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Toxolasma cylindrellus

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA BIVALVIA UNIONOIDA UNIONIDAE

Scientific Name: Toxolasma cylindrellus
Species Authority: (Lea, 1868)
Common Name(s):
English Pale Lilliput, Pale Lilliput Naiad, Pale Lilliput Pearly Mussel
Synonym(s):
Carunculina cylindrella Lea 1868
Carunculina glans Lea 1831
Unio cylindrellus Lea 1868
Taxonomic Notes: 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 A2ace; B1ab(i,ii,iii,iv) ver 3.1
Year Published: 2012
Date Assessed: 2012-06-07
Assessor(s): Cummings, K. & Cordeiro, J.
Reviewer(s): Seddon, M., Bohm, M. & Collen, B.
Contributor(s): Bogan, A., 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:
Toxolasma cylindrellus has been assessed as Critically Endangered under criteria A2 and B1, as this species has experienced an inferred 80% reduction in population over three generations, as a result of habitat loss. As a result, it currently exists within a single location, with an estimated extent of occurrence of less than 100 km2, which continues to be impacted by many threat processes including siltation, erosion and pollution from non-point sources. Monitoring of populations in the single location is required to keep check of the population numbers.

The species was assessed as Critically Endangered using the B1 criterion in 1996, based on habitat loss, number of location and population decline (Bogan and Seddon, pers. copmm., 2012).
History:
1996 Critically Endangered
1994 Endangered (Groombridge 1994)
1990 Endangered (IUCN 1990)
1988 Endangered (IUCN Conservation Monitoring Centre 1988)
1986 Endangered (IUCN Conservation Monitoring Centre 1986)

Geographic Range [top]

Range Description: This species is endemic to the middle reaches of the Tennessee River, in Alabama and Tennessee, and the Duck River in middle Tennessee; it is presumed to be extinct in Georgia (NatureServe 2009). It was found in Big Rock Creek and Duck River in Tennessee and Paint Rock River and Hurricane Creek in Northern Alabama (Parmalee and Bogan 1998). It is now thought to currently only exist in the Estill Fork of the Paint Rock River drainage, although it appears to be a viable population (P. Johnson pers. comm. 2009). As this species has become restricted to a single river reach, it is highly susceptible to extinction (P. Johnson pers. comm. 2009).
NatureServe (2009) estimates its extent of occurrence in this single location as less than 100 km2.
Countries:
Native:
United States (Alabama, Georgia - Possibly Extinct, Tennessee)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Although the species seems viable, there is currently only one known river fork population (Fobian et al. 2008). NatureServe (2009) estimate the observed declines in this species, which have led to its occurrence in a single location, to be between 70 and 90% (although potentially more than 90% over the long term).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: This species is found in small tributaries and streams, in less than three feet of water (Parmalee and Bogan 1998). It prefers gravel and sandy substrates in either a slow or medium current (Parmalee and Bogan 1998).
This is a very small naiad, usually around 35 mm in length and is elongate and ellipitical (Parmalee and Bogan 1998).

It is a long-term brooder, gravid from late summer or autumn to the following summer. Glochidia have been found to utilize Lepomis macrochirus (Bluegill) (Centrarchidae) as a host in laboratory trials (P. Johnson pers. comm. 2010, Williams et al. 2008).
Direct life-history data are not available for this species. Freshwater mussels are highly variable in their longevity from species to species (e.g. Haag and Rypel 2011). Studies in other species of the genus Toxolasma have shown ages of between 5 and 11 years being recorded (Toxolasma parva, T. pullus, T. texasensis: average of 8-9 years; Haag and Rypel 2011). Assuming a similar longevity for Toxolasma cylindrellus, and conservatively assuming age of maturity to be somewhere between 2 and 9 years (average of 5-6 years; Haag and Staton 2003), we estimate a generation length (estimated as the average age of a parent in the population) of around four to ten years, with three generations spanning around 12 to 30 years. However, this is likely to represent an underestimate of generation length, as it has been suggested that growth ring counts may underestimate age by a factor of between three and ten (Anthony et al. 2001). Assuming that the declines occurred over the past 75 or so years (or between the 1940s and the 1960s, see Threats section), then declines of 90% over this time period is likely to translate into around 80% declines over three generations (assuming constant population declines). Since generation length is likely to be an underestimate, we are confident that an 80% decline in population size has occurred within three generations.
Systems: Freshwater

Use and Trade [top]

Use and Trade: This species is not utilized.

Threats [top]

Major Threat(s): The Paint Rock River and its tributaries have been severely affected in past decades by impoundments, stream channelization, erosion, and agricultural runoff (Fobian et al. 2008). These habitat influences have led to the possible extirpation of 13 mussel and eight fish species from the river system, within the past 75 years (Fobian et al. 2008). The river has been channelized before, during the 1960s, by the U.S. Army, who also removed snags and riverbank timber in the upper Paint Rock River and the lower reaches of Larkin Fork, Estill Fork and Hurricane Creek (Fobian et al. 2008). This direct headwater habitat manipulation was probably a large contributor to freshwater mussel loss in the basin (Fobian et al. 2008).

Continued threats to the watershed include siltation and erosion due primarily to poor farming practices along with commercial and residential development (Fobian et al. 2008). Threats to this species arise from non-point source agricultural runoff, direct and inadvertent impacts from pesticides, herbicides, fertilizers and chemical spills (Fobian et al. 2008). As this species has become restricted to a single river reach, it is highly susceptible to extinction (P. Johnson pers.comm. 2009).

Conservation Actions [top]

Conservation Actions: The Paint Rock River Protection Project is designed to protect the river by creating a long-term protective forest corridor along each side of the river (The United States Department of Agriculture 2009). Once in place, this protective corridor will help stabilize the stream bank, reduce flood scour erosion, help keep the river water clean, and provide wildlife habitat. In addition, the program will provide a direct cash payment to the landowner (The United States Department of Agriculture 2009). Population monitoring in the single remaining site is recommended.

Williams et al. (2010) lists this species as endangered according to the American Fisheries Society (AFS) assessment.

Citation: Cummings, K. & Cordeiro, J. 2012. Toxolasma cylindrellus. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 23 October 2014.
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