ECOLOGY AND LIFE HISTORY
Crayfishes can be found in a variety of habitats including streams, lakes, marshes, roadside ditches, cave systems, and even in burrows that are sometimes well away from open water. In Georgia, particularly the northern part of the state, most species inhabit streams. As you move farther south in the state however, more and more species can be found in temporary habitats such as ditches and wet-weather ponds. As water recedes during dry times, the species in these habitats will burrow into the ground.
All crayfishes are capable of burrowing but some spend most of their lives in burrows. The latter, called primary burrowers, construct complex burrow systems and seem to be constantly moving soil around (Fig. 1). Their burrow openings are usually marked by mounds of soil referred to as chimneys. In Georgia, primary burrowing species are typically found in low, wet (“swampy”) areas, often in the floodplains of streams or rivers. Since these species are difficult to observe, they are poorly studied and several appear to be quite rare. About 15 of Georgia’s species are considered primary burrowers. As mentioned above, secondary burrowers spend part of their life in open water and retreat to relatively simple burrows to survive dry periods period. In some species, females likely burrow when it is time to release their eggs (tertiary burrower).
Figure 1. Crayfish burrow types. A) Primary burrower; B) Secondary burrower; C) Tertiary burrower. Illustrations by Carolyn Gast from Hobbs (1981).
There are approximately 30 obligate cave crayfish species in North America, one of which lives in Georgia. The Dougherty Plain Cave Crayfish is found in underground regions in the southwestern part of the state.
Crayfishes are typically inactive during the day and come out at night to feed. I have heard it said that “crayfishes eat everything and everything eats crayfishes”. This is largely true, and hence, crayfishes play a pivotal central role in freshwater food webs. Virtually all of the energy that sustains animals comes from nutrients produced by the photosynthetic activity of green plants. In stream and river systems however, there are often few plants growing directly in the water. Where does the energy come from that powers stream and river food webs? Well, most of the plant material that supports aquatic organisms comes from sources outside of the stream such as leaves and woody debris from trees. Crayfishes will eat this plant matter as well as insects that eat plant matter. Thus, they are converting “plant” energy into protein (large tail muscle) and thereby become an important food source for a host of organisms. Some crayfish predators include raccoons, kingfishers, wading birds, otters, minks, and of course, humans. There is a group of snakes of the genus Regina that are crayfish specialists. One Georgia species (Queen snake) eats only freshly molted individuals while others eat hard-shelled individuals. Crayfishes are a major food source for many fishes, particularly the basses and sunfishes in the family Centrarchidae.
Like other arthropods such as insects and spiders, crayfishes have a rigid exoskeleton. This hardened outer covering acts like our skeleton by providing attachment points for muscles, thereby facilitating movement. The problem is that the exoskeleton is largely composed of non-living tissue and therefore does not grow. In order for crayfishes to grow, they must periodically shed (molt) the exoskeleton. Once they molt, their body is soft (claws are useless) and they are extremely vulnerable to predation. The new exoskeleton will harden in about two days.
Male crayfishes (and possibly females) have a somewhat unusual pattern of molting once they become adults. Males molt back and forth between a reproductive form (Form I) and a non-reproductive form (Form II). The primary differences between Form I and Form II males are the development of the claws and the shape of the reproductive organ which is called the first pleopod, or gonopod. In juvenile and Form II males the gonopod appears to be the same color and consistency throughout its length and has more blunt and rounded features. In Form I males, at least one of the terminal elements is corneous (appears yellowish and brittle, Fig. 2).
Figure 2. Representative left gonopods of the most common Georgia crayfish genera. A) Form II Cambarus latimanus, B) Form I C. latimanus, C) Form II Orconectes spinosus, D) Form I O. spinosus, E) Form II Procambarus spiculifer, F) Form I P. spiculifer. Modified from Hobbs (1981).
Reproduction usually occurs during the spring and fall, but males in reproductive condition may be found at any time during the year. When female crayfish are ready to lay eggs, they usually find a secure hiding place and hence are rarely encountered. When the eggs are released, the female attaches them to her swimmerets (pleopods) and is said to be “in berry”.
Upon hatching, the juvenile crayfish are attached to the mother by a thread. After the juveniles molt for the second time, they are free of the mother, but stay close and will hold on to her for some time. Eventually they move off on their own. Crayfishes molt six or seven times during their first year of life and most are probably able to reproduce by the end of that year. They molt once or twice a year for the remainder of their lives. Stream dwelling species live only about three years in Georgia. Recent life history studies of burrowing crayfish indicate that they live at least 7-8 years and cave crayfishes are thought to live as long as 20 years.
Most stream crayfishes reach a total body length of no more than 3-4 inches. With the claws included though, some individuals can appear quite large. Individuals of the western genus Pacifasticus reach sizes of 7-8 inches while the Tasmanian lobsters (Astacopsis spp.) can weigh over 10 pounds!