Orangefoot pimpleback (Plethobasus cooperianus)

Orangefoot pimpleback shell
IUCN Red List species status – Critically Endangered CRITICALLY

Top facts

  • The orangefoot pimpleback is only found in four states in the United States.
  • The range of the orangefoot pimpleback was historically much larger.
  • The tubercles on the shell of the orangefoot pimpleback account for one part of its common name.
  • The host fish species used by the orangefoot pimpleback during juvenal development are not yet known.
Loading more images and videos...

Orangefoot pimpleback fact file

Orangefoot pimpleback description

GenusPlethobasus (1)

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).

Also known as
Orange-footed pearlymussel, Orange-footed pimpleback mussel.
Length: up to 10 cm (2)
Width: up to 4.6 cm (3)
Height: up to 7.8 cm (3)

Orangefoot pimpleback biology

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).


Orangefoot pimpleback range

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).


Orangefoot pimpleback habitat

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).


Orangefoot pimpleback status

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).

IUCN Red List species status – Critically Endangered


Orangefoot pimpleback threats

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).


Orangefoot pimpleback conservation

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

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:



Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
Litter formed from fragments of dead material.
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Describes species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, outcompete natives and take over the new environments.
Of or relating to the 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.
Interaction in which one organism derives its food from, and lives in or on, another living organism (the host) at the host's expense.
Situated behind or at the rear.
Light rapids where water flows across a shallow section of river.
In molluscs, a tube-like structure through which water passes into or out of the mantle cavity.
A small, rounded, wart-like bump on the skin or on a bone.


  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)

Image credit

Orangefoot pimpleback shell  
Orangefoot pimpleback shell

© Collection of the Illinois State Museum

Illinois State Museum


Link to this photo

Arkive species - Orangefoot pimpleback (Plethobasus cooperianus) Embed this Arkive thumbnail link ("portlet") by copying and pasting the code below.

Terms of Use - The displayed portlet may be used as a link from your website to Arkive's online content for private, scientific, conservation or educational purposes only. It may NOT be used within Apps.

Read more about



MyARKive offers the scrapbook feature to signed-up members, allowing you to organize your favourite Arkive images and videos and share them with friends.

Play the Team WILD game:

Team WILD, an elite squadron of science superheroes, needs your help! Your mission: protect and conserve the planet’s species and habitats from destruction.

Conservation in Action

Which species are on the road to recovery? Find out now »

This species is featured in:

This species is featured in the Illinois eco-region

Help us share the wonders of the natural world. Donate today!


Back To Top