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Pleuronaia dolabelloides (I. Lea, 1840)
Slabside Pearlymussel
Federal Protection: Listed Endangered
State Protection: Endangered
Global Rank: G2
State Rank: S1
Element Locations Tracked in Biotics: Yes
SWAP 2015 Species of Greatest Conservation Need (SGCN): No
SWAP 2025 Species of Greatest Conservation Need (SGCN): Yes
2025 SGCN Priority Tier: Highest Conservation Concern
Element Occurrences (EOs) in Georgia: 1
Habitat Summary for element in Georgia: small streams to large rivers with flowing water in TN Basin tributaries; stable gravel with interstitial sand
Pleuronaia dolabelloides can be highly variable but typically has a vaguely triangular outline. This species can be compressed to moderately inflated, which likely follows a cline from small to larger streams and as has been observed in many mussel species. The shell is typically rather thick and can vary from yellowish-brown to brown, often with interrupted green rays, and can reach 85 mm in length. The shell can have well defined, rough concentric rings, particularly in larger individuals, due to pronounced growth rests. Smaller individuals are typically lighter and have faded rays which are broken at growth rests. The Posterior slope is sharply rounded and smooth. The sharpness of the slope is often used to differentiate between P. dolabelloides and P. barnsiana, which has a more smoothly rounded posterior slope. The umbo extends above the hinge line but can be strongly eroded in larger individuals. P. dolabelloides has two pseudocardinal teeth and two lateral teeth in the left valve, and one pseudocardinal and one lateral tooth in the right valve. Pseudocardinal teeth are heavy and oriented at nearly 90 degrees to the lateral teeth. Lateral teeth are well defined, relatively short, and straight to slightly curved. The umbo pocket is present and moderately deep. Nacre is white, sometimes with peachy pigment around teeth and along margins.
Pleuronaia dolabelloides can a difficult species to identify due to its morphological variability and its similarity to several other co-occurring species across three genera. This species has been previously placed in the genera Pleurobema, Quadrula, Margaron, and Lexingtonia by different authors. One characteristic that often differentiates P. dolabelloides from similar species is the downward curved posterior slope.
Pleuronaia dolabelloides tends to have a more defined triangular shape as well as a sharper and steeper posterior slope which curves downward. The umbo of this species also appears to be positioned slightly closer to the anterior end of the shell. P. dolabelloides will also always have an orange or pale orange foot, while P. barnsiana can have an orange or white foot. In gravid female individuals, P. dolabelloides will have the outer pair of gills charged with glochidia (pink), while P. barnsiana will have all four gills changed (tan or orange) (Shilling 2015).
Fusconaia cor, Fusconaia cuneolus, and Fusconaia subrotunda look very similar to P. barnsiana however there are some subtle characters that are recognized to separate them. These Fusconaia species typically have heavier teeth and deeper umbo cavities than P. dolabelloides. F. cor also typically has a shiny periostracum.
Pleurobema oviforme can be very similar to P. dolabelloides, however, P. oviforme has considerably heavier teeth and the pseudocardinal teeth tend to form an acute angle with the lateral teeth instead of a perpendicular angle as seen in P. dolabelloides. In live individuals, P. oviforme has an orange or pale orange foot and gravid females have only the two outer gills charged with glochidia. The gravid gills appear white.
Below is a brief reference table for separating similar species using live, soft tissue characters.
Pleuronaia barnsiana
Foot color- Orange OR white
Gravid gill- all four, tan or orange
Pleuronaia dolabelloides
Foot color-Orange or pale orange
Gravid gills-outer two, pink
Fusconaia cor
Foot color- orange or pale orange
Gravid gills- all four, pink
Fusconaia cuneolus
Foot color- orange or pale orange
Gravid gills- all four, pink
Fusconaia subrotunda
Foot color- white
Gravid gills- all four, red
Pleurobema oviforme
Foot color- orange or pale orange
Gravid gills- outer two, white
P. dolabelloides can be found in a wide variety of flowing-water habitats from small tributary streams to medium sized rivers; Substrate preference is sandy gravel. Still and 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. dolabelloides 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. dolabelloides’ host attraction strategy is to release conglutinates into the water column, ostensibly in the hope that their host fish ingesting or encountering the mature glochidia. Glochidia has shovel shaped glochidia which 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. (Haag 2012). P. dolabelloides is assumed to be a host specialist and has been observed to successfully transform to juvenile mussels on minnow species in the genus Notropis. P. dolabelloides is a short-term brooder, spawning in spring and holding glochidia into mid-summer.
Surveys conducted during the mature gravidity period (May through July) are most likely to detect the species while individuals are at the surface to spawn.
Pleuronaia dolabelloides is endemic to the Tennessee River Basin in Tennessee, Alabama, Georgia, Mississippi, North Carolina, and Virginia. In Georgia, it is only known from Northwest Georgia in the Upper Tennessee River in the Chickamauga Creek system. P. dolabelloides appears to have been historically present in the Cumberland River basin, however it is now believed to be extirpated from there. Range-wide genetic analysis of P. dolabelloides has indicated that there are two relatively distinct populations of this species separated between the upper Tennessee River (Duck, Powell, Holston, Clinch) and the Duck River (Grobler et al. 2005)
Threats to P. dolabelloides 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. dolabelloides 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, and habitat requirements, and distribution of this species are still needed to guide management efforts. Conservation efforts directed at this species can be frustrated by the difficulty of accurate identification and taxonomic uncertainty.
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
Grobler, P. J., Jones, J. W., Johnson, N. A., Beaty, B., Struthers, J., Neves, R. J., & HALLERMAN, E. M. (2006). Patterns of genetic differentiation and conservation of the slabside pearlymussel, Lexingtonia dolabelloides (Lea, 1840) in the Tennessee River drainage. Journal of Molluscan Studies, 72(1), 65-75.
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
1/10/2022