Among all the invertebrates in Spring Creek, crayfish probably have the most profound effect on the ecology of the stream, yet they are among the least noticed. During the day, you can carefully observe a small area of stream bottom and never see a single crayfish. If you were to go back to that location at night and shine a flash light on the bottom, it would be crawling with crayfish ranging in length from one to four inches.
The Rusty Crayfish is characterized by a large brown spot on the side of its carapace. The claws usually do not have a purple tinge. Photo obtained from Nonindigenous Aquatic Species website, U.S. Geological Survey.
Spring Creek supports three species of crayfish: the Appalachian Brook Crayfish (Cambarus bartonii), the Allegheny Crayfish (Faxonius obscurus), and the Rusty Crayfish (Faxonius rusticus). The Appalachian Brook Crayfish is native to the Spring Creek drainage while the Allegheny Crayfish was introduced from western Pennsylvania and the Rusty crayfish was introduced from the Midwest. There are at least 17 species of crayfish in Pennsylvania (1).
Crayfish are important to the ecology of a stream, because they can attain high densities and they will eat many different types of foods; hence, they are classified as omnivores. Crayfish will eat other invertebrates, plants, partly decaying material (detritus), and fish eggs. Dead fish do not persist very long, because crayfish will quickly consume them.
In nearby Spruce Creek, densities of native crayfish were as high as four per square foot. When cages were installed in the stream bottom and crayfish were excluded, densities of macroinvertebrates were over three times higher than in adjacent areas inhabited by crayfish. In midwestern lakes, Rusty Crayfish drastically reduced the amount of rooted aquatic vegetation, which had direct and indirect effects on invertebrates and fishes.
Crayfish are an important food source for many other animals. Birds such as herons and barred owls readily feed on crayfish. Among mammals, raccoons, muskrats, minks, and river otters rely on crayfish for food. Aquatic predators include water snakes, large salamanders, frogs, and fishes. In Spring Creek, brown trout readily feed on crayfish, which are commonly found in stomachs of brown trout 11 inches and longer.
Compared to other invertebrates, crayfish are long-lived; with life spans typically ranging from 2 to 4 years. Most reproduce in spring, when females extrude about 200 eggs and they are fertilized externally. The female carries the egg mass on her underside with the aid of finger-like projections known as swimmerets. Eggs hatch in about four weeks, and the tiny crayfish are protected by the female for another one to two weeks.
As crayfish grow, they periodically shed their hard carapace or exoskeleton, and then form a new protective exterior. This process is called molting. Juveniles may molt six to ten times during their first year of life, while older crayfish molt three to five times a year. Rapid growth contributes to higher survival of crayfish, because larger individuals are less prone to predation by fishes. Large size also contributes to a crayfish’s competitive ability. This is particularly true for the Rusty Crayfish, an introduced species that can displace native species.
The Rusty Crayfish is displacing native species in many waters throughout Pennsylvania. The Rusty Crayfish has large claws (chelae), it is aggressive, and it occurs at densities as high as 18 per square foot. Besides driving out native crayfishes, this high density of predators must surely have a negative effect on stream invertebrates, the food source for fish and other organisms. We know that Rusty Crayfish are well established in Spring Creek, but we are not certain about their effect on our other two crayfish species. Maintaining a high density of large brown trout may be our best hope for keeping the Rusty Crayfish under control.
(1) For more information on Pennsylvania crayfishes go to http://community.lhup.edu/tnuttall/pennsylvania_crayfish_reference_.htm
Robert Carline is a fisheries biologist and the former leader of the U.S. Geological Survey’s Pennsylvania Cooperative Fish and Wildlife Research Unit at Penn State University