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Medionidus conradicus (I. Lea, 1834)
Cumberland Moccasinshell
Federal Protection: No US federal protection
State Protection: No Georgia state protection
Global Rank: G3
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: High Conservation Concern
Element Occurrences (EOs) in Georgia: 1
Habitat Summary for element in Georgia: Large creeks in TN Basin tributaries; shoal and run habitats; sand and gravel, frequently occurs under large, flat rocks
Medium sized, thin shelled mussel to 60 mm in length. Shell surface brown, gray, or gray-green with broken, wavy rays of inconsistent width on the entire shell. Rays often break into a series of chevron shapes. Corrugations on the posterior slope. These corrugations are often less pronounced than in other Medionidus species but are usually visible. The general shape is elliptical with rounded posterior and anterior ends. Older individuals can become slightly arcuate in shape. Interior of shell blue-green to light gray. Subtle sexual dimorphism with males being narrower at the posterior end and females being slightly more inflated. Pseudocardinal teeth triangular, two in left valve. One Peg-like tooth in right valve. Lateral teeth mostly straight, two in left valve one in right valve. This species has a mantle lure comprised of two small, elongated papillae that may resemble the cirri of mayfly or stonefly larvae to attract their darter host fish (Haag 2012).
Medionidus conradicus can be confused with some small individuals of Villosa sp., particularly Villoa iris, however, M. conradicus has distinct corrugations on the posterior slope. These corrugations can occasionally be obscured or eroded in older individuals; however, they will usually be some evidence of their presence remaining. M. conradicus is also usually more elongate and more dorso-laterally inflated. Female V. iris will have obvious broad inflation at the posterior end.
Small to large rivers; occurs often in sand, gravel, or bedrock (cracks) substrate or under flat rocks. This species seems to prefer swift run and riffle habitats. Often attached to substrate by one to three elastic byssal threads, particularly in juveniles.
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, M. conradicus 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. M. conradicus’ s host infection strategy is similar to that of other Lampsiline mussels consisting of active attraction using modified mantle margin tissue and papillae to simulate prey items. These structures are often pulsed or wiggled in order to entice predatory host fish to strike. When a host fish interacts with the gravid female mussel, the mussel will eject mature glochidia from the mantle cavity via the excurrent aperture. 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. M. conradicus is considered to be a host specialist with successful transformation of juvenile mussels in laboratory trials for several darter species in the genus Etheostoma (Luo and Layzer, 1993, Zale and Neves 1982). M. conradicus is a long-term brooder and has been reported gravid between September and May (Ortmann, 1921), however glochidia have been observed in drift throughout the year with the exception of August (Zale and Neves 1982a)
Surveys conducted around the mature gravidity period (August through June) are most likely to detect the species while individuals are at the surface to spawn.
Medionidus conradicus is endemic to the Tennessee and Cumberland River drainages in Tennessee, Alabama, Virginia, Kentucky, and Georgia. In Georgia, this species has been recorded only in the Lookout Creek Drainage.
Threats to M. conradicus include pollution, eutrophication, and sedimentation. The largest identifiable threat is the construction of impoundments. Numerous recorded populations have been eliminated due to inundation and habitat changes cause by altered flow patterns. Riffle, shoal, and swift water species like M. conradicus are particularly vulnerable to alterations caused by impoundments as riffle and shoal habitats are often selected as sites for dam construction and reductions in flow can cause rapid and severe changes in these habitats. An additional threat includes impacts from viral a 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 M. conradicus in Georgia will primarily rely on habitat protection in the Lookout 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.
Luo, M. and J.B. Layzer. 1993. Host fish of three freshwater mussels. (Abstract). Page 182 in K.S. Cummings, A.C. Buchanan, and L.M. Koch. (eds.). Conservation and Management of Freshwater Mussels. Proceedings of a UMRCC Symposium, 12-14 October 1992, St. Louis, Missouri. Upper Mississippi River Conservation Committee, Rock Island, Illinois. 189 pp.
Haag, W. R. (2012). North American freshwater mussels: natural history, ecology, and conservation. Cambridge University Press.
Parmalee, P. W., & Bogan, A. E. (1998). Freshwater mussels of Tennessee. University of Tennessee Press.
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.
Zale, A. V., & Neves, R. J. (1982). Reproductive biology of four freshwater mussel species (Mollusca: Unionidae) in Virginia. Freshwater Invertebrate Biology, 1(1), 17-28.
Zale, A.V. and R.J. Nevesb. 1982b. Identification of a fish host for Alasmidonta minor (Mollusca: Unionidae). American Midland Naturalist 107(2): 386-388.
Matthew Rowe
12/6/2021