Clubshell pearly mussel (Pleurobema clava)

Also known as: club naiad, clubshell, northern clubshell
GenusPleurobema (1)
SizeLength: up to 7.6 cm (2)
Top facts

The clubshell pearly mussel is classified as Critically Endangered (CR) on the IUCN Red List (1) and is listed on Appendix II of CITES (3).

The clubshell pearly mussel (Pleurobema clava) has a thick, triangular shell (2) (4) (5), which is relatively elongated at the posterior end (2) (4) and rounded at the anterior end (2). The shell is moderately inflated (2), giving the clubshell pearly mussel a wedge-shaped appearance when viewed from the top, and there is often a crease present near the centre of the shell which runs perpendicular to the mussel’s growth rings (4).

The smooth outer layer of the clubshell pearly mussel’s shell is tan (4) to yellowish-brown, although older individuals tend to be dark brown or black (2). Broad, dark green lines can be seen on the beak or ‘umbo’, stemming from a central point. The inner layer of the mussel shell, known as the ‘nacre’, is an iridescent, pearly white colour (2) (4). The shells of male and female clubshell pearly mussels are similar in appearance (4).

As in other mussel species, the clubshell pearly mussel has what are known as pseudocardinal and lateral teeth. Pseudocardinal teeth are structures found near the anterior-dorsal edge of the mussel (6), and in the clubshell pearly mussel these structures are small and well developed (2) (4), with two located in the left valve, and one in the right (2). The lateral teeth are interlocking ridge-like structures which are found along the hinge of the mussel valves (6). The lateral teeth of the clubshell pearly mussel are relatively long, and straight or slightly curved, and as with the pseudocardinal teeth, there are two in the left valve, and one in the right (2).

The clubshell pearly mussel is native to the United States (1). Historically, this species was found across an area stretching from Michigan south to Alabama, and from Illinois eastwards to Pennsylvania (4) (7), but its range has since been reduced by an alarming 95 percent (7). The clubshell pearly mussel has been extirpated from Alabama and Tennessee, and now only occurs in 12 or 13 streams (4) (7) in Ohio, Indiana and Illinois (2).

The clubshell pearly mussel is a freshwater species (1) which occurs in medium to large rivers or streams, typically those with a gravel or sandy substrate (2) (4) (7) in which it can bury itself (7). This species tends to prefer areas of clean water (7) with relatively little silt (4).

A burrowing species which spends much of its life buried several centimetres into the bottom substrate (4) (7) (8), the clubshell pearly mussel is a relatively little-known species (7).

Mussels are filter feeders, taking in water via one opening in the shell, known as the ‘inhalant aperture’, and expelling waste via another opening, known as 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 (9). To breathe and feed, the clubshell pearly mussel relies on the water percolating between the particles of the sandy substrate in which it lives (8). Mussels respire via gills that absorb oxygen from the water, and although they are fairly inefficient at absorbing oxygen, they make up for this by having large gills and, where possible, a high volume of water passing over them (9).

Breeding in the clubshell pearly mussel involves males discharging sperm into the river current, which then travels downstream where the females siphon in the sperm to fertilise their eggs (7). Following fertilisation, the eggs are stored in the female’s gill pouches where they develop and hatch out as larvae (4) (7), known as ‘glochidia’ (4). The clubshell pearly mussel then releases the glochidia into the water (4) (7). Interestingly, like most other freshwater mussels in the Unionidae family, the clubshell pearly mussel requires a fish host in order to complete its life cycle (4). The glochidia of the clubshell pearly mussel must use tiny clasping valves to attach themselves to the gills of a host fish species (7) such as the blackside darter (Percina maculata), central stoneroller (Campostoma anomalum), logperch (Percina caprodes) and striped shiner (Luxilus chrysocephalus) (4).

Mussel glochidia remain attached to the fish host for up to several months, depending on the species in question (4), and grow into juveniles with shells of their own. Once the adult form has been attained, the mussel detaches itself from its fish host (4) (7) and settles into the streambed (7). 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 (4).

The maximum lifespan of the clubshell pearly mussel is an impressive 50 years (4) (7).

It is estimated that the clubshell pearly mussel has been extirpated from more than 95 percent of its range in the last hundred years (4). The decline of this species has been attributed mostly to pollution from agricultural run-off and to the alteration of waterways (4) (7), with the clubshell pearly mussel being particularly sensitive to siltation (4).

An additional threat to the clubshell pearly mussel comes in the form of the introduced zebra mussel (Dreissena polymorpha) (4) (7), a fast-spreading invasive species which was accidentally introduced to the United States in ballast water taken from the Caspian Sea (7). The zebra mussel requires hard, stable substrates to attach itself to, often using native species for this purpose, which can eventually suffocate them (4) (7).

The clubshell pearly mussel is federally listed as ‘Endangered’ in the United States (4) (7), and is classified as ‘Endangered’ in Illinois, Indiana, Michigan and Ohio (2). In addition, this species is known to occur in Erie National Wildlife Refuge (10).

Given the clubshell pearly mussel’s reliance on fish hosts to complete its life cycle, it has been proposed that conservation efforts for this species should centre around measures to ensure the continued presence of associated fish communities. This could include the preservation of fish habitats and protection of fish food resources such as aquatic insects. Incentive programmes involving conservation tillage and reforestation are in place to minimise the impact of land use in areas surrounding the clubshell pearly mussel’s habitat (4).

Find out more about the clubshell pearly mussel:

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 (January, 2014)
  2. Illinois Natural History Survey - Pleurobema clava (January, 2014)
  3. CITES (January, 2014)
  4. Michigan Natural Features Inventory - Northern clubshell (January, 2014)
  5. Thorp, J.H. and Covich, A.P. (2010) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Massachusetts.
  6. Illinois State Museum - Mussel Glossary (January, 2014)
  7. U.S. Fish and Wildlife Service - Clubshell (January, 2014)
  8. Farris, J.L. and Van Hassel, J.H. (2006) Freshwater Bivalve Ecotoxicology. CRC Press, Boca Raton, Florida.
  9. Ruppert, E.E., Fox, R.S. and Barnes, R.D. (2004) Invertebrate Zoology. Thomson Brooks/Cole, California.
  10. U.S. Fish and Wildlife Service Species Profile - Clubshell (January, 2014)