Crayfish chimneys are a familiar site to most Arkansans. Seemingly springing up overnight in backyards, parks, creek banks, and roadside ditches, these structures are evidence that some of the most secretive and fascinating creatures in the Natural State are hard at work. A crayfish chimney consists of soft mud pushed to the surface by a crayfish during the excavation of its burrow and is simply the visible, uppermost portions of its burrow, which may extend many feet underground.

A closer inspection reveals that the chimney itself is actually composed of many small pellets of mud, which the crayfish has rolled up with its legs and mouthparts and carried to the surface, depositing them like a mason building a brick wall. While astacologists (scientists who study crayfish) do not fully understand the function of these chimneys, one idea is that constructing them in this way allows the crayfish to remove the displaced earth as it excavates its tunnel without actually leaving the tunnel (where it would be vulnerable to predators) to deposit it outside. Many burrowing crayfish are so secretive, spending nearly their entire lives underground, that if it were not for the presence of these structures, there would be no evidence of their existence.

Crayfish are often classified in terms of their behavior as it relates to burrowing. While nearly every species of crayfish is, to some extent, capable of building a burrow, the extent to which they rely on burrows and places they construct them varies greatly. Crayfish are categorized as either tertiary, secondary, and primary burrowers.

Tertiary burrowing crayfish are the species least adapted to burrowing. They construct only rudimentary burrows, which may often consist of a simple tunnel beneath a rock near the water, which may serve as a retreat or a refuge during times of drought.

Secondary burrowing crayfish build more elaborate excavations than tertiary burrowers, usually in areas that experience seasonal inundation such as the banks of intermittent streams, in seasonally flooded wetlands, and in other environments not far from areas with fluctuating surface waters. While secondary burrowers spend much of their time living below ground in their burrows, they may shift into surface waters when their burrows become seasonally inundated.

But typically, when we refer to “burrowing crayfish,” what we really mean are the primary burrowing species. Primary burrowers are the crayfish most highly specialized for excavating and living in elaborate burrow complexes. These are species that spend almost their entire lives below the ground deep within their burrows, which often include multiple chambers and tunnels extending many feet below the soil. These are the crayfish who often build their homes far from streams, wetlands, or other sources of surface water. While these crayfish are often thought of as terrestrial species, this is something of a misnomer. All crayfish, even primary burrowers, must live in the water, as crayfish can only respire by excavating oxygen from the water with their gills (although crayfish gills are sensitive enough to extract oxygen from moisture in the air as long as the gills remain somewhat moist). Primary burrowers therefore must typically burrow down to the water table to survive.

Primary burrowing crayfish have a number of specializations for their burrowing lifestyle, including larger gills that allow them to extract oxygen from muddy water below ground; deeper thoraxes to house these gills; and usually a smaller, reduced abdominal region, since they don’t require the powerful swimming muscles of stream-dwelling crayfish. Many of these species also have thick, heavy claws, which aid with excavation of burrows.

Arkansas has remarkable crayfish diversity, much of which is found among the primary burrowing species. While some, such as the unmistakable Painted Devil Crayfish (Lacunicambarus ludovicianus), distinguished by the vivid, almost neon-red stripes along its carapace, are wide-ranging, many others are narrow-range endemics with tiny ranges. Of the approximately 60 species of crayfish that call our state home, 13 are found only here. The entire known range of the state endemic Slenderwrist Burrowing Crayfish (Fallicambarus petilicarpus) lies along a single state highway at the boundary of two counties in southern Arkansas, where it is known only from burrows in a few roadside ditches.

A ditch along the highway might be the last place you would expect to find such a rare creature, but this is not uncommon for endemic primary burrowing crayfish, whose true habitat preferences and natural histories are incredibly difficult to infer due to their secretive lifestyles. Their presence may be documented here simply because these are places where chimneys can be more easily detected. In fact, new species of primary burrowing crayfish are still being described right here in Arkansas, such as the Caramel Crayfish (Fallicambarus schusteri), aptly named for its beautiful coloration. This species, known only from a handful of roadside ditches in southwestern Arkansas and southeastern Oklahoma, was only described in 2015, when it was named for Dr. Guenter Schuster, a prominent astacologist at Eastern Kentucky University.

Some primary burrowing crayfish appear to be associated with wet flatwoods, seeps, or shale flats. Others, such as the Osage Burrowing Crayfish (Procambarus liberorum), a species found primarily in Arkansas, is associated with seasonally wet prairie habitat. This species was described in 1978 from three specimens caught in a backyard by a cat in Bentonville, perhaps one of the few times that a cat has discovered a new species. Prairies are among our most threatened ecosystems and are home to many highly specialized species that live nowhere else. On wet prairie remnants in northwest Arkansas, species rare in Arkansas such as Crawfish Frogs (Lithobates areolatus) and Graham’s Crayfish Snake (Regina grahamii), as their names indicate, are entirely dependent on populations of Osage Burrowing Crayfish.

One of the state’s most unusual primary burrowers is the Boston Mountains Crayfish (Cambarus causeyi), a species found only in Arkansas. This crayfish is found in the Boston Mountains, which contain the most rugged topography in the state. This is perhaps not the place one might expect to find a species that lives below ground. This orange crayfish is covered in bristly setae, or sensory hairs, which give it a distinctive “spiky” appearance. Finding the burrows of C. causeyi in the region’s rocky soil can be a challenge for researchers studying this species, as they sometimes lack the distinct chimneys of other burrowers. The tunnels are sometimes located beneath large rocks in seeps, tiny ephemeral streams, and roadside ditches along the winding mountain roads, making them difficult to locate.

Despite the extreme diversity of burrowing crayfish in Arkansas, we still know very little about most of these species. This is a function of the difficulty inherent in studying them in their own environment. In order to even see some of these species, they must be extracted from their burrows, often after careful and patient excavation (it is not a simple matter of digging straight down, as the paths through the soil taken by the small tunnels are small, winding, and easy to lose track of).

I recently participated in a survey of burrowing crayfish at Downs Prairie Natural Area, a remnant of what was once the vast Grand Prairie region of eastern Arkansas. During this survey, I stumbled upon a huge crayfish chimney, one of the largest I’ve seen. On the same day, Theo Witsell, ANHC Chief of Research (who knew that I would be out looking for crayfish that day), sent me a photograph taken in the area around Downs Prairie nearly 100 years ago by an ecologist/botanist named Roland Harper. The photo showed an immense crayfish chimney jutting up from the prairie. I was struck by the similarity of the two photographs, taken a century apart. For all the many generations that have passed between the taking of these two photos, these secretive creatures have been living out their lives below the ground, far from human eyes. They have offered those of us on the surface only the tiniest glimpse that their world even exists by raising their monuments of mud.