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Amblema neislerii (I. Lea, 1858)
Fat Threeridge
Federal Protection: Listed Endangered
State Protection: Endangered
Global Rank: G1
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: 5
Habitat Summary for element in Georgia: Rivers with fine sediments with low-moderate gradient and slow-moderate current; pools and riffles; substrate gravel/cobble to sand and sandy mud
Shell is thick and heavy and quadrate to rounded in profile, rarely exceeding 100 mm (3⅞ inches) in length. Periostracum dark brown to black with 7 - 9 parallel plications positioned horizontally across the shell. Valves typically inflated with 2 heavy pseudocardinal teeth in the left and one heavy tooth in the right. Umbo pocket shallow and nacre typically white.
In the Apalachicola, Chattahoochee, Flint Rivers (ACF) basin, Fat Threeridge and the Washboard (Megalonaias nervosa) may co-occur but can be distinguished from one another as the plications in the fat threeridge tend to be more parallel to the ventral margin of the shell and lack additional sculpturing near the disk. The Fat Threeridge tends to be relatively inflated compared to Washboards of similar sizes. The Washboard may also have a dorsal wing, often becoming less apparent in large individuals.
Fat Threeridge typically occupies small to large rivers with moderate current and stable sandy to silty substrates. Smit and Kaeser (2016) reported that mid-channels and point bars were poor habitats for Fat Threeridge whereas high densities were found in inner recirculation zones, outer recirculation zones, and pools along outer bends. Populations appear to be most dense along gently sloping banks with relatively low levels of shear stress.
Unionid diets are poorly understood but are believed to consist of algae and 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).
Mature glochidia of the Fat Threeridge were collected in May and early June. Successful transformation of glochidia occurred on 23 species with the highest metamorphosis rates on darters and minnows (Fritts and Bringolf 2014; 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. Suitable habitat for the Fat Threeridge is available at few sites in the lower Flint River but availability of habitat greatly increases upstream of Lake Blackshear. Additional sampling efforts should be focused on this section of the river.
Historically occurring in the Apalachicola, Flint, and Chipola Rivers of Florida and Georgia. Within the Flint River of Georgia, this species was once known from Baker, Decatur, Dougherty, Macon, and Mitchell Counties. Surveys conducted during the early 1990’s found the Fat Threeridge at only 11 of 324 sites surveyed in the ACF basin. No sites surveyed in Georgia had live individuals and only relict shells were collected within Lake Seminole near Bainbridge. One extant and reproducing population is currently known from Flint River along the Baker/Mitchell county line (Wisniewski et al. 2013; Wisniewski et al. 2014).
The Fat Threeridge appears to thrive in waters with minimal anthropogenic disturbance. Insufficient water flow in the lower Flint River basin may be impacting undiscovered populations that could occur in some creeks in the basin. Excess sedimentation due to inadequate riparian buffer zones covers suitable habitat and could potentially bury mussels. Direct and indirect competition by the introduced flathead catfish may be reducing native mussel populations through direct consumption of mussels and their host fishes.
| Threat 1 | Threat 2 | Threat 3 | |
|---|---|---|---|
| General Threat | Natural system modifications | Pollution | Agriculture & aquaculture |
| Specific Threat | Dams & water management/use | None | Annual & perennial non-timber crops |
The Fat Threeridge is not known to occur on any state properties 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.
The Flint River upstream of Lake Blackshear should be assessed for the potential of reintroduction/augmentation of this species to meet objectives identified in the recovery plan for this species. This assessment should include detecting any extant populations and assessing the effective popualtion size to ensure that such actions do no negatively affect the genetic integrity of the population. Development of environmental DNA methods and techniques may greatly increase the probability of detecting this species. Techniques for captive propagation of this species should be explored in the event that captive rearing becomes a viable option for recovering this species.
Brim Box, J. and J.D. Williams. 2000. Unionid mollusks of the Apalachicola Basin in Alabama, Florida, and Georgia. Alabama Museum of Natural History Bulletin 21. 143 pp.
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.
Smit, R. and A. Kaeser. 2016. Defining freshwater mussel mesohabitat associations in a alluvial, Coastal Plain river. Freshwater Science 35:1276-1290.
Vaughn C.C. and C.C. Hakenkamp. 2001. The functional role of burrowing bivalves in freshwater ecosystems. Freshwater Biology 46:1431-1446.
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
2018-03-16
