal, watershed land use changes, impoundments, and pollution. While the specific host fish species for P. gibbosa 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. P. gibbosa, unlike most freshwater mussels in Georgia, is tolerant of conditions created by the construction of dams. This, combined with being a host generalist, puts P. gibbosa at a lower threat of extinction due to habitat modification.
Conservation of P. gibbosa will primarily rely on habitat protection in the Altamaha River basin in the form of drainage area, riparian, and water resource management. While dams are not as severe a concern for this species as many other in the Altamaha, dam construction could cause further barriers to dispersal and impede or prevent gene flow within the species. Further research into the specific life history, host fish, and habitat requirements of this species are still needed to guide management efforts.
Meador, J.R., J.T. Peterson, and J.M. Wisniewski. 2011. An evaluation of the factors influencing freshwater mussels capture probability, survival, and temporary emigration in a large lowland river. Journal of the North American Benthological Society 30:507-521.
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.
Wisniewski, J.M., B. Albanese, and G. Krakow. 2005. Current status of endemic mussels in the lower Ocmulgee and Altamaha Rivers. In K.J. Hatcher (ed.), Proceedings of the Georgia Water Resources Conference, Institute of Ecology, The University of Georgia, Athens.
Matthew Rowe
1/9/2020
