Crayfishes of Georgia


Introduction to the Crayfishes of Georgia

Crayfishes are freshwater decapod crustaceans that live in a variety of aquatic habitats worldwide. They are related to better known decapods such as lobsters, crabs, and shrimp. Crayfishes are native to all continents except Africa and Antarctica and are most common in North America. Of the approximately 625 species found on earth, over 400 live in North America, with the southeastern United States being the center of global diversity. We see this same pattern of biodiversity with other temperate aquatic organisms such as freshwater fishes and mussels. The other area with a major crayfish fauna is Australia with over 150 species. 

Georgia is home to approximately 70 species of crayfishes which ranks it fourth in diversity behind Alabama (~100), Mississippi (~80), and Tennessee (~95). Crayfishes exhibit high rates of endemism, which means that many species occur only within a specific and often small geographic area. For example, sixteen of Georgia’s species occur nowhere else in the world and many are restricted to a single or small number of river systems within the state.

The crayfish fauna of Georgia is well known because of the seminal work of Dr. Horton H. Hobbs Jr. titled The Crayfishes of Georgia (Hobbs 1981). Dr. Hobbs is considered the father of American crayfish work and described about 60% of the Georgia fauna as new species and nearly 40% of all the species found in North America! The Crayfishes of Georgia has been out of print for a long time but is now available online as a pdf from the Smithsonian Institution Press. This volume is full of detailed information on variation within species and is a must for the serious student of Georgia crayfishes. It is also an invaluable reference on species that are more widespread and found in Georgia and adjacent states.

For nearly 15 years after Hobbs (1981) publication, little work was conducted on Georgia crayfishes. In 1996, Taylor et al. published a paper outlining the conservation status of all the crayfishes found in the United States. In this article, the authors suggested that as many as 22 of the species found in Georgia were imperiled in some way. As a result of these findings, the Georgia Department of Natural Resources initiated studies to better assess the conservation status of Georgia crayfishes. This work, combined with a follow-up conservation paper in 2007 by Taylor et al., led to the protection of 20 species under Georgia’s Endangered Wildlife Act.

There is still much to learn about Georgia crayfishes and new information on life history, ecology, and distribution patterns continues to be discovered. We encourage you to email gabiodata@dnr.ga.gov with photographs and locations of crayfishes that you observe to help us update our database.


Ecology & 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 (Figs. 2, 3). 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. Secondary burrowers spend part of their lives in open water and retreat to relatively simple burrows for protection during the daylight hours; they may also use burrows to survive dry periods. Holes in creek banks that are sometimes called “snake holes” are more likely created and inhabited by a crayfish; snakes do not create their own holes. In some crayfish species, females burrow when it is time to release their eggs (tertiary burrower).

Crayfish burrow types
Figure 1. Crayfish burrow types. A) Primary burrower; B) Secondary burrower; C) Tertiary burrower. Illustrations by Carolyn Gast from Hobbs (1981).

 

Crayfish burrow
Figure 2. Crayfish burrow marked by three openings, only one of which shows fresh chimney material.

 

Crayfish chimney
Figure 3. Crayfish burrow marked by an impressive “chimney”.

 

There are approximately 30 obligate cave crayfish species in North America, two of which live in Georgia. Most of the cave species are not pigmented and some are blind; they rely on tactile and chemical senses to negotiate their lightless habitats. In Georgia, the Dougherty Plain Cave Crayfish is found in underground regions in the southwestern part of the state. The Pallid Cave Crayfish was discovered recently and is only known only from two locations in Brooks County.

Pallid Cave Crayfish
Figure 4. Pallid Cave Crayfish (Procambarus pallidus) from Brooks County, GA. Photo by Barry Mansell. Photo may be subject to copyright.

 

Crayfishes are typically inactive during the day and come out at night to feed. It has been said that “crayfishes eat everything and everything eats crayfishes.” This is largely true, and hence, crayfishes play a pivotal role in freshwater food webs. Virtually all 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 in the form of a large tail muscle and thereby become an important food source for a host of organisms. 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.

A great video that depicts a crayfish molting can be found at: https://thekidshouldseethis.com/post/a-molting-blue-crayfish

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 (see “Crayfish Identification” below for more detail). 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, Figs. 5, 6).

Gonopod chart
Figure 5. Representative left gonopods of the most common Georgia crayfish genera. A) Form II Cambarus latimanus, B) Form I C. latimanus, C) Form II Faxonius spinosus, D) Form I F. spinosus, E) Form II Procambarus spiculifer, F) Form I P. spiculifer. cp=central projection; mp=mesial process. Modified from Hobbs (1981).

 

Form II

Form I
Figure 6. Example terminal elements of the gonopod of the crayfish genus Cambarus. A) Form II, B) Form I.

 

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” (Fig. 7).

In berry female
Figure 7. Female crayfish with eggs (“in berry”).

 

Upon hatching, the juvenile crayfish are attached to the mother by a thread (Fig. 8). After the young 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 may live as long as 20 years!

Female crayfish with young attached
Figure 8. Female crayfish with attached young. Photo by Giff Beaton. Photo may be subject to copyright.

 

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. The largest species in Georgia are in the genus Procambarus and may reach a total length of six inches. Individuals of the western North American genus Pacifasticus reach sizes of 7–8 inches while the Tasmanian lobsters (Astacopsis spp.) can weigh over 10 pounds!


Conservation

While Georgia’s crayfish fauna is very diverse, it also threatened. Twenty species (~28% of Georgia species) are considered imperiled and are protected under Georgia’s Endangered Wildlife Act. Threats to crayfish are in part, similar to those of other aquatic organisms. For example, siltation from poor land use practices can fill spaces used by crayfish as daytime refugia and also limit good habitat for aquatic insects that crayfish feed upon. As mentioned above, many crayfish species are known from only a small geographic area. Just think of a species that is known from only a few populations, like many of Georgia’s rare crayfish species. An unforeseen event such as a chemical spill would sharply reduce the total numbers of a given species and increase the probability of local population loss (extirpation) or in some cases global extinction.

The introduction of nonindigenous invasive species is likely the most serious threat to Georgia’s crayfishes. Many people think of things like kudzu and pythons as being invasive. That is true, but in the case of crayfishes, just moving a species from one river system to another could have serious consequences. Crayfishes fight for space and other resources. If a species is introduced to a new river system, it has the potential to take over the new area if it has traits such as faster and larger growth, the ability to produce more young, or by being more aggressive. If an introduced species is better at taking over the best habitats, the native species will struggle to survive. Never move crayfishes (or any aquatic organism) to a waterbody away from the one in which it was captured. Whether you are an angler using crayfish as bait or you just purchased a sack of live crayfish for a crawfish boil, preventing the release of crayfishes into ponds, streams, or other waterbodies may be the single-most important thing you can do to protect Georgia’s rich crayfish fauna and the myriad of species that depend on it.

Find out more at Conserving Georgia’s Aquatic Species.

Red Swamp Crayfish
Figure 9. The Red Swamp Crawfish (Procambarus clarkii) is native to the south-central United States but has been introduced around the world as a food source. It is considered an invasive species and is now commonly found in Georgia. Photo by Chris Lukhaup. Photo may be subject to copyright.

Crayfish Identification

As with any group of organisms, the new student of crayfishes may find it difficult to distinguish between some species. Fortunately, there are extensive distribution records for crayfishes in Georgia. This is important because when attempting to identify an unknown crayfish, it is possible to dramatically reduce the number of choices based simply on the location that the crayfish was observed or captured. Thus, one way to try to determine the species of crayfish that you have seen is to visit the crayfish page on the Georgia Biodiversity Portal. You may search this database by county, quarter-quad, or watershed, and by doing so, generate a list of possible species in your area. Once a list is generated, you can click on an individual species name, which will take you to the account for that species. Each account includes a description, a color photo, and a range map. When first learning the crayfishes, you may just need to start at the beginning of the list you generated and click through the various species accounts to find your best match. 

Alternatively, you can attempt to identify your crayfish using a dichotomous key, also accessible through the crayfish page on the Georgia Biodiversity Portal. On the dichotomous key page, you will find a clickable drainage map that will take you to a list of species and the dichotomous key for your particular drainage. There are also tips for using keys, general information on crayfish anatomy, and discussion of the primary characteristics used for identification.


Literature Cited

  • Hobbs, H.H., Jr. 1981. The crayfishes of Georgia. Smithsonian Contributions to Zoology 318:1–549.
  • Taylor, C.A., M.L. Warren Jr., J.F. Fitzpatrick Jr., H.H. Hobbs III, R.F. Jezerinac, W.L. Pflieger, and H.W. Robison. 1996. Conservation status of crayfishes of the United States and Canada. Fisheries 21:25–38.
  • Taylor, C.A., G.A. Schuster, J.E. Cooper, R.J. DiStefano, A.G. Eversole, P. Hamr, H.H. Hobbs III, H.W. Robison, C.E. Skelton, and R.F. Thoma. 2007. A reassessment of the conservation status of crayfishes of the United States and Canada after 10+ years of increased awareness. Fisheries 32:372–389.