Theliderma metanevra (Rafinesque, 1820)
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.
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.
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).
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.