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Plethobasus cyphyus

(Rafinesque, 1820)

Sheepnose

G2Imperiled (G2G3) Found in 2 roadless areas NatureServe Explorer →
G2ImperiledGlobal Rank
EndangeredIUCN
HighThreat Impact
Sheepnose Mussel (Plethobasus cyphyus). 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.2.113633
Element CodeIMBIV34030
Record TypeSPECIES
ClassificationSpecies
Classification StatusStandard
Name CategoryInvertebrate Animal
IUCNEndangered
Endemicendemic to a single nation
KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderUnionoida
FamilyUnionidae
GenusPlethobasus
Synonyms
Unio aesopusGreen, 1827
Other Common Names
Bullhead (EN) Clear Profit (EN) Sheepnose Mussel (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
Ortmann (1918) noted a "subspecies" from the upper Tennessee river basin, Plethobasus cyphyus compertus (Frierson, 1911). This form differs from typical specimens by a more elongated shell, weak development of the medial row of knobs and sulcus, and the pale color (instead of orange) of the tissues. Ortmann (1918) speculated that compertus might be a local race but also reported that the two forms were sympatric. An in-depth systematic examination (morphological and electrophoretic) of these two forms should be performed to determine if any taxonomic significance is warranted. The specific epithet of this species has been variably spelled cyphya, scyphius, cyphius, cyphia, cyphyum, and ultimately as cyphyus. The sheepnose or its synonyms have been placed in the genera Unio, Pleurobema, Margarita, and Margaron. It was ultimately placed in the genus Plethobasus by Ortmann (1919), where it remains today (Turgeon et al., 1998).
Conservation Status
Rank Method Rank calculation - Biotics v2
Review Date2024-01-08
Change Date2024-01-08
Edition Date2024-01-08
Edition AuthorsWhittaker, J.C. (1998); rev. K.S. Cummings (2000); rev. J. Cordeiro (2009); rev. T. Cornelisse (2024)
Threat ImpactHigh
Range Extent20,000-2,500,000 square km (about 8000-1,000,000 square miles)
Number of Occurrences21 - 80
Rank Reasons
This species has a moderate range and number of occurrences, but many ongoing threats and both long and short term population declines.
Range Extent Comments
This species occurs in the Upper and Lower Mississippi, Ohio, and Tennessee Rivers in the United States in Alabama, Illinois, Indiana, Iowa, Kentucky, Minnesota, Mississippi, Missouri, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Wisconsin (USFWS 2022).
Occurrences Comments
This species is known from 37 occurrences or extant populations using records from 2000 to 2020 (USFWS 2022).
Threat Impact Comments
This species is threatened by oil and gas exploration, coal mining, and the heavy metal and chemical pollutants that result from those activities, as well as habitat loss and degradation from energy production and development, invasive species, and dams or other barriers that alter its habitat and prevent habitat connectivity and recolonization (USFWS 2020, 2022).
Ecology & Habitat

Description

SHELL EXTERIOR: Shell thick, oval or oblong, somewhat elongate, and slightly inflated. Anterior end rounded, posterior end bluntly pointed. Dorsal margin straight, ventral margin curved anteriorly, straight posteriorly. Umbos slightly elevated above the hinge line. Beak sculpture of two heavy ridges, visible only in young shells. Shell smooth, except for a row of knobs or tubercles on the center of the valve, running from the umbo to the ventral margin (sometimes obscure). A shallow sulcus or furrow present between the row of tubercles and the posterior ridge. Periostracum yellow or light brown in juveniles, becoming chestnut to dark brown in adults. Length to 12.7 cm.

SHELL INTERIOR: Pseudocardinal teeth rather small relative to overall shell size; two in the left valve, one in the right (occasionally with a smaller tubercular tooth on either side). Lateral teeth long, straight or slightly curved; two in the left valve, one in the right Beak cavity shallow. Nacre white, occasionally tinged with pink or salmon. (see Butler, 2003; Cummings and Mayer, 1992; Oesch, 1995; U.S. Fish and Wildlife Service, 2003)

Diagnostic Characteristics

This species is rather distinctive with its oval shape and flattened posterio-ventral margin, its knobby medial ridge, wide sulcus between the medial and posterior ridges, and shiny yellow priostracum. Obliquaria reflexa (Rafinesque, 1820) is similarly shaped but is a smaller, more inflated shell with medial knobs which alternate in position between the right and left valves, a costate posterior slope, and a brownish periostracum. it also superficially resembles Epioblasma torrulosa s.s. (Rafinesque, 1820) but that shell was highly rayed and apparently is now extinct. Butler (2003) lists key distinguishing characters as shell color, occurrences of central tubercles, and its outline.

Habitat

This species is found in medium to large rivers with moderate to swift currents associated with riffles and gravel, cobble, and clay substrates (USFWS 2023).

Reproduction

This species is a short-termed brooder with ectobranchous marsupia and females are gravid between mid-May and early August (Gordon and Layzer 1989; USFWS 2020).
Other Nations (1)
United StatesN2
ProvinceRankNative
MississippiS1Yes
VirginiaS1Yes
MinnesotaS1Yes
West VirginiaS2Yes
PennsylvaniaS1Yes
WisconsinS1Yes
IndianaS1Yes
TennesseeS2Yes
OhioS1Yes
KansasSNRYes
KentuckyS1Yes
IowaS1Yes
IllinoisS1Yes
AlabamaS1Yes
MissouriS2Yes
Threat Assessments
ThreatScopeSeverityTiming
1 - Residential & commercial developmentRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
1.2 - Commercial & industrial areasRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
3 - Energy production & miningRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
3.1 - Oil & gas drillingRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
7 - Natural system modificationsRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
7.2 - Dams & water management/useRestricted (11-30%)Moderate or 11-30% pop. declineHigh (continuing)
8 - Invasive & other problematic species, genes & diseasesLarge (31-70%)Moderate or 11-30% pop. declineHigh (continuing)
8.1 - Invasive non-native/alien species/diseasesLarge (31-70%)Moderate or 11-30% pop. declineHigh (continuing)
8.1.2 - Named speciesLarge (31-70%)Moderate or 11-30% pop. declineHigh (continuing)
9 - PollutionLarge - restrictedSerious or 31-70% pop. declineHigh (continuing)
9.2 - Industrial & military effluentsLarge - restrictedSerious or 31-70% pop. declineHigh (continuing)

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Roadless Areas (2)
Idaho (1)
AreaForestAcres
Bear CreekCaribou-Targhee National Forest118,582
Illinois (1)
AreaForestAcres
Burke BranchShawnee National Forest6,231
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