OCHLOCKONEE MOCCASINSHELL                            

Medionidus simpsonianus




STATUS: On March 16, 1998, the Ochlockonee moccasinshell was designated as Endangered throughout its entire range (USFWS 1998).  A recovery plan addressing the Ochlockonee moccasinshell was finalized on October 1, 2003 (USFWS 2003).


SPECIES DESCRIPTION:   The Ochlockonee moccasinshell is a small species, generally under 5.6 cm (2.2 in) in length.  It is slightly elongate-elliptical in outline, the posterior end obtusely rounded at the median line, and the ventral margin broadly curved.  The posterior ridge is moderately angular and covered in its entire length with well developed, irregular plications.  Sculpture may also extend onto the disk below the ridge.  The periostracum (outside surface of the shell) is smooth.  The color is light brown to yellowish green, with dark green rays formed by a series of connected chevrons or undulating lines across the length of the shell.  Internal characters include thin straight lateral teeth and compressed pseudocardinal teeth.  There are two pseudocardinals and two laterals in the left valve and one pseudocardinal and one lateral in the right valve.  The nacre is bluish white.  The taxonomic confusion that has surrounded the genus Medionidus is summarized elsewhere (USFWS 2003).


Like other freshwater mussels, adults are filter-feeders, orienting themselves in the substrate to facilitate siphoning of the water column for oxygen and food (Kraemer 1979).  Mussels have been reported to consume detritus, diatoms, phytoplankton, zooplankton, and other microorganisms (Coker et al. 1921, Churchill and Lewis 1924, Fuller 1974).   Juvenile mussels employ foot (pedal) feeding, and are thus suspension feeders (Yeager et al. 1994).  Food of juvenile freshwater mussels up to two weeks old include bacteria, algae, and diatoms with amounts of detrital and inorganic colloidal particles (Yeager et al. 1994).  Specific food habits of the Ochlockonee moccasinshell are unknown, but are likely similar to those of other freshwater mussels.


REPRODUCTION AND DEVELOPMENT: The extreme rarity of the lampsiline Ochlockonee moccasinshell has precluded any opportunities to explore its life history.  It can only be assumed that this species has similar reproductive biology traits of its congener, the Gulf moccasinshell.  Therefore, it may be a parent overwintering, summer releasing species that probably utilizes darters as hosts, as does the Gulf moccasinshell, Alabama moccasinshell (Haag and Warren 1997), and Cumberland moccasinshell (Lea, 1834) (Zale and Neves 1982).


RANGE AND POPULATION LEVEL: The Ochlockonee moccasinshell was described from the Ochlockonee River, Calvary, Grady County, Georgia.  This Ochlockonee River system endemic mussel was known historically from the main stem in Georgia and Florida, and the Little River (Johnson 1977, Butler 1993, Williams and Butler 1994).  The Service has provided a table of occurrences of Ochlockonee moccasinshell (Table 4, USFWS 2003).


Museum records for the Ochlockonee moccasinshell indicate that it was historically common, including two Ochlockonee River sites (21 and 24 specimens collected twice from a single site in the early 1930s, 19 from another collected in 1969) (J.D. Williams, USGS, unpub. data).  Even as late as the early 1970s this species was found in some numbers above Talquin Reservoir, Florida (W.H. Heard, Florida State University, pers. comm., 1994). This species may now be the rarest mussel currently inhabiting the Apalachicolan Region and is one of the rarest mussels nationwide (USFWS 2003).  Only three live specimens are known to have been collected since 1974 despite concerted efforts by numerous investigators (J.D. Williams, USGS, unpub. data).  The most recent live specimen was collected during the status survey in 1 of 4 hand-picked 97-square foot quadrats (J. Brim Box, USGS, unpub. data).  Currently, this species persists in only a relatively short reach of Ochlockonee River above Talquin Reservoir (Table 4, USFWS 2003).


HABITAT:  The Ochlockonee moccasinshell inhabits large creeks and the Ochlockonee River main stem in areas with current.  Typical substrates are sand with some gravel (Williams and Butler 1994).


PAST THREATS:  The abundance and distribution of the Ochlockonee moccasinshell decreased historically from habitat loss and degradation (Williams et al. 1993, Neves 1993) caused by impoundments (Talquin Reservoir), sedimentation and turbidity, dredging and channelization, and contaminants contained in numerous point and nonpoint sources.  A comprehensive review of these past threats is provided elsewhere (USFWS 2003, Brim Box and Williams 2000, Butler 1993, Richter et al. 1997, Watters 1997, Neves et al. 1997).  However, the histories of anthropogenic impacts specifically to the Ochlockonee river drainage have not been summarized.  These habitat changes have resulted in significant extirpations (localized loss of populations), restricted and fragmented distributions, and poor recruitment of young. 


CURRENT THREATS: Due to the extreme rarity of the Ochlockonee moccasinshell, any adverse impacts to its habitat or an individual could result in the potential demise of the species. 


Habitat loss and degradation (Williams et al. 1993, Neves 1993) primarily caused by contaminants contained in point and nonpoint source discharges, sedimentation and erosive land practices, water quantity and withdrawal, construction of new impoundments and alien species are primary threats to the Ochlockonee moccasinshell (USFWS 2003).  Low populations levels and restricted ranges now render these mussels extremely vulnerable to toxic chemical spills and other catastrophic events, and the deleterious effects of genetic isolation.


Although effluent quality has improved with modern treatment technologies and a phosphate detergent ban, hundreds of miles of streams in the Ochlockonee basin in Georgia and Florida, as identified in reports prepared by the water quality agency of the states under Section 305(b) of the Clean Water Act, do not meet water use classifications. 


Many southern streams have increased turbidity levels due to siltation (van der Schalie 1938).   The Ochlockonee moccasinshell attracts host fishes with visual cues, luring fish into perceiving that their glochidia are prey items.  Such a reproductive strategy depends on clear water during the critical time of the year when mussels are releasing their glochidia (Hartfield and Hartfield 1996).  Turbidity is a limiting factor impeding sight-feeding fishes (Burkhead and Jenkins 1991).  In addition, mussels may be indirectly affected when turbidity levels significantly reduce light available for photosynthesis and the production of unionid food items (Kanehl and Lyons 1992).


Maintaining vegetated riparian buffer zones adjacent to stream banks is a well-known method of reducing stream sedimentation and other runoff (Allan and Flecker 1993, Lenat and Crawford 1994).  Buffers reduce impacts to fish and other aquatic faunas (Armour et al. 1991, Naiman et al. 1988, Osborne and Kovacic 1993, Belt and O’Laughlin 1994, Penczak 1995, Rabeni and Smale 1995), and are particularly crucial for mussels (Neves et al. 1997).  Riparian forest removal in southeastern streams and subsequent sedimentation has been shown to be detrimental to fish communities (Burkhead et al. 1997, Jones et al. 1999).  Particularly affected in the study by Jones et al. (1999) were benthic-dependent species (e.g., darters, benthic minnows, sculpins), which were found to decrease in abundance with longer deforested patches of riparian area.  Benthic-dependent fishes, themselves disproportionately imperiled (Burkhead et al. 1997), commonly serve as hosts for numerous imperiled mussel species (Watters 1994), probably including the Ochlockonee moccasinshell. 


Water quantity is becoming more of a concern in maintaining mussel habitat in the Apalachicolan Region.  The potential impacts to mussels, their host fishes, and their respective habitats from ground water withdrawal may be profound.  Low DO conditions in stagnating stream pools due to drought conditions are having a disastrous effect on these mussels.  Mussel mortality increases dramatically as DO decreases below 5 mg/L (Johnson et al. 2001). 




Exposure Scenario Summary Table for the Ochlockonee Moccasinshell




Life Stage





















contact with

water, diet


fish body



host fish(es)









contact & ingestion of

water, diet,



filter feeder


algae, detritus,









Allan, J.D. and A.S. Flecker. 1993. Biodiversity conservation in running waters.  BioScience 43(1):32-43.


Armour, C.L., D.A. Duff, and W. Elmore.  1991.  The effects of livestock grazing on riparian and stream ecosystems.  Fisheries 16(1):7-11.


Belt, W.G., and J.O’Laughlin.  1994.  Buffer strip design for protecting water quality and fish habitat.  Western Journal of Applied Forestry 9(2):41-45.


Brim Box, J., and J.D. Williams.  2000.  Unionid mollusks of the Apalachicola Basin in Alabama, Florida, and Georgia.  Bulletin of the Alabama Museum of Natural History No. 22. 143 pp.


Burkhead, N.M. and R.E. Jenkins. 1991. Fishes.  Pages 321-409 in: K. Terwilliger, coordinator, Virginia’s endangered species.  McDonald and Woodward Publishing Co., Blacksburg, Virginia.


Burkhead, N.M., S.J. Walsh, B.J. Freeman, and J.D. Williams.  1997.  Status and restoration of the Etowah River, an imperiled Southern Appalachian ecosystem.  Pages 375-441 in: G.W. Benz and D.E. Collins, eds.  Aquatic fauna in peril: the southeastern perspective.  Special Publication 1, Southern Aquatic Research Institute, Chattanooga, Tennessee.


Butler, R.S. 1993.  Results of a status survey for eight freshwater mussels (Bivalvia: Unionidae) endemic to eastern Gulf Slope drainages of the Apalachicolan Region of southeast Alabama, southwest Georgia, and north Florida.  Unpublished report, U.S. Fish and Wildlife Service, Jacksonville, Florida.  41 pp.


Churchill, E.P., Jr., and S.I. Lewis.  1924.  Food and feeding in freshwater mussels.  Bulletin of the Bureau of Fisheries 39:439-471.


Coker, R.E., A.F. Shira, H.W. Clark, and A.D. Howard.  1921.  Natural history and propagation of freshwater mussels.  Bulletin of the U.S. Bureau of Fisheries 37:77-181.


Fuller, Samuel L.H. 1974. Chapter 8: Clams and mussels (Mollusca: Bivalvia), in: Pollution ecology of freshwater invertebrates. pp. 215‑73, Hart and Fuller (eds.) Academic Press.


Haag, W.R., and M.L. Warren, Jr. 1997.  Host fish and reproductive biology of six freshwater mussel species from the Mobile Basin, USA.  Journal of the North American Benthological Society 16:576-585.


Hartfield, P.D., and E. Hartfield.  1996.  Observations on the conglutinates of Ptychobranchus greeni (Conrad, 1834) (Mollusca: Bivalvia: Unionoidea).  American Midland Naturalist 135:370-375.


Johnson, R.I.  1977.  Monograph of the genus Medionidus (Bivalvia: Unionidae) mostly from the Apalachicolan Region, southeastern United States.  Occasional Papers on Mollusks, Museum of Comparative Zoology 4(56):161-187.


Johnson, P.M., A.E. Liner, S.W. Golladay, and W.K. Michener.  2001.  Effects of drought on freshwater mussels and instream habitat in coastal plain tributaries of the Flint River, southwest Georgia (July-October 2000).  Final Report to The Nature Conservancy. 



Jones, E.B.D., III, G.S. Helfman, J.O. Harper, and P.V. Bolstad.  1999.  Effects of riparian forest removal on fish assemblages in Southern Appalachian streams.  Conservation biology 13(6):1454-1465.


Kanehl, P., and J. Lyons.  1992.  Impacts of in-stream sand and gravel mining on stream habitat and fish communities, including a survey on the Big Rib River, Marathon County, Wisconsin.  Unpublished report, Wisconsin Department of Natural Resources Research Report 155.  32 pp.


Kraemer, L.R. 1979.  Corbicula (Bivalvia: Sphaeriacea) vs. indigenous mussels (Bivalvia: Unionacea) in U.S. rivers: a hard case for interspecific competition?  American Zoologist 19:1085-1096.


Lenat, D.R., and J.K. Crawford.  1994.  Effects of land use on water quality and aquatic biota of three North Carolina Piedmont streams.  Hydrobiologia 294:185-199.


Naiman, R.J., H. Decamps, J. Pastor, and C.A. Johnston.  1988.  The potential importance of boundaries to fluvial ecosystems.  Journal of the North American Benthological Society 7:289-306.


Neves, R.J. 1993. A state-of-the-unionids address.  Pages 1-10 in: K.S. Cummings, A.C. Buchanan, and L.M. Koch, eds.  Conservation and management of freshwater mussels.  Proceedings of the UMRCC symposium, 12-14 October 1992, St. Louis, Missouri.  Upper Mississippi River Conservation Committee, Rock Island, Illinois.


Neves, R.J., A.E. Bogan, J.D. Williams, S.A. Ahlstedt, and P.D. Hartfield.  1997.  Status of aquatic mollusks in the southeastern United States: a downward spiral of diversity.  Pages 43-85 in G.W. Benz and D.E. Collins, eds.  Aquatic fauna in peril: the southeastern perspective.  Special Publication 1, Southeast Aquatic Research Institute, Lenz Design and Communications, Decatur, Georgia. 


Osborne, L.L., and D.A. Kovacic.  1993.   Riparian vegetated buffer strips in water-quality restoration and stream management.  Freshwater Biology 29:243-258.


Penczak, T.  1995.  Effects of removal and regeneration of bankside vegetation on fish population dynamics in the Warta River, Poland.  Hydrobiologia 303:207-210.


Rabeni, C.F., and M.A. Smale.  1995.  Effects of siltation on stream fishes and the potential mitigating role of the buffering riparian zone.  Hydrobiologia 303:211-219.


Richter, B.R., D.P. Braun, M.A. Mendelson, and L.L. Master.  1997.  Threats to imperiled freshwater fauna.  Conservation Biology11:1081-1093.

U.S. Fish and Wildlife Service.  2003.  Recovery Plan for Endangered Fat Threeridge (Amblema neislerii), Shinyrayed Pocketbook (Lampsilis subangulata), Gulf Moccasinshell (Medionidus penicillatus), Ochlockonee Mocccasinshell (Medionidus simpsonianus), and Oval Pigtoe (Pleurobema pyriforme); and Threatened Chipola Slabshell (Elliptio chipolaensis), and Purple Bankclimber (Elliptoideus sloatianus).  Atlanta, Georgia.  142 pp.


U.S. Fish and Wildlife Service.  1998.  Endangered and threatened wildlife and plants; determination of endangered status for five freshwater mussels and threatened status for two freshwater mussels from the eastern Gulf Slope drainages of Alabama, Florida, and Georgia.  Federal Register 63:12664-12687.


van der Schalie, H. 1938. Contributing factors in the depletion of naiades in eastern United States.  Basteria 3(4):51-57.


Watters, G.T.  1994.  An annotated bibliography of the reproduction and propagation of the Unionoidea (primarily of North America).  Ohio Biological Survey Miscellaneous Contributions No. 1.  158 pp.


Watters, G.T.  1997.  Freshwater mussels and water quality: a review of the effects of hydrologic and instream habitat alterations.  In P.D. Johnson and R.S. Butler, ed.  Freshwater Mollusk Symposium Proceedings-Part II: Proceedings of the 1st Symposium of the Freshwater Mollusk Conservation Society, March 17-19, 1999, Chattanooga, Tennessee.  Ohio Biological Survey, Columbus.


Williams, J.D. and R.S. Butler.  1994.  Class Bivalvia, freshwater bivalves.  Pages 53-128, 740-742 in R. Ashton, ed.  Rare and endangered biota of Florida.  Volume 6.  Invertebrates.  University of Florida Press, Gainesville.


Williams, J.D., M. L. Warren, Jr., K.S. Cummings, J.L. Harris, and R.J. Neves. 1993.  Conservation status of freshwater mussels of the United States and Canada.  Fisheries (Bethesda) 18(9):6-22.


Yeager, M.M., D.S. Cherry, and R.J. Neves.  1994. Feeding and burrowing behaviors of juvenile rainbow mussels, Villosa iris (Bivalvia: Unionidae).  Journal of the North American Benthological Society 13(2):217-222.


Zale, A.V., and R.J. Neves.  1982. Fish hosts of four species of lampsiline mussels (Mollusca: Unionidae) in Big Moccasin Creek, Virginia.  Canadian Journal of Zoology 60:2535-2542.