Basis for Listing

Monkeyface mussels were once widely distributed in the larger streams of the Mississippi basin, although they were among the less common mussels where they occurred (Fuller 1978). They are no longer found in the Minnesota River (Bright et al. 1990) and are very rare in the Mississippi River (Thiel 1981; M. Davis, Minnesota DNR, pers. comm.). Only the St. Croix River appears to still support a viable monkeyface population (Doolittle 1988; Heath 1990), where Hornbach et al. (1995) found it comprised 3% of the specimens they collected. The monkeyface was listed as a threatened species in Minnesota in 1996.

  Description

The shell of the monkeyface can reach up to 12.7 cm (5 in.) long. It is squarish in shape with thick valves and a prominent posterior ridge, which often has a series of large knobs surrounded by scattered pustules (bumps). The posterior slope of the shell is flattened, appearing winged, often with a series of small ridges that curve upward. The posterior shell margin is indented. The outside of the shell is yellowish, greenish or brown, and usually marked with green chevrons (V-shaped markings). The pseudocardinal and lateral teeth are heavy, and the inside of the shell is white. The monkeyface resembles the mapleleaf (Quadrula quadrula), pimpleback (Q. pustulosa), purple wartyback (Cyclonaias tuberculata), wartyback (Q. nodulata), and winged mapleleaf (Q. fragosa) mussels, but can be distinguished from all of these species by its large, knobbed posterior ridge and green V-shaped markings.

  Habitat

Hornbach et al. (1996) reported that densities of the monkeyface mussels in the St. Croix River peaked in habitats dominated by stable substrates in water over 2 m (6.6 ft) deep.

  Biology / Life History

Mussels are long-lived animals. Members of many species may live for several decades and in some instances, a century or more. They spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds (Sietman 2003).

Mussels are primarily sedentary, but they can move around with the use of their foot, which is a hatchet shaped muscle that can be extended out between the valves (shells). A mussel will burrow its foot into the sediment and then contract it to pull itself slowly along the bottom of its aquatic habitat (Sietman 2003).

Mussels eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They draw water into their body through their incurrent siphon, remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hairlike cilia located on the gills. Waste is expelled through the excurrent siphon (Sietman 2003).

Mussels have a complex and distinctive reproductive cycle. Males release sperm into the water, which are drawn in by females through their incurrent siphon. Fertilized eggs are brooded in the female's gills, where they develop into tiny larvae called glochidia. The monkeyface is a short-term breeder, and females may contain glochidia in their gills from May to July (Parmalee and Bogan 1998). Once the glochidia are expelled from the female's gills, they attach to fish gills or fins by clamping onto them with their valves. The glochidia live as parasites on the host fish until they develop into juvenile mussels, at which point they detach from the fish and fall to the streambed as free-living mussels. Fish hosts for the glochidia of the monkeyface are known to include sunfish (Lepomis spp.), bluegill (L. macrochirus), and sauger (Stizostedion canadense) (Parmalee and Bogan 1998).

  Conservation / Management

The monkeyface is declining or extirpated throughout most of its former range. The viability of remaining populations is jeopardized by the continuing decline in habitat conditions on the Mississippi River associated with its management as a navigation canal, and from non-point and point source water and sediment pollution. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. The monkeyface is also being impacted by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach in large numbers to the shells of native mussels, eventually causing death by suffocation. If observed trends cannot be reversed, the monkeyface will likely become endangered in the future.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the monkeyface's ecology and current status in Minnesota. Additionally, 40 monkeyface mussels were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas of the Mississippi River south of the Twin Cities, where habitats were devoid of zebra mussels.

  References and Additional Information

Doolittle, T. C. J. 1988. Distribution and relative abundance of freshwater mussels in the Saint Croix National Scenic Riverway. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resouces. Unpaged.

Fuller, S.L.H. 1978. Freshwater mussels (Mollusca: Bivalva: Unionidae) of the upper Mississippi River: Observations at selected sites within the nine-foot channel navigation project on behalf of The United States Army Corps of Engineers. Final report prepared for the U.S. Army Corps of Engineers no. 78-33, Academy of Natural Sciences of Philadelphia. 401 pp.

Heath, D. J. 1990. Identification of distribution, abundance, and critical habitat for Lampsilis higginsi and Category 2 species of mussels - performance report - Ocober 1, 1987 to September 30, 1988. Wisconsin Endangered Resources Report #65. 11 pp. + tables and figures.

Hornbach, D. J., J. G. March, T. Deneka, N. H. Troelstrup, Jr., and J. A. Perry. 1996. Factors influencing the distribution and abundance of the endangered Winged Mapleleaf Mussel Quadrula fragosa in the St. Croix River, Minnesota and Wisconsin. The American Midland Naturalist 136:278-286.

Hornbach, D. J., P. Baker, and T. Deneka. 1995. Abundance and distribution of the endangered mussel Lampsilis higginsi in the lower St. Croix River, Minnesota and Wisconsin. Final report submitted to the U.S. Fish and Wildlife Service, Minneapolis, Minnesota. 68 pp.

Parmalee, P. W., and A. E. Bogan. 1998. The freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.

Sietman, B. E. 2003. Field guide to the freshwater mussels of Minnesota. Minnesota Department of Natural Resources, St. Paul, Minnesota. 144 pp.

Thiel, P. 1981. A survey of unionid mussels in the upper Mississippi River (pools 3-11). Technical Bulletin 124. Wisconsin Department of Natural Resources, Madison, Wisconsin. 24 pp.