Quadrula fragosa (Conrad, 1835)
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Basis for Listing
The winged mapleleaf was thought to be extinct until a small population was discovered in the St. Croix River in 1987 (Doolittle 1988). Extensive surveys in the St. Croix River over the last 15 years indicate that the species is restricted to a 20 km (12 mi.) stretch south of Taylors Falls, Minnesota (Hornbach et al. 1996; Hove et al. 2003). This extant population occurs in runs over clean gravel, sand, and rubble substrates and in clear areas of high water quality. Hornbach (1992) concluded that the winged mapleleaf is only found under conditions that would be considered high quality for mussel habitat. The age structure of the individuals sampled indicates that recruitment to this population has been low in recent years. The youngest sizeable cohort is from 1984. Given it's extreme rarity, the winged mapleleaf was listed as federally endangered by the U.S. Fish and Wildlife Service in 1991. It has been state listed as endangered since 1996.
The shell of the winged mapleleaf is squarish, thick, moderately inflated, and up to 10 cm (4 in.) long. Externally it is brown, and young mussels have faint rays. The shell has a well-developed posterior wing with radiating ridges, and 2 rows of pustules or larger tubercles separated by a sulcus (depression or furrow) extending from the beak to the ventral margin. The pseudocardinal and lateral teeth are well-developed, and the inner shell is white. The winged mapleleaf is very similar to the mapleleaf (Quadrula quadrula), but the wing and 2 rows of pustules or larger tubercles on the winged mapleleaf's shell usually distinguish it from the mapleleaf.
Historically, the winged mapleleaf has been described as a large river species (Baker 1928). It has been found in the St. Croix River in riffles dominated by gravel, sand, and rubble substrates in water averaging about 1 m (3 ft.) deep (Hornbach et al. 1996). In general, winged mapleleafs have similar habitat requirements as the other mussel species residing in the St. Croix River mussel community. Three species of mussels, the deertoe (Truncilla truncata), the fawnsfoot (T. donaciformis), and the monkeyface (Quadrula metanevra), have been found to be significantly associated with winged mapleleafs (Hornbach et al. 1996).
Biology / Life History
Mussels are long-lived animals. Members of many species may live for several decades and in rare instances, a century or more. The maximum lifespan of the winged mapleleaf is not known, but individuals up to 22 years of age have been collected from the St. Croix River (U.S. Fish and Wildlife Service 1997).
Conservation / Management
Endangered throughout its range, the following factors potentially threaten the continued existence of the winged mapleleaf: narrow range, sparse population and low reproduction, probability of inbreeding which could weaken the species genetically, low streamflow episodes, high streamflow variations on the St. Croix River caused by a hydroelectric dam operating on a seasonal peaking regime, and the threat of non-point and point source water and sediment pollution (U.S. Fish and Wildlife Service 1997). Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movements of fish hosts. The winged mapleleaf 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 themselves in large numbers to the shells of native mussels, eventually causing death by suffocation. The health of winged mapleleaf host fish populations is also crucial to the recovery of this species.
Conservation Efforts in Minnesota
A recovery team consisting of biologists from the U.S. Fish and Wildlife Service, Minnesota and Wisconsin DNRs, the U.S. Geological Survey, and Macalaster College completed a winged mapleleaf recovery plan in 1997. The plan identifies and priortizes actions necessary for the recovery of wing mapleleaf populations and provides specific criteria for delisting (U.S. Fish and Wildlife Service 1997). In 1992, the MDNR Division of Ecological Services conducted an instream flow study that resulted in stream flow recommendations aimed at protecting and conserving the winged mapleleaf and other mussel populations residing in the St. Croix River (Johnson 1995). Consequently, the operator of the dam just upstream of the the winged mapleleaf population is now coordinating with the Minnesota and Wisconsin DNRs and the U.S. Fish & Wildlife Service to insure that dam discharge and resultant instream flow levels are high enough to be protective of this rare mussel. Research is also being conducted by personnel from the Minnesota DNR, the Wisconsin DNR, the University of Minnesota, and Macalester College to determine habitat preferences and population dynamics of the winged mapleleaf. Since 2003, an interagency team has been conducting propagation work for this species. Channel catfish are being infested with glochidia and placed in cages in the Mississippi River, following a methodology developed for the Higgins eye mussel (Lampsilis higginsii). Juveniles have been documented in some cages, and have been left to develop to a larger size. Research is also being conducted by federal agency staff to develop tests of infestation of wild catfish. The National Park Service has posted signs at Interstate State Park prohibiting the handling of mussels. Lastly, multiple state and federal agencies are working to keep zebra mussels out of habitat occupied by winged mapleleafs in the St. Croix River.