Orangefoot pimpleback (Plethobasus cooperianus)

Also known as: Orange-footed pearlymussel, Orange-footed pimpleback mussel
GenusPlethobasus (1)
SizeLength: up to 10 cm (2)
Width: up to 4.6 cm (3)
Height: up to 7.8 cm (3)
Top facts

The orangefoot pimpleback mussel is classified as Critically Endangered (CR) on the IUCN Red List (1) and is listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) (4).

The orangefoot pimpleback (Plethobasus cooperianus) is a medium-sized freshwater mussel (3) with a large, heavy shell which is almost circular in shape (2) (3). The colouration of the shell can vary between yellowish-brown and rusty or chestnut brown, and older individuals generally have a darker colouration (2) (3). The shell is marked with dark concentric ridges, which correspond to seasonal variation in growth, and the posterior of the shell is covered with numerous smooth pimple-like tubercles (2) (3), from which this species gained its common name. Young individuals of this species have green ray-like markings on the shell (3). The inside of the shell, known as the ‘nacre’, ranges in colouration between white and pink (3).

The orangefoot pimpleback is currently found in rivers in Alabama, Tennessee, Kentucky, and Illinois (5), although the historical range of this mussel was much more extensive (3).

The orangefoot pimpleback can be found in medium to large rivers, generally in riffle areas with a gravel or mixed sand and gravel substrate (2) (6). This mussel prefers silt-free, clean and fast-flowing water (3) (6).

Although little is known about the early development of the orangefoot pimpleback, it is likely to be similar to closely related species whose glochidia parasitise the gills of host fish throughout their development into adults. The glochidia usually remain on the gills of the host fish throughout their development into juveniles (3). Glochidia generally have a limited set of host species that they are capable of parasitising (3). After a period of parasitisation, the glochidia fall from the gills of the host fish onto the substrate, where they develop into adult mussels. The parasitic stage of the lifecycle of mussels provides a method of dispersal, which is important because the adult mussels are largely immobile (3) (6).

One of the greatest threats to the orangefoot pimpleback, and other freshwater mussels in North America, is the invasive zebra mussel (Dreissena polymorpha) (7). Hundreds to thousands of minute zebra mussels can attach to the shell of native mussels, which can eventually kills the contaminated individual by interfering with its ability to feed, respire, excrete, and move (7).

Orangefoot pimpleback populations are also threatened by intensive agriculture and increasing urbanisation, particularly due to the heavy loadings of silt, sediments, nutrients and pollution that they cause, which destroy mussel habitats, clog their gills, and reduce water quality (3) (7).

Dams and water impoundments are particularly harmful to freshwater mussels as their construction and presence degrades habitats and eliminates the ability of populations to disperse, subsequently isolating them (3). Between the 1930s and 1980s, 51 impoundments were built in the Tennessee and Cumberland Rivers (3), which contain some of the last remaining populations of the orangefoot pimpleback (3) (5).

The orangefoot pimpleback is classified as federally endangered in the United States which gives the species some protection from direct harm (5). The orangefoot pimpleback is also listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) (4) meaning that international trade in this species is strictly prohibited without a permit. The United States Fish and Wildlife Service has released several summaries and action plans for the conservation of this freshwater mussel (5) (7), which include surveying areas with known populations and establishing a captive breeding and reintroduction programme (7).

Find out more about freshwater mussels in the Order Unionoida:

Find out more about mussel conservation in North America at:

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:

  1. IUCN Red List (May, 2014)
  2. Illinois Natural History Survey - Plethobasus cooperianus (May, 2014)
  3. United States Fish & Wildlife Service Recovery Plan for the Orange-footed pearly mussel (May, 2014)
  4. CITES - Plethobasus cooperianus (May, 2014)
  5. United States Fish and Wildlife Service Species Profile - Orangefoot pimpleback (May, 2014)
  6. United States Fish & Wildlife Service Endangered Species - Orange-Footed Pearly Mussel (May, 2014)
  7. United States Fish & Wildlife Service - Orangefoot pimpleback mussel Recovery Action Plan (May, 2014)