Arcidens confragosus (Say, 1829)
Basis for Listing
Historically a resident of the Mississippi River and its largest tributaries, where Dawley (1947) characterized it as "not common", the rock pocketbook was apparently both more common and more widely distributed in Minnesota in the past than it is today. Van der Schalie and van der Schalie (1950), reporting on survey work done by Ellis in the early 1930s, characterized the species as having a wide range, but seldom found in large numbers. Evidence of the rock pocketbook's past distribution can be found in turn-of-the-century shell middens left by pearl hunters near Red Wing, where the species is no longer found (M. Davis, Minnesota DNR, pers. comm.), and by relict shells found along the Minnesota River (Bright et al. 1990). The rock pocketbook continues to be rare in Minnesota waters, but recent surveys have found it to be repopulating portions of Pools 2 and 3 of the Mississippi River (Kelner and Davis 2002). It has never been reported from the St. Croix River, one of the last stable habitats for large river mussel fauna in the Upper Midwest. The rock pocketbook was listed as an endangered species in Minnesota in 1996.
The rock pocketbook is a fairly thin-shelled mussel species, which can be up to 15 cm (6 in.) long. The outside of its shell is green to dark brown, and it is heavily sculptured. The beak sculpture consists of 2 rows of large knobs or heavy, double-looped ridges, which become irregular folds or ridges as the individual matures. The pseudocardinal teeth are present, the lateral teeth are poorly developed, and the inner shell is white. The rock pocketbook resembles the threeridge (Amblema plicata) and the washboard (Megalonaias nervosa), but is distinguishable from them by its distinct coarse beak sculpture, thin shell, and reduced lateral teeth.
The rock pocketbook inhabits medium to large rivers. It may be found in fine substrates such as silt or sand in slow current areas (Parmalee and Bogan 1998).
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). Because the rock pocketbook is a rare species in Minnesota, its populations consist of a few individuals scattered through its habitat.
Conservation / Management
The viability of remaining rock pocketbook populations in Minnesota is jeopardized by the continuing decline in habitat conditions on the Mississippi River associated with its management as a navigation canal, and with 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 rock pocketbook 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 rock pocketbook is a thin-shelled species making it especially vulnerable to zebra mussel mortality. It is not considered commercially valuable to the cultured pearl industry, therefore the threats of poaching are thought to be minimal.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the rock pocketbook's ecology and current status in Minnesota. Additionally, 88 rock pocketbooks 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.