Ohio pigtoe (Pleurobema cordatum)

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Ohio pigtoe shell
IUCN Red List species status – Near Threatened NEAR
THREATENED

Top facts

  • The Ohio pigtoe is a freshwater mussel species typically found in large rivers.
  • The triangular shape of the Ohio pigtoe is further accentuated by the presence of a broad, shallow groove on the shell.
  • The shell of the adult Ohio pigtoe is chestnut to dark brown, although older specimens often turn black.
  • Pollution, habitat destruction and the introduction of invasive species are all potential threats to the Ohio pigtoe.
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Ohio pigtoe fact file

Ohio pigtoe description

KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderUnionoida
FamilyUnionidae
GenusPleurobema (1)

The Ohio pigtoe is a medium to large freshwater mussel (3) with a relatively thick (2), triangular shell (2) (3) (4) (5). The somewhat angular shape of this species is further accentuated by the presence of a broad, shallow groove in front of a ridge at the posterior end of the mussel (2) (3) (6). This posterior section is bluntly pointed, while the anterior end is rounded (2).

The two segments of the Ohio pigtoe, known as valves, are swollen anteriorly, with the diameter generally being greater than 50 percent of the length of the shell (3). The beak, or ‘umbo’, of the Ohio pigtoe is curved (4), relatively swollen in appearance (2) (3), and projects forwards (2) (3) (4).

The outer surface of the Ohio pigtoe’s shell is smooth (2) (3) and dark brown or chestnut in adults (2) (3), often becoming black in very old specimens (3). In juvenile individuals, the shell tends to be lighter, and is marked with green lines stemming from a central point (2). Growth lines tend to be rather distinct and are darker than the rest of the shell (3). The inner surface of the shell, known as the ‘nacre’, is usually white (2) (3), although occasionally it may be pinkish (2). The valves of male and female Ohio pigtoes are similar in appearance (3)

As in other mussel species, the Ohio pigtoe has what are known as ‘pseudocardinal’ and ‘lateral’ teeth. Pseudocardinal teeth are structures found near the anterior-dorsal edge of the mussel (7), and in the Ohio pigtoe these structures are well developed, with two located in the left valve and one in the right (2). The lateral teeth are straight or slightly curved (2).

Also known as
pigtoe.
Synonyms
Fusconaia cordata, Margarita obliqua, Pleurobema obliquum, Quadrula obliqua, Unio cordatus, Unio mytiloides, Unio obliqua, Unio obliquus.
Size
Length: up to 10.2 cm (2)
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Ohio pigtoe biology

Little is known about the natural history of the Ohio pigtoe. However, as in other species in the Pleurobema genus, the Ohio pigtoe is thought to be tachytictic, meaning that reproduction involves spawning followed by the release of larvae (4). Males discharge sperm into the river current, which then travels downstream where the females siphon in the sperm to fertilise their eggs (8). In the Ohio pigtoe, this relatively synchronous spawning typically occurs in April and early May (9). Following fertilisation, the eggs are stored in the female’s gill pouches where they develop and hatch out as larvae (8) (10), known as ‘glochidia’ (10). The female then expels the glochidia into the water (8) (10), with June being the peak month for glochidia release in the Ohio pigtoe (9).

Interestingly, freshwater mussels in the Unionidae family, to which the Ohio pigtoe belongs, require a fish host in order to complete their life cycle (10). The glochidia must use tiny clasping valves to attach themselves to the gills of a host fish species (8), which in the Ohio pigtoe includes the rosefin shiner (Notropis ardens) and bluegill (Lepomis macrochirus) (3).

Mussel glochidia remain attached to the fish host for between a few weeks and several months, depending on the species in question (10), and grow into juveniles with shells of their own. Once the adult form has been attained, the mussel drops off its fish host (8) (10) and settles into the streambed (8). It is thought that one benefit of having a fish host is dispersal, helping to ensure that mussels are transported to new habitats and that gene flow between populations is facilitated (10).

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Ohio pigtoe range

The Ohio pigtoe is native to the United States, and occurs in the upper Mississippi and Saint Lawrence River drainages, from western New York west through Michigan, Wisconsin and Kansas, and southwards to Arkansas and Alabama (1) (4).

Although still considered to be a relatively widespread species (2), the Ohio pigtoe is now presumed to have been extirpated in Virginia, Pennsylvania and Iowa, and is at severe risk of extinction in Illinois and Ohio (4). In the Ohio River, this species now only occurs sporadically (1) (2).

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Ohio pigtoe habitat

The Ohio pigtoe is generally found in large rivers (1) (2) (3) (4) (6), but is also known to occur in medium-sized rivers (1) (2) (3). Although this species typically inhabits flowing streams (2) (3) (4) (6), it is reported to be tolerant of some reservoir environments (1). The Ohio pigtoe tends to prefer habitats with mud, sand, gravel or cobble substrates (2) (3) (4), and is usually found within the first few kilometres of the river upstream of its mouth (6).

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Ohio pigtoe status

The Ohio pigtoe is classified as Lower Risk/near threatened (LR/nt) on the IUCN Red List, and requires a reassessment (1).

IUCN Red List species status – Near Threatened

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Ohio pigtoe threats

Potential threats to the Ohio pigtoe include the destruction of its habitat through the creation of impoundments, waterway modification and siltation, as well as pollution and the introduction of non-native species (4). In particular, the zebra mussel (Dreissena polymorpha) could pose a serious threat to the Ohio pigtoe (3) (4) by interfering with its feeding, breathing and reproduction (4).

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Ohio pigtoe conservation

There are currently no known conservation measures specifically in place for the Ohio pigtoe. However, this species is classified as ‘Endangered’ in Ohio (2) (4), Illinois and Virginia (4), and is considered to be a Species of Special Concern in Illinois and Indiana (2).

Recommended conservation measures for the Ohio pigtoe include preserving existing populations and allowing them to expand, or actively expanding the range of this species through translocation of individuals from captive-breeding programmes (4).  

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Find out more

Find out more about the Ohio pigtoe:

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

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Glossary

Anterior
Situated at or near the front.
Dorsal
Relating to the back or top side of an animal.
Fertilisation
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Genus
A category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
Larva
Immature stage in an animal’s lifecycle, after it hatches from an egg and before it changes into the adult form. Larvae are typically very different in appearance to adults; they are able to feed and move around but are usually unable to reproduce.
Posterior
Situated behind or at the rear.
Siphon
In molluscs, a tube-like structure through which water passes into or out of the mantle cavity.
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References

  1. IUCN Red List (January, 2014)
    http://www.iucnredlist.org/
  2. Illinois Natural History Survey - Pleurobema cordatum (January, 2014)
    http://wwx.inhs.illinois.edu/collections/mollusk/publications/guide/index/62
  3. Terwilliger, K., Tate, J.R. and Woodward, S.L. (Eds.) (1995) A Guide to Endangered and Threatened Species in Virginia. University of Tennessee Press, Knoxville, Tennessee.
  4. Roe, K.J. (2002) Conservation Assessment for the Ohio Pigtoe (Pleurobema cordatum) Rafinseque, 1820. United States Department of Agriculture, USA. Available at:
    http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsm91_054189.pdf
  5. Thorp, J.H. and Covich, A.P. (2010) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Massachusetts.
  6. Haag, W.R. (2012) North American Freshwater Mussels: Natural History, Ecology, and Conservation. Cambridge University Press, Cambridge.
  7. Illinois State Museum - Mussel Glossary (January, 2014)
    http://www.museum.state.il.us/ismdepts/zoology/mussels/mussel_glossary.html
  8. U.S. Fish and Wildlife Service - Clubshell (January, 2014)
    http://www.fws.gov/midwest/endangered/clams/pdf/clubshell.pdf
  9. Dillon, R.T. (2010) The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge.
  10. Michigan Natural Features Inventory - Northern clubshell (January, 2014)
    http://mnfi.anr.msu.edu/abstracts/zoology/Pleurobema_clava.pdf
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Image credit

Ohio pigtoe shell  
Ohio pigtoe shell

© Collection of the Illinois State Museum

Illinois State Museum
http://www.museum.state.il.us/

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