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Pleurobema pyriforme (I. Lea, 1857)
Oval Pigtoe
Federal Protection: Listed Endangered
State Protection: Endangered
Global Rank: G2
State Rank: S1S2
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: Highest Conservation Concern
Element Occurrences (EOs) in Georgia: 50
Habitat Summary for element in Georgia: Large rivers to small creeks with slow to moderate current in pool, run, and riffle habitats; combinations of clay, sand, and gravel substrate
Shell profile is elliptical to oval in outline and the shell is moderately inflated with a maximum length of approximately 65 mm (2⅝ inches). Anterior margin broadly rounded and posterior margin is bluntly pointed to round. Ventral margin is straight to broadly rounded. Umbos positioned slightly anterior to the middle of the shell and elevated above hingeline. Posterior ridge is rounded with a steep posterior slope. The periostracum is yellow to dark brown, sometimes with several dark rays present. Dark and prominent growth rings often present on surface of the shell. Pseudocardinal teeth are compressed and angled anteriorly. Lateral teeth long and straight or slightly curved. Umbo cavity shallow. Nacre color varies from white to salmon.
Little Spectaclecase (Villosa lienosa) and Downy Rainbow (Villosa villosa). The Oval Pigtoe can be distinguished from the Little Spectaclecase by umbo sculpture, which tends to consist of concentric rings for the oval pigtoe while the Little Spectaclecase typically has double-looped sculpturing. The Oval Pigtoe differs from the Downy Rainbow in that the posterior terminus of the Oval Pigtoe tends to be below the midline of the shell whereas the Downy Rainbow terminates at the midline and typically has prominent rays.
Typically occupies small streams to large rivers with moderate flow and sand or gravel substrates.
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 though pedal feeding and then developing the ability to filter feed during adulthood (Vaughn and Hakenkamp 2001).
Gravid females were found from May until early July. Glochidia of this species successfully transformed on 6 cyprinid species native to the Apalachicola River Basin (Fritts and Bringolf 2014). In addition, the Sailfin Shiner (Pteronotropis hypselopterus) and the Eastern Mosquitofish (Gambusia holbrooki) successfully transformed glochidia (O'Brien and Williams 2002). It is unlikely that the Eastern Mosquitofish frequently comes in contact with this unionid.
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 my increase probability of detection.
This species occurs from Econfina Creek east to the Hillsborough River in Florida. In Georgia, this species is currently restricted to Sawhatchee Creek, a tributary to the Chattahoochee River, Spring Creek, and the Flint River and its tributaries upstream to Line Creek near Peachtree City, Georgia.
Excessive water withdrawals in the Lower Flint River basin coupled with severe drought could result in the extirpation of this species from Georgia. Habitat fragmentation may isolate populations and prevent fish movement, limiting the distribution of host fishes carrying glochidia. Additionally, construction of impoundments may further fragment populations and inundate suitable habitat. Excess sedimentation due to inadequate riparian buffer zones also covers suitable habitat and potentially bury individuals.
| Threat 1 | Threat 2 | Threat 3 | |
|---|---|---|---|
| General Threat | Pollution | Natural system modifications | Natural system modifications |
| Specific Threat | None | Dams & water management/use | Dams & water management/use |
The Oval Pigtoe is known from Chickasawhatchee Creek in the vicinity of Chickasawhatchee and Elmodel Wildlife Management Areas in Georgia. However, unlike terrestrial species, the occurrence of an aquatic species on state or federal lands may not eliminate habitat degradation due to the influences of upstream and downstream disturbances.
Development of environmental flow criteria and investigation of dissolved oxygen and thermal tolerance of this species was identified as a high management priority in the 2015 Georgia Wildlife Action Plan. In addition, development of environmental DNA (eDNA) methodologies may aid in identifying additional populations of this species.
Fritts, A.K., and R.B. Bringolf. 2014. Host fishes for four federally endangered freshwater mussels (Unionidae) in the Apalachicola-Chattahoochee-Flint Basin. Walkerana 17:51-59.
O’Brien, C.A. and J.D. Williams. 2002. Reproductive biology of four freshwater mussels (Bivalvia: Unionidae) endemic to the eastern Gulf Coastal Plain drainages of Alabama, Florida, and Georgia. American Malacological Bulletin 17: 147-158.
Shea, C.P., J.T. Peterson, M.J. Conroy, and J.M. Wisniewski. 2013. Evaluating the influence of land use, drought, and reach isolation on the occurrence of freshwater mussel species in the lower Flint River Basin, Georgia (U.S.A.). Freshwater Biology 58:382-395.
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., N.M. Rankin, D.A. Weiler, B.A. Strickland, and H.C. Chandler. 2013. Occupancy and detection of benthic macroinvertebrates: a case study of unionids in the lower Flint River, Georgia, USA. Freshwater Science 32:1122-1135.
Wisniewski, J.M., C.P. Shea, S. Abbott, and R.C. Stringfellow. 2013. Imperfect recapture: a potential sources of bias in freshwater mussel studies. The American Midland Naturalist 170:229-247.
Wisniewski, J.M., N.M. Rankin, D.A. Weiler, B.A. Strickland, and H.C. Chandler. 2014. Use of occupancy modeling to assess the status and habitat relationships of freshwater mussels in the lower Flint River, Georgia, USA. Walkerana 17:24-40.
Jason Wisniewski
2018-03-20
