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Medionidus simpsonianus Walker, 1905
Ochlockonee Moccasinshell
Federal Protection: Listed Endangered
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: 2
Habitat Summary for element in Georgia: Medium sized river to large creeks with moderate current; muddy sand, sand, and gravel substrates
Shell profile is sub-rhomboidal to elliptical in outline, and the shell is rather delicate and rarely exceeds 55 mm (2⅛ inches) in length. 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 broadly rounded with well developed corrugations present on the posterior slope. The periostracum is dark brown to green with broken rays. Pseudocardinal teeth are short and triangular while lateral teeth are slightly curved. Umbo cavity shallow. Nacre color bluish-white to iridescent.
None
Typically occupies small streams to large rivers with moderate flow and sandy 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).
The life history of Ochlockonee Moccasinshell is unknown mainly as a result of the rarity of this species. However, it is assumed to be similar to that of other individuals in the genus Medionidus, which brood glochidia from October to mid-summer and parasitize darters.
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.
Ochlockonee Moccasinshell is endemic to the Ochlockonee River of Florida and Georgia. Historically, this species was known from 7 locations in the Ochlockonee River. It was thought to be extinct until summer 2014 when several live individuals were collected in the lower Ochlockonee River in Florida (Holcomb et al. 2015). No recent records of this species have been found in 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. Excessive water withdrawals in Southwest Georgia coupled with severe drought could cause this species to become extirpated from Georgia. Excess sedimentation due to inadequate riparian buffer zones also covers suitable habitat and potentially bury individuals. This species is also though to be impacted by discharges from wastewater treatment plants located in several large municipalities in the headwaters in Georgia.
The Ochlockonee Moccasinshell is not known from any state or federal lands in Georgia. 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.
In order to better assess the status of the Ochlockonee Moccasinshell in Georgia, extensive surveys are needed in the Ochlockonee River Basin. Developing and implementing environmental DNA (eDNA) methods may be useful to help locate extant populations of the species. The Ochlockonee River of Georgia is one of the most under-surveyed basins in the state. In addition, the 2015 State Wildlife Action Plan technical team recommended developing environmental flows criteria for the Ochlockonee River Basin.
Holcomb, J., M. Rowe, J. Williams, and S. Pursifull. 2015. Discovery of the Ochlockonee Moccasinshell, Medionidus simpsonianus, in the Lower Ochlockonee River, Florida. Southeastern Naturalist 14:714-720.
Vaughn C.C. and C.C. Hakenkamp. 2001. The functional role of burrowing bivalves in freshwater ecosystems. Freshwater Biology 46: 1431-1446.
Jason Wisniewski
2018-03-19