Crayfishes in the Classroom: A New Look at Some Old Friends

Abstract

Crayfish are fascinating animals that inhabit virtually every freshwater ecosystem in the United States. Only Alaska, Hawaii, and portions of the western Great Plains and desert Southwest are without natural populations of these organisms. However, because crayfish are secretive animals, their presence frequently goes unnoticed. Nevertheless, they do play an important role in the ecology of many aquatic communities and are worthy of more attention by biology teachers and their students. Crayfishes are members of the class Crustacea, a group of arthropods with gills and heavy, crust-like exoskeletons. Included in the Crustacea are such diverse forms as the microscopic water fleas and copepods, the larger sowbugs and amphipods, and the more familiar decapods including the crayfishes and river shrimp. In the United States, the crayfishes are represented by two families (Astacidae and Cambaridae), 10 genera, and nearly 300 species (Hobbs 1974). Crayfishes occupy almost every conceivable aquatic habitat from streams, rivers, and lakes to springs, marshes, and temporary ponds. According to Crocker and Barr (1968), most species are stream dwellers, usually hiding under cover, waiting to seize any food that happens by. Because their gills must be kept moist to function properly, no crayfishes are permanently terrestrial. However, crayfishes may venture on shorelines at night and some can travel considerable distances over land if the grass is moist with dew or rain. Some crayfishes even make their own aquatic habitat by digging burrows or tunnels below the water table. These burrows are usually located along stream banks or near ponds, but if the water table is not too deep, they may be found in fields far from any body of water. Different species dig burrows of different design (Crocker and Barr 1968). Some tunnels are simple and unbranched while others are equipped with side branches and enlarged chambers. When digging a tunnel, the crayfish brings mud to the surface and deposits it around the opening of the burrow forming a chimney that may be several inches high. The importance of crayfishes in the food webs of most aquatic ecosystems is due primarily to their tremendous reproductive potential and their omnivorous feeding habits. According to Meredith and Schwartz (1960), the natural diet of crayfishes includes insect larvae, worms, smaller crustaceans, snails, small fish, tadpoles, and aquatic plants. Burrowing species often eat large amounts of young terrestrial plant stems and roots. Adults usually feed at night, but immatures may frequently forage during the day. Crayfishes also consume large quantities of dead plant and animal material and as a result, represent an efficient means of main taining the flow of energy within aquatic communities. Conversion of detritus to food for vertebrate animals is completed without the loss of energy and substance which a more complex pathway would entail. Despite their formidable appearance, nocturnal habits, and other protective devices, crayfishes lead precarious lives as they are preyed upon by a host of other animals. Their greatest enemies are probably fish, but they are also eaten by frogs, turtles, water snakes, and raccoons. Birds such as kingfishers and herons also take their toll (Pennek 1978). Crayfishes occasionally eat each other; in such cases, recently molted individuals are usually the victims.