Mussel, fat pocketbook pearly

Potamilus capax

phylum: Mollusca

class: Bivalvia

order: Unionoida

family: Unionidae

status: Critically Endangered, IUCN Endangered, ESA

range: USA (Arizona, Illinois, Indiana, Iowa, Kentucky, Minnesota, Missouri, Mississippi, New York, Texas, Wisconsin)

Description and biology

The fat pocketbook pearly mussel is a fresh water mussel that grows to about 4 to 5 inches (10 to 13 centimeters) long. It has a smooth and shiny yellow, tan, or brown outer shell that is round and inflated. In young mussels, the shell is thin, but in adults it is thick. The inside of the shell is pink at the center and bluish white toward the shell edges.

The fat pocketbook lives at the bottoms of large rivers in places where the water is less than eight feet deep. It buries itself in the sand or mud at the bottom of the river with only its feeding siphons (tubular organs used to draw in fluids) exposed to the water. It then feeds by pumping water through its siphon, gathering nutrition from the tiny plant and animal life in the water.

The fat pocketbook spawns (produces eggs) in late August or September. To breed, the male fat pocketbook discharges his sperm into the river's current. The female, located downstream from the male, siphons the sperm to fertilize her eggs. The eggs are then kept in the female's gill (the organ used for obtaining oxygen from the water) pouches, where they develop into larvae (an immature stage of development, like the caterpillar stage of an insect). The larvae, called the glochidia (pronounced glow-KID-ee-), hatch in June or July, and the female fat pocketbook expels them into the water. In order to survive, each larva must find a host fish and clamp onto it with its tiny clasping valves. Only certain fish, such as freshwater drum and white crappie, are suitable hosts for fat pocketbooks, and if the larva does not find a host fish, it will fall to the bottom of the river and die. The larva that finds a host will remain attached to the fish for about two to four weeks, until it has grown its own shell. At that point, it unclasps itself from the fish and, if it is lucky enough to be in an appropriate habitat, buries itself in the sand or mud at the bottom of the river or stream. There it will probably remain for its long life. The life span of a fat pocketbook may be up to 50 years.

Habitat and current distribution

The fat pocketbook is found in drainage ditches or in sand, gravel, or muddy streams with flowing water. It is known to occur in the upper Mississippi River, the lower Wabash and Ohio rivers, the Cumberland River, and the White River and St. Francis River in Arizona.

History and conservation measures

The fat pocketbook has ceased to exist in much of its former range. For instance, the species inhabited the Mississippi River in Missouri up until the 1960s, but it has not been seen there for several decades. Elsewhere the population is declining, mainly due to severe loss of habitat.

Human efforts to change the course or nature of the waterways in which the fat pocketbook lives are responsible for most of the reduction in population. The construction of dams and channels and the practice of dredging (digging up the river bottom) for navigation and flood control present major threats to the fat pocketbook. These practices destroy the habitat at the bottom of the rivers and also result in siltation—filling the water with too much mud or sediment. When fat pocketbooks siphon in silt, it can block up their feeding organs and it can also suffocate them. All changes in water temperature, water flow, and water quality are potentially harmful to these mussels. Pollution from farming and industry has also caused their decline. The fat pocketbook is a filter-feeder, and chemicals or contaminants in the water will eventually poison it. The destruction of the river habitat may also be reducing the number of the host fish that are essential to the fat pocketbook's life cycle.

The U.S. Fish and Wildlife Service has put together a recovery plan for the fat pocketbook. It is working to protect and preserve the population of the species that is currently living in the St. Francis River in Arizona, particularly by protecting this habitat from dredging and other destructive practices. There is ongoing research into the habits of the mussel and a search for other populations in the wild. Fat pocketbook populations are being reintroduced into the former habitats of the species, by releasing both adult fat pocketbooks and fish with fat pocketbook larvae attached into rivers where the habitat is deemed stable.