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Plethobasus cyphyus

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA BIVALVIA UNIONOIDA UNIONIDAE

Scientific Name: Plethobasus cyphyus
Species Authority: Rafinesque, 1820
Common Name(s):
English Sheepnose
Taxonomic Notes: Plethobasus cyphyus (Rafinesque, 1820), the "Sheepnose", was placed in the genus Plethobasus by Ortmann (1919) having previously been assigned to other genera such as Unio, Pleurobema, Margarita, and Margaron. There are various spellings of the epithet of this species such as cyphya, scyphius, cyphius, cyphia, cyphyum and cyphyus
A subspecies, Plethobasus cyphyus compertus (Frierson, 1911), has been noted in the upper Tennessee river basin by Ortmann (1918) and has been considered a headwaters form. This form is described as having weak development of the medial row of knobs and sulcus, an elongated shell and pale coloured tissues. It is recommended that a systematic examination of the two forms should be undertaken in order to determine any taxonomic significance. It is also recommended to examine all species in the genus Plethobasus to confirm relatedness.

Assessment Information [top]

Red List Category & Criteria: Endangered A2ce ver 3.1
Year Published: 2014
Date Assessed: 2013-12-04
Assessor(s): Bogan, A.E. & Cummings, K.
Reviewer(s): Böhm, M. & Seddon, M.B.
Contributor(s): Ledger, S., Carrete-Vega, G. & Cordeiro, J.
Justification:

Plethobasus cyphyus is classified as Endangered under A2ce. This species occupies a large extent of occurrence exceeding threshold values under criterion B, although within this, its geographic range has declined by approximately two thirds from its historically recorded range. The habitat in which it occupies has declined in quality due to pollutants, impoundments, channelization, mining and sediments. The population is estimated to be between 2,500 and 100,000 individuals and it is inferred to be rare in occurrence and recorded to experience sharp declines in numbers. It is estimated that the species has undergone a decline in population by 50% since the 1940s (or three generation lengths). In light of this reduction of habitat quality and population numbers of the Sheepnose, all known populations should be protected. This species is listed in the U.S. Federal Endangered species list (U.S. Fish and Wildlife Service 2012). The water quality and river designation should be monitored and improved by working with local authorities. Further research into the species life history and ecology which particular attention to the larval stages and age at which the species reaches maturity should be undertaken. Investigation into the tolerance levels to pollutants and environmental changes would also be recommended. The reintroduction or relocation of Sheepnose populations into areas where the species has been extirpated could aid an increase in population numbers.

The species was previously assessed as Near Threatened, based on the limited data set, which has since been revised, and is now more extensive, and hence the difference in category is due to better information compiled on decline rates and generation length.

Geographic Range [top]

Range Description:

The Sheepnose is found in certain North American rivers and reservoirs. Historically this species was found throughout the Mississippi River system apart from the upper Missouri River system and the lowland tributaries in the lower Mississippi River system. These included 77 streams and one canal in 15 states and three Service regions (3, 4, and 5). These states are Minnesota, Wisconsin, Iowa, Illinois, Missouri, Ohio, West Virginia, Indiana, Kentucky, Tennessee, Alabama, Mississippi, Pennsylvania, Virginia and Arkansas (U.S. Fish and Wildlife Service, 2003). The Sheepnose is currently known from the Mississippi, Ohio, Cumberland, Tennessee and Ohio main stems and tributary streams throughout the range. It is also still found in the Yazoo River drainage of Big Sunflower River, but only archaeological remains remain in Big Black (Jones et al. 2005). It is no longer known to occur in the State of Arkansas (USFWS 2003, Butler 2003, Cummings and Mayer 1997, Parmalee and Bogan 1998). It now occurs in geographically isolated populations which may or may not be viable given that the remaining populations are generally small.

Countries:
Native:
United States (Alabama, Arkansas - Regionally Extinct, Illinois, Indiana, Iowa, Kentucky, Minnesota, Mississippi, Missouri, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, Wisconsin)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population:

The population size is difficult to estimate due to the species' historically wide distribution and occurrences in large rivers. However, it is estimated to be between 2,500 and 100,000 individuals. The remaining populations have been observed to undergo sharp declines and are considered to be rare in the ecosystems in which they occur. This trend has been observed by qualitative sampling and via their absence from commercial shell harvests. Densities of these remaining populations have been estimated to be 0.03 - 0.02 mussels per square meter (Jenkinson and Ahlstedt 1988). It now occurs in geographically isolated populations which may or may not be viable given that the remaining populations are generally small. 

Taking the species average of three generation lengths at its most conservative estimate of 75 years (see Habitats and Ecology section), the percentage decline in geographic range from the 1940s would be equal to approximately 60%. To account for any discrepancy between the percentage decline in geographic range and population we can infer the decline in population to be 50%. This species is very rare where it does occur in the range, thus we can assume that the percentage decline in population relatively closely follows that of the percentage decline in its geographic range.

Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology:

This mussel is considered to be a large-river species, however, it has also been found in medium sized rivers. It may be associated with riffles and gravel or cobble substrates but usually has been recorded in deep water at depths greater than two meters in slight to swift currents, sand, mud or gravel bottoms (Gordon and Layzer 1989). Specimens in larger rivers may occur in deep runs (Parmalee and Bogan 1998) and it also appears capable of surviving in reservoirs, such as upper Chickamauga Reservoir immediately below Watts Bar Dam (Ahlstedt 1989). 

To get an accurate estimate of the species life span where no data are directly available, we infer the life history and longevity from related species. Plethobasus was previously listed as a subgenus of Pleurobema and a study by Haag and Rypel in 2010 found the maximum age of populations of three Pleurobema species (Pleurobema coccineum, Pleurobema collina and Pleurobema decisum) examined to range from 19 to 45 years old (average of 31 years). However, other unionid species have longer life expectancies and age assessments based on counting the growth rings can be an underestimate of a factor of between three and 10 (Anthony et al. 2001), thus we use the upper estimate of age. A study which investigated the fecundity and maturity of freshwater mussels found that age at maturity ranged from one to nine years across five species of molluscs studied (Haag and Staton 2003). However, there were no Plethobasus or Pleurobema species represented in the investigation, therefore estimates based on this study must be used conservatively. Conservatively assuming age of maturity to be around five years (though this may be less in reality), if fecundity does not tail off over time, the average age of a parent (our estimate of generation length in the absence of more detailed information) is estimated to be between 12 and 25 years. Any population decline in this species would therefore have to be assessed over a time period of three generations, or between 36 (lower estimate) and 75 years (upper estimate). Because of issues with underestimating age based on ring counts, we use precaution and err on the side of the upper estimate.

Taking the species average of three generation lengths at its most conservative estimate of 75 years, the percentage decline in geographic range from the 1940s would be equal to approximately 60%. To account for any discrepancy between the percentage decline in geographic range and population we can infer the decline in population to be 50%. This species is very rare where it does occur in the range, thus we can assume that the percentage decline in population relatively closely follows that of the percentage decline in its geographic range.


Systems: Freshwater

Use and Trade [top]

Use and Trade: It is unlikely that the species will be over-exploitation for commercial, scientific, or educational purposes.

Threats [top]

Major Threat(s):

The Sheepnose has reduced its total range size by two-thirds and it currently occupies 26 streams compared to 77 streams historically. This species has also disappeared from its former habitat across rivers such as the Illinois and Cumberland and several reaches of the Mississippi and Tennessee Rivers. Furthermore, the Sheepnose is no longer known to occur in the State of Arkansas (USFWS 2003, Butler 2003, Cummings and Mayer 1997, Parmalee and Bogan 1998). It is reported by Sietman (2003) to be extirpated from the lower Minnesota River in Minnesota.

It is considered unlikely that the species is threatened by exploitation, invasive species, genetic isolation or inadequate regulations (U.S. Fish and Wildlife Service 2003, Williams et al. 1993, Neves 1993, Neves et al. 1997, Watters 2000). However, invasive species such as the Asiatic clam Corbicula fluminea, Zebra mussel Dreissena polymorpha and Black carp Mylopharyngodon piceus are reported to be minor threats as the Sheepnose occurs in the same areas as the Asiatic clam and potentially as the Zebra mussel (NatureServe 2012). However, as of yet there is no evidence to suggest that there is competition between the Sheepnose and the Asiatic clam. If the Black carp was introduced into areas with the Sheepnose, smaller individuals could be considered at risk. The Zebra mussel has depleted other mussel populations in lakes and rivers in North America and therefore could become a greater threat should its numbers increase to a threshold level in the same distribution as the Sheepnose (NatureServe 2012). There is little known about the threats caused by disease or predation which may warrant further study.

The Sheepnose's decline in the Mississippi River system has been primarily due to habitat loss and degradation (Butler 2003). Impoundments are estimated to be the greatest contributing factor to their decline. In addition to impoundments channelization, chemical contaminants, mining and sedimentation have all played their part in the species decline. For example, heavy metal rich drainage from coal mining and associated sedimentation have had a negative effect on sections of the upper Tennessee River system in Virginia.

Conservation Actions [top]

Conservation Actions:

In light of the reduction of habitat quality and distribution of Sheepnose populations, all known populations should be protected. Furthermore, this species is listed in the U.S. Federal Endangered species list (U.S. Fish and Wildlife Service 2012). The water quality and river designation should be monitored and improved by working with local authorities. A national wildlife refuge on the Clinch River is under consideration. Other wildlife refuges in river systems where Sheepnose populations occur should also be established which would improve water and habitat quality for the species. Improvements in reservoir releases from TVA dams could enable the reintroduction of extirpated populations of species such as the Sheepnose, which could help boost population numbers (U.S. Fish and Wildlife Service 2003).

It would also be recommended to monitor and regulate any land use upstream from population to reduce erosion of silt to rivers. Continuation of the conservation outreach program on the St. Croix River and its mussel fauna would also be advisable. To reduce the impact of development projects such as channel dredging on mussel populations, Sheepnose populations should be relocated to avoid local extinctions. Further investigation into the tolerance levels towards pollutants and environmental changes such as siltation would also be required.

To effectively manage mussel species such as the Sheepnose, it is necessary to have a full understanding of their life history characteristics especially as this species has a parasitic larval stage. Thus, it is important to understand which host species the Sheepnose depends upon and investigate the nature of its relationship. Further life history studies need to be undertaken to identify the age and size at sexual maturity, recruitment success and age class structure.


Citation: Bogan, A.E. & Cummings, K. 2014. Plethobasus cyphyus. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 01 September 2014.
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