Arcidens confragosus    (Say, 1829)

Rock Pocketbook 


MN Status:
endangered
Federal Status:
none
CITES:
none
USFS:
none

Group:
mussel
Class:
Bivalvia
Order:
Unionoida
Family:
Unionidae
Habitats:

(Mouse over a habitat for definition)


Arcidens confragosus

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Arcidens confragosus
Minnesota range map
Map Interpretation
North American range map
Map Interpretation

  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.

  Description

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.

  Habitat

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.

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. Fuller (1978) reports that the rock pocketbook is bradytictic, with females brooding their young long-term, from September through June, before they are released as glochidia. 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. The freshwater drum (Aplodinotus grunniens), rockbass (Ambloplites rupestris), and white crappie (Pomoxis annularis) have been reported to be viable host fish species for the glochidia of the rock pocketbook (Fuller 1978).

  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.

  References and Additional Information

Bright, R. C., C. Gatenby, D. Olson, and E. Plummer. 1990. A survey of the mussels of the Minnesota River, 1989. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 106 pp.

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697.

Fuller, S. L. 1978. Fresh-water mussels (Mollusca: Bivalvia: Unionidae) of the Upper Mississippi River: observations of selected sites within the 9-foot channel navigation project on behalf of the United States Army Corps of Engineers (USACE). Report submitted to the USACE, No. 78-33. Academy of Natural Sciences of Philadelphia, Division of Limnology and Ecology, Philadelphia, Pennsylvania. 401 pp.

Kelner, D., M. Davis. 2002. Final report: mussel (Bivalvia:Unionidae) survey of the Mississippi National River and Recreation Area Corridor, 2000-01. Final report submitted by the Minnesota Department of Natural Resources. 44 pp. + appendices.

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.

van der Schalie, H., and A. van der Schalie. 1950. The mussels of the Mississippi River. American Midland Naturalist 44:448-464.