Northern Riffleshell (Epioblasma torulosa)

Life History & Michigan History

This member of the pearly mussel family is listed as endangered by both the U.S. Fish and Wildlife Service and the State of Michigan. The historic range was confined to southeastern Michigan, in several of the major river systems including the Detroit, St. Clair, and Raisin rivers. Recent surveys have found the riffleshell in only the Black, St. Clair, and Detroit rivers. The future of the Detroit River population is in serious condition.

The ovate shape of an adult riffleshell will reach 2 inches in diameter. They are light green-yellow to olive green and have dark, narrowly spaced rays. Their habitat is swiftly flowing, well-oxygenated water. Coarse gravel runs provide the best bottom in these rivers.

Most mussels develop through a secondary fish host. Females carry eggs from late summer until the following spring when they release the glochidia (young) into the water. The glochidia attach themselves to certain species of darters, sculpins, and trout. These fish act as hosts while the glochidia grow. After development, the glochidia become mussels and drop off into the gravel substrate. Anular ring counts indicate they may live up to 15 years.

Mussels are filter feeders. They feed on the microscopic zooplankton and phytoplankton found in the water system. Because they are filter feeders, heavy amounts of siltation from runoff can smother mussels. Other causes of declines include water pollution, dredging, and the invasive zebra mussel.

Conservation of these unique animals will require the protection and preservation of their high quality habitat and their host fish species. The zebra mussel invasion poses a significant threat to several of the remaining populations. Transplantation of populations to more protected habitats may be needed. Many answers remain in how best to preserve these mussels as part of Michigan's native biodiversity.

Non-DNR Links

Species Profile (U.S. Fish & Wildlife Service)

Epioblasma rangiana (Illinois Natural History Survey)