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Medionidus walkeri (Wright, 1897)
Suwannee MoccasinshellFederal Protection: Listed Threatened
State Protection: Endangered
Global Rank: G1
State Rank: SH
Element Locations Tracked in Biotics: Yes
SWAP High Priority Species (SGCN): Yes
Element Occurrences (EOs) in Georgia: 1
Habitat Summary for element in Georgia: Medium creeks and rivers in slow to moderate current; muddy sand, sand, and gravel
Small and elongate to 53 mm. Anterior margin rounded, posterior margin bluntly pointed. Sculpture always present on posterior slope. Shell disk generally smooth and moderately inflated. Young individuals yellowish green sometimes with faint rays. Older individuals chestnut to dark brown and generally lacking rays (Williams et al., 2008; Williams et al., 2014). M. walkeri is subtly sexually dimorphic with females being less dorsoventrally compressed giving them a taller appearance as compared to the more elongate males. Periostracum is generally smooth but can appear clothy in some cases. Nacre white to blueish white. Pseudocardinal teeth moderate in size and umbo pocket is shallow. Soft tissue for this species is tan to pale orange. Gravid females display small patches of elongated papillae as a host fish attractant (see life history).
M. walkeri is distinct from all other species in the Suwannee River basin and is easily identified by the combination of its oval shape and sculpture on the posterior slope. C. kleiniana also has sculpture on the posterior slope but is rounded or square in profile.
M. walkeri prefers stable sand or sand-silt mixtures in small creeks and rivers. M. walkeri has been observed frequently on sloping banks away from stream margins. They have also been detected frequently within proximity of spring outflows; however, this could be coincidental. This species prefers moderate flows and has been rarely, if ever, observed in low flow areas, oxbows, ponds, or impoundments.
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, M. walkeri has an obligate parasitic life cycle. Larval mussels, known as glochidia, develop in water tube present in the gills of the female mussel. This species utilizes a modified structure and active luring behavior to attract host fish. The lure of M. walkeri is a combination of a small patch of elongated papillae along the posterior mantle margin and a bright blue-white mantle coloration. M. walkeri is considered a long-term brooder and has been observed gravid between fall and early spring. This brooding period is consistent with other members of the genus Medionidus. A displaying female will rapidly flash the bright white interior of the mantle tissue open and closed while simultaneously flexing the elongate papillae on the mantle margin creating a color, motion, and structure which piques the interest of this species’ fish host. This lure is thought to mimic wiggling insect larvae. Glochidia are released when disturbed by an appropriate host fish. 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 sand where they feed on detritus using their foot and eventually grow into filter-feeding, adult mussels. Host fish species which have successfully transformed M. walkeri larvae in laboratory inoculations include Percina nigrofasciata and Etheostoma edwini. This result, combined with the insect larva mimic lure of M. walkeri, seems to indicate that darter species may be its primary host (Johnson et al., 2016). The list of fish species exposed to M. walkeri glochidia is far from exhaustive, and other fish species may be suitable or ever preferred hosts in a natural setting.
Surveyors should consider sampling during periods when female individuals are spawning or brooding as this species may have higher detection rates during this period. Investigators should conduct tactile searches of unconsolidated, but stabilized, habitats (i.e. sand around an embedded log) away from stream margins in moderate flow areas. Special attention should be paid to habitats where host species might be likely to spend time.
M. walkeri is endemic to the Suwanee River basin in GA and FL. In Georgia, this species is only known to have occurred in the upper Withlacoochee River. The most recent record for M. walkeri from Georgia was collected in 1968 from just above the FL/GA state line.
Like most native freshwater mussels in Georgia, M. walkeri is threatened by habitat destruction in the form of flow alteration, excessive water withdrawal, watershed land use changes, impoundments, and pollution. Freshwater mussels are dependent on fish hosts to complete their life cycle; thus, threats to host fish or barriers to host fish movement are also serious threats.
Conservation of M. walkeri will primarily rely on habitat protection in the Suwannee River basin in the form of drainage area, riparian, and water resource management. Further research into the specific life history, host fish, and habitat requirements of this species are still needed to guide management efforts. Additional surveys need to be conducted to confirm whether M. walkeri is still extant in the state of Georgia.
Johnson, N.A., Mcleod, J., Holcomb, J.M., Rowe, M., & Williams, J.D. (2016). Early life history and spatiotemporal changes in distribution of the rediscovered Suwannee moccasinshell Medionidus walkeri (Bivalvia: Unionidae).
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.
Matthew Rowe
1/9/2020