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Medionidus walkeri 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Unionidae

Scientific Name: Medionidus walkeri (Wright, 1897)
Common Name(s):
English Suwannee Moccasinshell
Taxonomic Notes: 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).

A list of synonyms for this species can be found on The MUSSEL project web site (Graf and Cummings 2011).

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv) ver 3.1
Year Published: 2012
Date Assessed: 2012-03-05
Assessor(s): Cummings, K. & Cordeiro, J.
Reviewer(s): Bohm, M., Collen, B. & Seddon, M.
Contributor(s): Richman, N., Dyer, E., Soulsby, A.-M., Whitton, F., Kasthala, G., McGuinness, S., Milligan, HT, De Silva, R., Herdson, R., Thorley, J., McMillan, K., Collins, A., Offord, S. & Duncan, C.
Justification:
Medionidus walkeri has been assessed as Critically Endangered under criterion B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), as it is a localised endemic and has experienced significant declines in populations, demonstrates a limited distribution, restricted habitat, and has a decreasing number of extant occurrences due to deteriorating habitat and water quality in some portions of its range and faces many threats. Only one individual from a single site has been collected in the last decade, although multiple sites throughout the Suwannee River system have been surveyed for unionid mussels (Brim Box pers. obs. 2000). Previously, it was also collected from a second site, but again in very low numbers. Whether it still exists at that site is questionable. Because of this, we employ a precautionary approach, and use lower margins of extent of occurrence and area of occupancy estimates, and lower numbers of locations, for the purpose of this assessment. Further research is required regarding this species' habitat and population trends, and the threats to this species. If it is found in more locations during future surveys, this may lead to a reassessment in the Endangered category. Site protection and restoration would enable this species a degree of security.
Previously published Red List assessments:

Geographic Range [top]

Range Description:This species is endemic to the Florida portion of the Suwannee River system (Johnson 1977), and known from 11 historical occurrences. For example, it is listed from the Withlacoochee, Suwannee, and Santa Fe drainages. It occurs in the lower Withlacoochee, from its confluence with the main stem downstream to about Levy County, and in the upper Santa Fe river system (above the sinks), including both main stem and New River sites. It is apparently absent from large sections of these areas, particularly in the Suwannee main stem. It is one of only a few unionids restricted to Florida. Its extent of occurrence is estimated to be between less than 100 km2 to 250 km2 and its area of occupancy is likely to be between 4 and 20 km2.
Countries occurrence:
Native:
United States (Florida)
Additional data:
Estimated area of occupancy (AOO) - km2:4-20
Number of Locations:1-2
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:This species is endemic to the Florida portion of the Suwannee River system, where it is known from eleven historical occurrences. It was historically known from four occurrences on the main stem of the Suwannee River, one on the Withlacoochee River, five in the Santa Fe River and one in the New River. The only recent live collections of this species are from the Santa Fe River in 1981 and the New River in 1987 and 1994. In addition, a dead shell was located at the type locality (Suwannee River, Ellaville) in 1981. It may be extirpated from the Withlacoochee River and main stem of the Suwannee River.

Johnson (1977) stated it was abundant at the type locality in the Suwannee River in Florida but known from only small numbers of specimens elsewhere. Recent collections have been infrequent and few so the species has little or no viability remaining. Of the fourteen historical records located, only one contained over twenty specimens. The last site to harbour large populations yielded only three specimens in two surveys during 1987. Only one live specimen has been collected in the last decade throughout its historical range.

This species is exceedingly rare and is in significant decline. Only one individual from a single site has been collected in the last decade, although multiple sites throughout the Suwannee River system have been surveyed for unionid mussels (J. Brim Box pers. obs. 2000).
Current Population Trend:Decreasing
Additional data:
Population severely fragmented:Unknown

Habitat and Ecology [top]

Habitat and Ecology:It has been reported from mud and sand in slight to moderate current (Heard 1979, Johnson 1977), and in medium-sized creeks and rivers in muddy sand, sand and gravel, in slow to moderate current (Deyrup and Franz 1994). Like most members of the genus, it is most often found in mid-channel habitats in coarser sediments (Brim Box pers. obs. 2000).
Systems:Freshwater
Congregatory:Congregatory (and dispersive)

Use and Trade [top]

Use and Trade: It is likely that the species has been overharvested by collectors and biologists in the past. For example, it has previously been collected for museum collections.

Threats [top]

Major Threat(s): The main threats to this species are sedimentation due to agricultural and silvicultural activities; phosphate mining (upper Suwannee River main stem); industrial pollution (pulp mill in the Withlacoochee watershed) and localized municipal pollution; fairly high density residential development; watershed development. These threats have contributed to excessive eutrophication in the Suwannee main stem and have caused declines in the population. Overharvesting by shell collectors and biologists has been a distinct possibility; in the past 20 years, 20 specimens of this threatened species have been retained for collections at the GEXEMPSITE alone. Given already stressed populations throughout most or possibly all of its range, overcollecting can potentially contribute significantly to this species' decline.

The species is probably highly sensitive to runoff, sedimentation and habitat modifications; typical liabilities of filter-feeders (e.g., to excessive pollutants, eutrophication, etc.). Disappearance of host fish is a possibility.

Conservation Actions [top]

Conservation Actions: A historic occurrence is known from O'Leno State Park on the Santa Fe River. This species has been assigned a NatureServe Global Heritage Status Rank of G1 - Critically Imperiled, and a State/Province Status Rank of S1 - Critically Imperiled for Florida (NatureServe 2009). Williams et al. (2010) lists this species as endangered according to the AFS assessment. Further research is required regarding this species' habitat and population trends, and the impacts of threats to this species. Site protection and restoration would enable this species a degree of security.

Citation: Cummings, K. & Cordeiro, J. 2012. Medionidus walkeri. The IUCN Red List of Threatened Species 2012: e.T12930A510583. . Downloaded on 15 October 2018.
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