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Pleurobema curtum

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA BIVALVIA UNIONOIDA UNIONIDAE

Scientific Name: Pleurobema curtum
Species Authority: I. Lea, 1859
Common Name(s):
English Black Clubshell, Curtus's Mussel
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 B1ab(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:
Pleurobema curtum has been assessed as Critically Endangered as this regionally endemic species has declined significantly throughout its range and its extent of occurrence is now is believed to be less than 100 km2. There is no evidence of recruitment and no live specimens found within 10 years at the single known extant site and the species continues to be impacted by major threats. Any impact on the species is significant and the species may already be extinct. Further survey work it needed to determine whether this species is still extant.
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: Historically, this species was known from the Tombigbee River near Pickensville, Alabama, and the East Fork Tombigbee River downstream of its confluence with Bull Mountain Creek. Mirarchi et al. (2004) cite former distribution in Alabama as the mainstem of the Tombigbee River. A short section of the upper Tombigbee River in Mississippi (Hartfield and Jones 1989) contained a small persistent population until the mid-1980s but is now extirpated (Mirarchi et al. 2004). A single record from the Big Black River, Mississippi (Hartfield and Rummel 1985), is believed to be an error (USFWS 1989). The current range of the species appears to be limited to the East Fork Tombigbee River in Mississippi (USFWS 1989, 2000; Hartfield pers. comm. 1992) although no recent specimens were found at this single known extant site in over a decade, despite intensive surveys in 1990, 1991, 1992, 1993, 1997, 1999, and 2001 with the last dead shells collected in 1989 and 1990 (Hartfield pers. comm. 2003). It is extirpated from Alabama following construction of Tennessee-Tombigbee Waterway (Mirarchi et al. 2004). It is considered historical in Mississippi in the Tombigbee River drainage (Jones et al. 2005) with the unlikely potential that a living population might be found. The extent of occurrence is believed to be <100 km2 (NatureServe 2009).

Although specimens have not been found recently, this species may still be holding on with one or two populations but the exact area of occupancy, although very small, is not precise.
Countries:
Possibly extinct:
United States (Alabama, Mississippi)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Recent populations are all but extirpated with the last holdout at East Fork of the Tombigbee River (Itawamba/Monroe Co., Mississippi) (USFWS 2000) showing no live specimens in over a decade (Hartfield pers. comm. 2003). Population numbers are extremely low and live specimens have not been reported for over a decade (USFWS 2000, Hartfield and Jones 1989, USFWS 1989, Hartfield pers. comm. 2003).

The decline of the species has been over the last 25-50 years and has been hastened by the construction of the Tennessee-Tombigbee Waterway. Construction of the waterway has adversely impacted the species by physical destruction during dredging, increasing sedimentation, reducing water flow, and suffocating juveniles with sediment (USFWS 1989). A small population persisted until the mid-1980s in a short section of the upper Tombigbee River in Mississippi not directly modified by the waterway (Hartfield and Jones 1989) but it is believed to be extirpated as are all Alabama records (Mirarchi et al. 2004). The species is nearly extinct and is considered extirpated in Alabama by construction of the Tennessee-Tombigbee Waterway (Mirarchi et al. 2004).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: This species is found in riffles and shoals on sandy gravel to gravel-cobble substrates and with moderate to fast currents in lotic habitat (USFWS 2000, Mirarchi et al. 2004). This species requires clean freshwater. It is known only from flowing water in medium to large rivers. The preferred substrate of P. curtum is a mixture of sand and gravel or pure sand and may be found in water less than 1 m deep. It is probably a short-term brooder, gravid in spring and summer. Its glochidial hosts are unknown, but it may utilise members of the Cyprinidae (Haag and Warren 2003, Williams et al. 2008).
Systems: Freshwater

Use and Trade [top]

Use and Trade: This species is not utilised.

Threats [top]

Major Threat(s): The continued existence of Pleurobema curtum is dependant upon habitat in the tributaries of the Tombigbee River. The East Fork Tombigbee River is threatened by a clearing and snagging project, sand and gravel mining, the continued diversion of flows, and water removal for municipal use. Runoff of fertilisers and pesticides may adversely affect the species. Such runoff may exceed the streams' ability to assimilate, resulting in algal blooms and excesses in other aquatic vegetation. Pesticides which enter the stream are ingested by filter feeders such as P. curtum while being transported downstream (USFWS 1989). The species is nearly extinct and is considered extirpated in Alabama by construction of the Tennessee-Tombigbee Waterway (Mirarchi et al. 2004).

The principal cause of population decline is habitat modification for navigation. Waterway construction adversely affected this species by physical destruction during dredging, increasing sedimentation, reducing water flow, and suffocating juveniles with sediment. Deposition continues in remaining portions of the Tombigbee River that have not already been impacted severely. Water diversion is a continuing threat along with associated accumulation of sediment. Runoff of fertilisers and pesticides adversely affects mussels and leads to eutrophication (USFWS 1989).

In the Mobile River basin, the greatest threats are dams (for navigation, water supply, electricity, recreation, and flood control), channelisation (causing accelerated erosion, altered depth; and loss of habitat diversity, substrate stability, and riparian canopy), dredging (for navigation or gravel mining), mining (for coal, sand, gravel, or gold) in locally concentrated areas, pollution- point source (industrial waste effluent, sewage treatment plants, carpet and fabric mills, paper mills and refineries in mainstem rivers), and pollution- nonpoint source (construction, agriculture, silviculture, urbanization) USFWS 2000).

Low population levels cause increased difficulty in completing successful reproduction. When individuals become scattered, the opportunity for the female to become gravid is greatly diminished. With low population levels, any impact is a major threat (USFWS 1989). Isolated imperiled populations in the Mobile River basin are likely to be vulnerable to random accidents, such as toxic spills, and to naturally catastrophic events, such as droughts and floods, even if land use and human populations were to remain constant within isolated watersheds (USFWS 2000).

Conservation Actions [top]

Conservation Actions: This species was listed as federally endangered in the U.S. in 1987.

A recovery plan has been created for the species (and four others in the Tombigbee River) (USFWS 1989) which contains the following objectives: (1) protect the habitat where the species occurs, (2) determine habitat requirements and management needs and correct as necessary and feasible, (3) monitor existing populations at not more than 3-year intervals and recommend additional actions as needed, (4) solicit the assistance of the states, other Federal agencies, municipalities and conservation organizations in protecting the remaining habitat.

A specific recovery plan has been created for the Mobile River basin (USFWS 2000) which contains the following objectives: (1) protect habitat integrity and quality of river and stream segments that currently support or could support imperiled aquatic species, (2) consider options for free-flowing river and stream mitigation strategies that give high priority to avoidance and restoration, (3) promote voluntary stewardship as a practical and economical means of reducing nonpoint pollution from private land use, (4) encourage and support community-based watershed stewardship planning and action, (5) develop and implement programs and materials to educate the public on the need and benefits of ecosystem management, and to involve them in watershed stewardship, (6) conduct basic research on endemic aquatic species and apply the results toward management and protection of aquatic communities, (7) develop and implement technology for maintaining and propagating endemic species in captivity, (8) reintroduce aquatic species into restored habitats, as appropriate, (9) monitor listed species' population levels and distribution and periodically review ecosystem management strategy, (10) coordinate ecosystem management actions (more detail in USFWS 2000).

There are no known protected areas for the species. Research is needed into the continued distribution of this species, as well as threats; continued monitoring is needed, but is already implemented under the species recovery plan (see above).

This species has also been assigned a NatureServe Global Heritage Status Rank of GH - Possibly Extinct, and State/Province Status Ranks of SH - Possibly Extinct for Mississippi and SX - Presumed Extinct in Alabama (NatureServe 2009). Williams et al. (2010) lists this species as possibly extinct according to the AFS assessment.

Citation: Cummings, K. & Cordeiro, J. 2012. Pleurobema curtum. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 24 July 2014.
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