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Mussel image: Adult Fat Pocketbook mussels can measure up to 5 inches across. Photo by Bruce Reid.

Mussel image: Adult Fat Pocketbook mussels can measure up to 5 inches across. Photo by Bruce Reid.

The endangered Fat Pocketbook mussel is one of three priority species listed in a  Strategic Habitat Conservation Plan for the Lower Mississippi River. Although it was listed as federally endangered in 1976, and at that time known only from the St. Francis River system in Arkansas, it was first found in the main channel of the lower river about two decades later. Since then, the U.S. Fish and Wildlife Service and the U.S. Army Corps of Engineers have collected adult Fat Pocketbook mussels at multiple sites between river mile 346 (south of Natchez, Mississippi) and river mile 670 (north of Helena, Arkansas), and reproduction has been documented at some sites.

Freshwater mussels are related to marine mussels, clams and oysters. They are similar in that they have two shells connected by a hinge-like ligament. They differ in that the larvae of most species of freshwater mussels, called glochidia, must attach to and parasitize a fish to transform into a juvenile mussel, whereas the larvae of marine species are free-living, floating with currents. Fat pocketbook glochidia parasitize Freshwater Drum, a fish found in many flowing water habitats.  The Fat Pocketbook spawns from late August through September and releases glochidia the following year in June and July.

The Fat Pocketbook appears to be expanding its range because of the current practice of notching rock dikes along the river to restore more consistent water flows to side channels. Hundreds of dikes have been installed perpendicular to the banks of the lower river to force river water toward the navigation channel. As a result, flows to side channels have been restricted. Notches, or gaps in the dikes, allow river water to flow into the side channels for most of the year without harming navigation. Recent collections of live Fat Pocketbook mussels and fresh dead shells are associated with side channels stabilized by notched dike fields, where there is clearer, flowing water the species needs to survive. Since 2006, the Lower Mississippi River Conservation Committee has worked with the Corps and the Fish and Wildlife Service to complete 14 dike-notching projects affecting more than 50 miles of side channels. The Corps is also designing notches into new dikes and retrofitting existing dikes with notches.

Species: Fat Pocketbook mussel (Potamilus capax)

Status: First listed as endangered in 1976, based on the only known viable population at the time in the St. Francis River system in Arkansas. A review of its status was published in 2012.

Size: Shells of adult mussels reach up to 5 inches across.

Diet: Filters food particles from water. Specific food items unknown, but other freshwater mussels are known to feed on plankton, diatoms and detritus.

Habitat: Requires flowing water and found on a broad range of substrates. Most recent collections on the lower river in side channels associated with notched rock dikes. Larvae disperse by attaching themselves to the gills of fish. The Freshwater Drum appears to be the primary host fish.