Sometimes biologists get a chance to play detective, as we did on a particular “case” in early 2019. For the last two years, I’ve collaborated closely with Brian Wagner, aquatic wildlife diversity biologist at the Arkansas Game and Fish Commission (AGFC), on a project to re-assess conservation ranks and statuses of every crayfish species that occurs in Arkansas (see Reassessment of Arkansas Crayfish Conservation Ranks). During our weekly ranking sessions, Brian and I often come across gaps in our knowledge, questionable records, issues of complicated taxonomy, and situations where we aren’t entirely clear which species are found in particular watersheds. More often than not, our response to these questions turns out to be “Let’s go take a look for ourselves!” and thus these meetings have at times turned into impromptu fieldwork planning sessions.

In the clear, beautiful waters of certain streams in the Ouachita Mountains of southwest Arkansas, a truly striking-looking crayfish bearing a “leopard-print” spotting pattern is sometimes seen. In the past, Brian had collected some of these beautiful crayfish in the Cossatot River and determined them to be Little River Creek Crayfish (Faxonius leptogonopodus) based on anatomical features. This small crayfish, first described in 1948 from specimens collected in a small tributary of the Mountain Fork River southwest of Mena, AR, is endemic to the Little River watershed in the Ouachita Mountains of southwest Arkansas and southeast Oklahoma (the Cossatot and Mountain Fork are both tributaries of the Little River). Not all populations of this species bear the dramatic pattern though; in fact, the existing literature simply described the species as being uniformly gray or brown in coloration.

Another species endemic to streams in the Ouachita Mountains is the Mena Crayfish (Faxonius menae), first described from specimens collected in the Irons Fork of the Ouachita River in 1933. Unlike the Little River Creek Crayfish, the Mena Crayfish actually is described in the literature as having a maculate (spotted) pattern. Many collections of F. menae have come from tributaries of the Upper Ouachita River, which is an entirely different major river system than the Little River (a tributary of the Red River), but other collections of this species have come from tributaries of the Mountain Fork in Arkansas and Oklahoma, the same watershed that is home to F. leptogonopodus. Although some aquatic species are found only in one watershed or the other, the headwater creeks of these two major river systems actually lie in fairly close geographic proximity in places.

When it comes to identifying animals, it is only natural for us to focus on things that first catch our eye, like coloration or pattern. While these can be useful clues in crayfish, it is important to remember that color and pattern can vary dramatically within many species. The color of crayfish can be influenced by environmental conditions such as diet or habitat, can change from one developmental stage to the next, and can also vary considerably between populations. For these reasons, definitive identifications between otherwise similar-looking crayfish are most often made using anatomical features that do not vary as much as color.

One of the main features that astacologists (biologists who study crayfish) use in species determination is the shape of the male reproductive organs, or gonopods. Gonopods are the greatly elongated, forward-projecting first pair of swimming legs. In male crayfish, the gonopods are used to transfer sperm to female crayfish. Differences in gonopod structure feature heavily into dichotomous keys for crayfish, but in order to make these identifications, it is important to examine the gonopods of males in the Form I condition. Male crayfish cycle between Form I, or active breeding condition and Form II, the nonbreeding condition, and the differences in gonopod shape become more pronounced and distinctive in Form I males. In fact, F. leptogonopodus derives its scientific name from the unusually long and slender shape of its gonopods (“lepto” is a Greek prefix meaning slender or fine).

On a cold day in January 2019, Brian and I set out along with two other biologists from the AGFC, Justin Stroman and Katie Morris for the Ouachita Mountains of southwestern Arkansas. Our first destination was the Cossatot River near the visitor center of Cossatot River State Park-Natural Area. The Cossatot River is home to many rare endemic species, including fishes such as the federally threatened Leopard Darter (Percina pantherina) and the Ouachita Mountain Shiner (Lythrurus snelsoni), both of which, like the Little River Creek Crayfish, are found only in the Little River watershed. On several occasions, including my very first survey for ANHC, I have observed beautiful leopard-spotted crayfish at this very spot. We kick-seined in the riffles (shallow, fast-flowing portions of the stream) not far from the visitor center, and it didn’t take us long to find what we were looking for. We found six females and three Form I male crayfish, all beautifully covered in the leopard spotted pattern. Examination of the noticeably long, slender gonopods quickly confirmed that these were indeed F. leptogonopodus.

After a quick stop for lunch in Mena, we hopped watersheds over to Powell Creek, a tributary of the Mountain Fork of the Little River west of town, at a site where Mena Crayfish had been collected in the past. We were curious to see what these crayfish would look like if we could find them.

The more challenging stream-bed, strewn with large cobbles and boulders, meant that we had to work a bit harder to find what we were after at this site, often reaching into the cold water to flip boulders up by hand. This technique was effective, resulting in the discovery of a variety of interesting crayfish. We found several beautiful Western Painted Crayfish (Faxonius palmeri longimanus), a widespread species in Arkansas aptly named for its beautiful coloration. Under a particularly large boulder, we found a single Ouachita Mountain Crayfish (Fallicambarus tenuis), a mysterious and rare endemic belonging to a genus otherwise entirely consisting of primary burrowers, i.e. species that primarily live underground in burrows reaching the water table. This species is intriguing in that it is sometimes found burrowing in ditches and seeps far from permanent streams, and at other times can be found tunneling beneath boulders in swift permanent creeks such as Powell Creek. And crayfish were not the only crustaceans we found on the survey; clinging to the rocks we found tiny freshwater Isopods, distant aquatic relatives to roly-polys.

But most importantly, we collected several more “leopard-spotted crayfish” bearing a close resemblance to those we had collected in the Cossatot. When we examined the gonopods of the Form I males, they were quite different, lacking the long slender processes of F. leptogonopodus, and in fact matching the description of F. menae. So despite the incredibly similar coloration, we confirmed that these crayfish actually belonged to a different species, and the answer to the question of which species had the leopard-spotted pattern ended up being “both."