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Medionidus penicillatus (I. Lea, 1857)
Gulf Moccasinshell
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): Yes
SWAP 2025 Species of Greatest Conservation Need (SGCN): Yes
2025 SGCN Priority Tier: Highest Conservation Concern
Element Occurrences (EOs) in Georgia: 37
Habitat Summary for element in Georgia: Large rivers to small creeks; found in a variety of substrates
Shell profile is sub-rhomboidal to elliptical in outline. Shell rather delicate with a maximum length of approximately 55 mm (2¼ inches). Anterior margin broadly rounded while posterior margin is pointed and terminates near the posterior-ventral margin. Ventral margin is straight to slightly arcuate. Umbos positioned anterior of the middle of the valves and elevated to or just slightly above the hingeline. Posterior ridge is sharply developed with well developed plications present on the posterior slope. Pseudocardinal teeth are short and triangular while lateral teeth are slightly curved. The periostracum is yellow with fine, broken rays radiating from the umbo to the margin of the shell. Nacre color typically white.
None
Typically occupies small streams to large rivers with moderate flow and sandy substrates. This species has also been found in gravel and cobble 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 collected in Georgia from October until mid-summer. The Blackbanded Darter (Percina nigrofasciata) Halloween Darter (P. crypta), Gulf Darter (Etheostoma swaini), and Turquoise Darter (E. inscriptum) all successfully transformed glochidia with the Blackbanded Darter and Turquoise Darter having transformation rates in excess of 39%. However the Turquoise Darter does not occur in the ACF Basin (Fritts and Bringolf 2014). A previous host trial found that the Brown Darter (E. edwini) successfully transform glochidia of this species (O'Brien and Williams 2002).
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 may increase probability of detection.
This species is endemic to the Apalachicola River basin of Alabama, Georgia, and Florida. Historically known from the mainstem and tributaries of the Chipola, Chattahoochee, and Flint Rivers. Currently, this species appears to be rare or extirpated outside of the Chattahoochee and Flint River drainages of Georgia and has drastically declined within these drainages. Recent collections of this species have been made in Kirkland and Sawhatchee creeks in the Chattahoochee River Basin and in Ichawaynochaway, Chickasawhatchee, Whitewater, and Swift creeks as well as several smaller tributaries to the Flint River.
Water withdrawals in the Lower Flint River Basin coupled with severe drought could cause this species to become extirpated 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 | Natural system modifications | Natural system modifications | Pollution |
| Specific Threat | Dams & water management/use | Dams & water management/use | None |
The Gulf Moccasinshell 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 in-stream flow criteria and investigation of the dissolved oxygen and thermal tolerances of this species were identified as a high priority actions in the State Wildlife Action Plan. Suitable but unoccupied habitats appear to be relatively abundant in the basin, which may provide an opportunity to recover this species using re-introduction/augmentation techniques. However, prior to the initiation of any reintroduction/augmentation activities, studies of the effective population size should be completed to ensure that the genetic integrity of the species is not compromised by this management activity. Expanding monitoring efforts into one additional occupied stream was recommended by the 2015 State Wildlife Action Plan Mollusk technical team.
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., 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., 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
2018-03-19
