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Home/Species/ Ochlockonee Moccasinshell

Medionidus simpsonianus

Walker, 1905

Ochlockonee Moccasinshell

G1Critically Imperiled Found in 2 roadless areas NatureServe Explorer →
G1Critically ImperiledGlobal Rank
Critically endangeredIUCN
Very highThreat Impact
Ochlockonee moccasinshell (Medionidus simpsonianus). Photo by U.S. Fish & Wildlife Service, Public Domain (U.S. Government Work), via ECOS.
U.S. Fish & Wildlife Service, https://www.usa.gov/government-works
Identity
Unique IDELEMENT_GLOBAL.2.120661
Element CodeIMBIV28070
Record TypeSPECIES
ClassificationSpecies
Classification StatusStandard
Name CategoryInvertebrate Animal
IUCNCritically endangered
Endemicendemic to a single nation
KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderUnionoida
FamilyUnionidae
GenusMedionidus
Other Common Names
Ochlockonee moccasinshell (EN)
Concept Reference
Turgeon, D. D., J. F. Quinn, Jr., A. E. Bogan, E. V. Coan, F. G. Hochberg, W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G. Thompson, M. Vecchione, and J. D. Williams. 1998. Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks. 2nd Edition. American Fisheries Society Special Publication 26, Bethesda, Maryland. 526 pp.
Taxonomic Comments
Clench and Turner (1956) recognized only one species of Medionidus from the Suwannee, Ochlockonee, and Apalachicola River systems. Their records of Medionidus penicillatus from the Ochlockonee River are now recognized as Medionidus simpsonianus, and their Suwannee River records are now recognized as Medionidus walkeri (Johnson, 1977).
Conservation Status
Rank Method Rank calculation - Biotics v2
Review Date2025-04-05
Change Date2000-08-31
Edition Date2025-04-05
Edition AuthorsCordeiro, J. (2007); Jackson, D. R. (2014); rev. T. Cornelisse (2025)
Threat ImpactVery high
Range Extent100-250 square km (about 40-100 square miles)
Number of Occurrences1 - 5
Rank Reasons
This species is critically imperiled due to its limited distribution and substantial long-term population declines caused by many ongoing threats.
Range Extent Comments
The species occurs in the Ochlockonee River in Florida and, at least historically, in adjacent southern Georgia USA with a historical range of 216 km (USFWS 2003; 2019).
Occurrences Comments
This species is known from only one population since 2014 (USFWS 2019).
Threat Impact Comments
This species is threatened by habitat loss and degradation associated with dams and other waterway barriers, channelization, and mining; pollutants in wastewater discharges, including from sewage treatment plants and industrial operations; runoff of silt, fertilizers, pesticides, and other pollutants from land disturbance activities, such as development and agriculture implemented without adequate measures to control runoff; and climate change resulting in droughts and salt water intrusion (USFWS 2003; FNAI 2020; USFWS 2019; Pursifull et al. 2021).
Ecology & Habitat

Description

See Johnson (1977) and Deyrup and Franz (1994). This species is slightly elongate-elliptical in outline, the posterior end obtusely rounded at the shell's median line and the ventral margin broadly curved. The posterior ridge is moderately angular and covered in its entire length with well developed, irregular ridges. Sculpture may also extend onto the disk below the ridge. Surface texture is smooth. 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. Lateral teeth are thin and straight and pseudocardinal teeth are compressed. There are two laterals and two pseudocardinals in the left valve and one lateral and one pseudocardinal tooth in the right valve. Nacre is bluish white (Butler and Alam, 1999).

Diagnostic Characteristics

Unique outline with ventral margin broadly curved, posterior point medial in position, dark shell with dark green rays forming chevrons, some sculpture on posterior slope.

Habitat

This species is found in a main river stem in areas with current in sandy substrates with some light gravel in mid-channel areas (USFWS, 2003).
Other Nations (1)
United StatesN1
ProvinceRankNative
FloridaS1Yes
GeorgiaS1Yes
Threat Assessments
ThreatScopeSeverityTiming
3 - Energy production & miningLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
3.2 - Mining & quarryingLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
7 - Natural system modificationsLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
7.2 - Dams & water management/useLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
9 - PollutionLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
9.1 - Domestic & urban waste waterRestricted (11-30%)Serious or 31-70% pop. declineHigh (continuing)
9.2 - Industrial & military effluentsRestricted (11-30%)Serious or 31-70% pop. declineHigh (continuing)
9.3 - Agricultural & forestry effluentsRestricted (11-30%)Serious or 31-70% pop. declineHigh (continuing)
11 - Climate change & severe weatherPervasive (71-100%)Moderate or 11-30% pop. declineHigh (continuing)
11.2 - DroughtsPervasive (71-100%)Moderate or 11-30% pop. declineHigh (continuing)
11.4 - Storms & floodingRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)

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Roadless Areas (2)
Florida (2)
AreaForestAcres
Gum BayApalachicola National Forest11,645
Long BayApalachicola National Forest5,726
References (34)
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  2. Clench, W. J. and R. D. Turner. 1956. Freshwater mollusks of Alabama, Georgia, and Florida from the Escambia to the Suwanee River. Bulletin of the Florida State Museum Biological Sciences 1(3):97-239.
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