White Catspaw Pearlymussel

Epioblasma obliquata perobliqua

Description

The white catspaw pearlymussel (Epioblasma obliquata perobliqua ) rarely exceeds 1.7 in (4.3 cm) in shell length. The outer surface of the shell is greenish-yellow with numerous light green rays that radiate from posterior to anterior. The species is sexually dimorphic, meaning that males and females differ in structure and appearance. The larger female is more pointed at the valve end and bears little teeth along the shell margin. The teeth give the appearance of tiny claws, hence the name "cat's paw."

Behavior

The life of mussels is complex, and reproduction often depends upon a stable habitat—unaltered stream conditions, clean water, and an undisturbed stream bottom. The cycle also depends upon the abundance of suitable fish hosts to complete the mussel's larval development.

To reproduce, males discharge sperm, which are dispersed by stream currents. In the process of feeding, females nearby or downstream take in sperm, which fertilizes eggs stored in their gills. The gills serve as brood pouches (marsupia), where the glochidia hatch and begin to develop. After a time, these glochidia are released into the stream. A few mussels have inner parts that resemble a tiny minnow and can be manipulated to lure host fish. When a fish gets close to the shell, the mussel expels its glochidia.

Glochidia have tiny bean-or spoon-shaped valves that attach to the gill filaments of host fish. Glochidia can only progress to the juvenile stage while attached to the fish's gills. Those that do not fortuitously encounter a host fish do not survive when released by the female mussel. They sink to the bottom and die.

When the juvenile has developed a shell and is large enough to survive on its own, it detaches from the host fish and falls to the stream bottom, beginning a long association with a single stretch of stream. Maturing mussels bury themselves in riffles and shoals with only the shell margins and feeding siphons exposed to the water. Some mussels live as long as 50 years or more.

The family Unionidae, which includes all of the freshwater mussels in the United States, is separated into two groups based on the length of time the glochidia remain in the female's marsupia. The eggs of the short-term (tachytictic) breeders are fertilized in the spring, and glochidia are released by late summer of the same year. Long-term (bradytictic) breeders hold developing glochidia in the brood pouch over winter and release them in the spring.

Freshwater mussels feed by siphoning phytoplankton and other plant matter from the water. Indigestible particles are expelled from the shell by reverse siphoning. Silt in the water can kill mussels by clogging their feeding siphons.

There are no known interspecific differences in feeding among freshwater mussels. The glochidia are obligate parasites on the gills or fins of fish. Adult mussels are filter-feeders and consume particulate matter in the water column. Identifiable stomach contents almost invariably include desmids, di-atoms, algae, protozoa, and zooplankton.

Most freshwater mussel species display seasonal variations in activity associated with water temperature and reproduction. Metabolic rate is, in part, positively correlated with temperature. Many ectothermic species have the capacity to adjust their metabolic rates in response to long-term changes in temperature. Thus, metabolic rates do not continue to rise as temperatures rise in the summer, and they do not continue to fall during the winter as temperatures decline.

Some freshwater mussels also show diurnal changes in metabolic rates that indicate a tendency toward nocturnal activity patterns. Mussels may move to the surface to feed at night and move deeper into the substrate during the day; this is one way to avoid predators that hunt by visual contact.

Freshwater mussels are nonmigratory.

Habitat

This mussel inhabits freshwater streams and rivers and favors stable, sandy-gravel bottoms. It prefers reaches where the water is fairly shallow and the current strong enough to keep silt scoured from the bottom.

Distribution

The white catspaw pearlymussel was carried into its current range during the warming period after the last glaciation. As the Wisconsin Glacier melted, fish carried mussel eggs north into the Maumee River and its tributaries (such as the St. Joseph and Auglaize rivers, which drain into Lake Erie). There is evidence to suggest that a population was established at the edge of Lake Erie in the vicinity of Toledo, Ohio. Other populations were found in the Wabash River, which arises close to the Maumee River but drains instead to the southwest, eventually emptying into the Ohio River west of Evansville. Populations in this drainage were probably centered near the confluence of the Eel River in Cass County, Indiana.

The current distribution of the white catspaw pearlymussel is extremely limited relative to its historic range. Since 1985 surveys have located this species in the St. Joseph River that joins the St. Marys River at Fort Wayne, Indiana, to form the Maumee. Mussels were found in both the Indiana and Ohio portions of the river. Scattered individuals were also found in Fish Creek in Indiana. The catspaw survives in a few viable beds in fairly low numbers.

Threats

The great forests that once stretched across most of Ohio and Indiana have largely been cleared to support intensive agriculture. Poor agricultural practices in many cases have clouded the rivers with topsoil runoff, raising turbidity levels and smothering once-favorable stretches of mussel habitat with silt. This is probably the primary cause of the overall decline of the white catspaw pearlymussel. Pesticide and fertilizer contamination probably played a role in degrading habitat. Filter-feeders like mussels must siphon many gallons of water to extract food; if the water is contaminated, poisonous residues concentrate in their tissues.

Along with existing threats, perhaps the greatest potential for damage to the mussel population may be a catastrophic event such as a toxic chemical spill, which could extirpate a large percentage of the species. Such an event has already taken place, although fast clean-up measures limited the damage to the habitat. On September 15, 1993, a pipeline break discharged an estimated 30,000 gal (113,562 l) of #2 diesel fuel in a crop field in DeKalb County, Indiana. Spilled fuel made its way to a small drainage ditch that discharges into Fish Creek. While response action limited potential damage, it did not prevent injury, and the catastrophe may have long-term residual effects, since fuel accumulated along the banks and in detrital organic matter and sediment.

Conservation and Recovery

The U.S. Fish and Wildlife Service encourages local landowners to follow more ecologically sound agricultural practices to reduce the amount of suspended solids in habitat waters. Known populations are being monitored at least every two years to determine trends, and potential habitat in the basin is being surveyed to locate additional populations and to identify possible sites for reintroduction. The immediate goal for the recovery of this subspecies is to protect the only remaining population. Other goals include increasing present distribution and adding to current knowledge of life history.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360
http://midwest.fws.gov/

References

Johnson, R.I. 1980. "Zoogeography of North American Unioniacea (Mollusca: Bivalvia) North of the Maximum Pleistocene Glaciation." Bulletin of the Museum of Comparative Zoology 149 (2): 77-189.

Stansbery, D.H. 1971. "Rare and Endangered Freshwater Mollusks in Eastern North America." In S. Jorgensen and R. Sharp, eds., Rare and Endangered Mollusks (Naiads) of the U.S. U.S. Fish and Wildlife Service, Twin Cities, Minnesota.

U.S. Fish and Wildlife Service. 1990. "White Cat's Paw Pearly Mussel Recovery Plan." U.S. Fish and Wildlife Service, Twin Cities, Minnesota.