Cumberland bean pearly mussel (Villosa trabalis)

Also known as: Cumberland bean, Purple bean
KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderUnionoida
FamilyUnionidae
GenusVillosa (1)
SizeLength: up to 5.5 cm (2)

Classified as Critically Endangered (CR) on the IUCN Red List (1)

The thick, elongated, oval shell of a Cumberland bean pearly mussel is a pleasure to behold; when cleaned the outer surface is smooth and glossy with a gentle iridescence. When not clean, the shell may be dull at first glance, but on further inspection is an intricate blend of dark greens and browns banded with black rays. The inside of the shell (the nacre) is pearly white with a delicate bluish tinge. Female Cumberland bean pearly mussels usually possess rounder shells than the males and are also slightly larger (2).

The Cumberland bean pearly mussel is endemic to North America. Today, this species inhabits a small fraction of its former range and is now only found in four river systems: the Hiwassee River in Tennessee, Buck Creek, the Little South Fork of the Cumberland River, and the Rockcastle River system in Kentucky (3) (4).

The Cumberland bean pearly mussel is a freshwater species, which can be found occupying very clean, fast-flowing rivers and streams, where it dwells attached to rocky or gravely substrate at depths of one metre or less (5). Riffle areas, where the river is shallow and rapid with many protruding rocks, are perfect for this species (2).

Mussels are filter feeders, taking in water via one opening in the shell (the inhalant aperture) and expelling waste via another opening (the exhalant aperture). These apertures, along with the foot, aid in breathing, feeding and reproduction. They are the only parts of the body that emerge from the shell but can be withdrawn very quickly to prevent predation (6). Mussels respire via gills that absorb oxygen from the water; they are fairly inefficient at absorbing oxygen but make up for this set back by having large gills and a high volume of water passing over the gills (6). This particular mussel is an omnivore, feeding on planktonic organisms such as algae and microscopic freshwater animals (6).

During reproduction, male Cumberland bean pearly mussels release sperm into the water column, where the flow of the river transports the sperm to the female. The female collects the sperm by extruding a tentacle-like organ called the siphon. The sperm then enters the marsupium, which is a specially adapted water tube near the gills that stores eggs to be fertilized and houses the developing larvae. This is an important adaptation as if larvae were released straight away they would be washed downstream and end up at an unsuitable habitat (6). Measuring just 0.2 millimetres (7), the larvae of mussels, also known as glochidia, are parasites of fish; the Cumberland bean pearly mussel distributes its larvae on to the river bed in spidery mucous strands where they attach on to the gills or fins of a host fish (2). Here the larvae complete development into juvenile mussels, before dropping off the fish to the streambed (2).

Historically the Cumberland bean pearly mussel inhabited ten river systems, but now occupies only four (3). Unfortunately, like many other species of mussel, the Cumberland bean pearly mussel is under threat due to human activities, namely siltation, channelization and water pollution, such as from toxic chemical spills and run-off from urban and agricultural areas (8). Being a filter feeder the Cumberland bean pearly mussel must have exceptionally clean water to feed, reproduce and respire, thus any pollution or siltation seriously jeopardises the mussel’s chance of survival as the water becomes dirty or poisonous. In addition, channelization destroys suitable river habitats, changing the dynamics of the water downstream so that it is no longer suitable for the mussel (2). Population numbers are hard to estimate as little data are available; however, it is thought that there are around 1,000 to 2,000 individuals left in the wild, meaning that this species has suffered a decline of around 75 to 90 percent since it was first discovered (4).

This species was first listed as Endangered by the U.S. Fish and Wildlife Service in June 1976 and a conservation plan was approved in August 1984. The aims of the recovery plan are to re-establish the Cumberland bean pearly mussel within two river systems that were formerly part of its range, and to prevent the loss or degradation of existing habitats (4) (8). It is essential for more research to be carried out on the life history of this mysterious mussel, including studies to identify the host fish species of the larvae, to understand how to rear the species in a laboratory to aid with reintroduction, and to identify potential habitats for reintroduction (4).

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

  1. IUCN Red List (March, 2010)
    http://www.iucnredlist.org
  2. Biggins, R.G. (2000) The Cumberland Pearly Bean Mussel in North Carolina. US Fish and Wildlife Service, Washington DC. Available at:
    http://www.fws.gov/nc-es/mussel/cumberland.html
  3. Jones Jr, M.P. (2001) Endangered and threatened wildlife and plants; establishment of nonessential experimental population status. Federal Register, 66(115): 33250-32264.
  4. NatureServe (November, 2009)
    http://www.natureserve.org/explorer
  5. Gordon, M.E.and Layzer.J.B. (1989) Mussels (Bivalvia: Unionoidea) of the Cumberland River review of life histories and ecological relationships. US Fish and Wildlife Service Biological Report, 89(15): 1-99.
  6. Ruppert, E.E., Fox, R.S. and Barnes, R.D. (2004) Invertebrate Zoology. Thomson Brooks/Cole, Belmont, USA.
  7. Hoggarth, M.A. (1999) Descriptions of some of the Glochidia of the Unionidae (Mollusca: Bivalvia). Journal of Malacologia, 41(1): 88.
  8. US Fish and Wildlife Service. (1984) Cumberland Bean Pearly Mussel Recovery Plan. US Fish and Wildlife Service, Atlanta, Georgia.