As with anything, the new student of crayfishes will find it difficult in the beginning to distinguish between some species. Fortunately, there is a good record of crayfish distribution for Georgia (in The Crayfishes of Georgia by H.H. Hobbs Jr., see links) so it is possible to dramatically reduce the number of choices based on the location where the crayfish is observed or captured. In the past, I have gone page by page through the book to find out what species occur in a given area, and then looked through the illustrations at the possibilities. This is a somewhat time consuming but fairly reliable way to identify crayfishes in Georgia.
I have posted lists of species by drainage elsewhere on this site to help narrow the range of possible species you encounter. There is currently a color photo on a species account page for almost all of the species in the state, and a detailed account for the protected species. Eventually, there will be an account for all of the species to include a range map and pertinent illustrations. I am also developing dichotomous keys for various drainages in the state that correspond to the “species lists by drainage” that are already posted. Until the keys are complete you can easily click through the species account pages to try and identify your crayfish by matching it with the photos.
If possible, it is best to use adults for identification purposes. Virtually all of the illustrations you can find on crayfishes use adults, particularly adult males. Below are some tips to look for when trying to identify crayfishes.
Crayfish are composed of two main parts; the front half is the cephalothorax and the rear half is the abdomen (Fig. 1). The hard outer covering of the cephalothorax is called the carapace. The walking legs (pereiopods) are attached to the cephalothorax and smaller feathery appendages called pleopods (swimmerets) are attached to the underside of the first five segments of the abdomen. In males, the first two pleopods are modified for sexual reproduction with the first pleopod (gonopod) being the sperm transfer organ. At a glance these pleopods resemble small legs (Fig. 2). All of the pleopods look similar on females and a sperm receptacle called the annulus ventralis lies between the fourth and fifth walking legs (Fig. 3).
Figure 1. Lateral view of crayfish. Modified from Hobbs (1981) and Eversole and Jones (2004).
Figure 2. Ventral view of male crayfish. Modified from Hobbs (1981).
Figure 3. Ventral view of female crayfish. Modified from Hobbs (1981).
The most obvious feature of the crayfish has to be the claws (Fig. 4). You may see the word chela (plural chelae) written for claw, and some people call them pincers. The claws are really just another pair of legs that have a moveable segment at the end (dactyl) which allows the animal to grab things. When you look closely at the other crayfish legs, you will notice that the second and third pairs of legs have small claws at the end as well. The claws can be an important feature for species identification. As you gain experience, you will often be able to identify your crayfish to the genus and subgenus level (based on the claws) which will greatly speed the identification process. Important features of the claw are the length of the moveable finger (dactyl) relative to the length of the mesial margin of the palm, the number of rows of tubercles along the mesial margin of the palm, and in live individuals, color can be a useful characteristic. Although the claws can help with identification, they can also be very confusing. Crayfish frequently lose appendages during their life, but like many invertebrates, they can re-grow a claw or another leg. The claw they re-grow (called regenerated) can look quite different from the original claw. You will know for sure if you have at least one regenerated claw if one is much larger than the other. It becomes a problem when both were lost at the same time and then regenerated; they will look identical to one another buy may have odd proportions or a different number of tubercles relative to a normal claw. Another possibility is that one may have been lost early in the life of the crayfish and is virtually the same size as the original claw. Again, the regenerated claw will likely have a shorter palm and the wrong marginal tubercle count relative to the normal claw. These last two situations can be tough to ascertain and only experience can help.
Figure 4. Dorsal view of crayfish claw. Modified from Eversole and Jones (2004) and Prins and Hobbs (1972).
Another useful feature is the areola (Fig 5). The word areola means “space” and this refers to the space on the dorsal surface of the crayfish carapace (outer shell) that is bounded by shallow (usually curved) grooves oriented longwise on the body. It often has an hourglass-shaped appearance. These grooves mark the dorsal boundary of the branchial chambers which house the gills. The width and length of the areola are usually very similar among individuals of a species (and often different between species) and can help quickly narrow the field of suspects. The areolae of some species are said to be obliterated because there is no open space between the grooves.
Figure 5. Dorsal view of crayfish. Modified from Hobbs (1981).
The next characteristic that is fairly easy to observe is the rostrum (Fig 5). The rostrum is “nose” of the crayfish. Depending on which species you are looking at, it can be long and pointed, rounded, have an extra extension called an acumen, or be with or without marginal spines or tubercles.
One more “easy” characteristic that helps with identification is whether or not the animal possesses cervical spines. These projections are found on both sides of the body (cephalothorax) and can be a good clue for identification (Fig. 1). One group in GA, consisting of five species, has two cervical spines on each side of the body. So, if you find a crayfish with two cervical spines, you have the subgenus Pennides in the genus Procambarus. Many other species will have one distinct cervical spine while others will have only a tubercle or nothing at all.
Arguably the most difficult thing to learn concerning crayfish identification concerns the study of the male genitalia (gonopods). Some species that are encountered will only be separable by examining these structures. With a little practice, you will be able to easily place your specimen into the correct genus just by looking at the terminal projections of the male reproductive organ. Using the gonopods to differentiate between species of the same genus can be more difficult.
The male reproductive organs are the paired first pleopods and are attached to the first segment of the abdomen (Fig. 2). Except during the reproductive process, these appendages are oriented anteriorly and lie (sometimes hidden by setae) between the fourth and fifth legs of the male crayfish. The gonopods of all species in Georgia have at least two terminal elements, the central projection and the mesial process (Fig. 6). These are easy to pick out on the genera Cambarus and Orconectes, but are much more difficult on the complex gonopods of Procambarus. Further complicating this is the fact that adult male crayfish molt back and forth between a reproductive form called Form I and a subadult, non-reproductive form termed Form II. 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, the terminal elements are better defined and the central projection is partly or wholly corneous (appears yellowish and brittle, Fig. 6). The most complex gonopods are found on some species in the genus Procambarus. The Procambarus gonopod typically has several terminal projections and they are often hidden by long setae (hair-like structures; Fig. 6G). Most illustrations of Procambarus gonopods do not show the setae. You can use a fine tipped forcep to remove the setae if you need to see the detailed structures of the gonopod.
Figure 6. Representative left gonopods of the most common Georgia crayfish genera with central projection (cp) and mesial process (mp) indicated for Form I males. 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 with setae removed; G) Form I P. spiculifer with setae present. Modified from Hobbs (1981).