Smooth Softshell Turtles: A Focal Species of the Illinois Wildlife Action Plan

One could argue that some of the strangest looking turtles in Illinois might be the spiny softshell turtle and the smooth softshell turtle. Both of these river turtles lack the hard, domed shells that shield other Illinois turtles from predators. Instead, as their name implies, softshell turtles have leathery, flat shells which provide little protection.

What their suppler, large pancake-shaped shells lack in protection, they make up for in speed. According to Mike Dreslik, Interim Director of the Illinois Natural History Survey (INHS) part of the Prairie Research Institute, and Director of the Population and Community Ecology (PACE) Lab at INHS, their shells are very hydrodynamic. This allows the softshells to move through the water fast and make quick turns. That’s important because these turtles are carnivores and need to be able to quickly maneuver to catch their aquatic prey.

Illinois’ Two Softshell Turtles

The spiny softshell (Apalone spinifera) is a common turtle in large rivers and is found statewide in Illinois. They inhabit rivers and bodies of still water that have sand bars or mud bars where they can bask in the sun. As an aquatic turtle, spiny softshells are strong swimmers, but they are also surprisingly agile on land. They are commonly found buried in sand or mud with only their head sticking out, just far enough for their noses to breach the surface for air. When they are underwater, they can remain submerged for long periods of time because they can absorb oxygen through their skin. Spiny softshells get their name from the small, sharp spines that cover the front of their upper shell.

The smooth softshell (Apalone mutica) can be found in rivers throughout much of the state with the exception of northeastern and east central Illinois. This species seems to be more vulnerable to anthropogenic changes to river ecosystems and was listed as endangered in Illinois in 2009. Where they occur, they select moderate- to fast-moving rivers with plenty of sand bars and need clean, sandy river bottoms. Based on data from additional studies, the Illinois status of this turtle was changed from endangered to threatened in 2020.

Smooth Softshell a Focal Species

Because of their geographic distribution throughout the state and their sensitivity to their environment, the smooth softshell turtle serves as one of several focal species monitored by the Illinois Department of Natural Resources as part of the Illinois Wildlife Action Plan (IWAP).

The Streams Campaign is one of seven campaigns in the IWAP, which serves as a roadmap for conserving wildlife and their habitats before they become too rare or too expensive to restore. The Streams Campaign focuses on species monitoring and assessments, habitat protection, water quality and flow management, and invasive species control. The IWAP lists the smooth softshell turtle as one of 52 focal species of the Streams Campaign and one of 24 reptiles designated as a Species in Greatest Need of Conservation. The recovery efforts for the smooth softshell are managed by the IDNR Wildlife Diversity Program.

Studies of Smooth Softshells

The successful conservation of threatened populations requires knowledge of their habitat preferences, seasonal and daily movements, and home range size. Adequate distribution and abundance information is lacking for many aquatic turtles, but there have been several studies of the smooth softshell turtle which provide snapshots of their distribution over time. Here I highlight the findings from a handful of those research studies.

In 1937, Alvin Cahn published a review of the then-named Amyda mutica, having examined specimens from the following locations: Rock Island, Meredosia, Havana, Crooked Creek in Schuyler County, Robinson, Shawneetown, Carmi, Horseshoe Lake in Alexander County, Chester, Murphysboro, Centralia, Grafton, Carlyle, Vandalia, Louisville, Newton, Beardstown, Petersburg, Sterling and East St. Louis. He also provided other distribution records within Illinois from 1892 to 1921. Such a relatively broad distribution around the state means that these turtles were found in most of the Illinois Natural Divisions including the Upper Mississippi River and Illinois River Bottomlands.

Jumping ahead 40 years in the published records, in 1977 Michael Plummer conducted a 3-year study of the then-named Trionyx muticus. Using a capture-mark-recapture method he found that male smooth softshells preferred areas with emergent sandbars, while females selected deeper open water outside of the nesting season. During the nesting season the females moved to shallower waters with sandbars where they could lay their eggs. From his sampling of 8.5 miles of the Kansas River downstream from Lawrence, Kansas, he estimated 1,900 turtles would be in an approximately one mile stretch of the river.

Skipping ahead almost 30 years, in 2006 Valerie Barko and Jeffrey Briggler published a study of smooth and spiny softshell turtles in the Middle Mississippi River which flows through the Middle Mississippi Border Natural Division. They used six years of Long Term Resource Monitoring Program (LTRMP) turtle by-catch data collected from the middle Mississippi River to investigate the abundance of smooth and spiny softshells. They reported 150 captures of the smooth softshell. Barko and Briggler focused on five habitat types: main channel border, wing dikes, tributary, open side channel and closed side channel. They found that smooth softshells were most abundant in open side channels and main channel borders, while spiny softshells were most abundant in tributaries and in closed side channels. Water velocity and visibility were both found to impact abundance of the turtles, with smooth softshells more common in deep, fast waters. Spiny softshells were more abundant in shallower waters with higher visibility and slower water velocity.

The reasons for adding the smooth softshell to the Illinois endangered species list were outlined in Endangered and Threatened Species of Illinois: Status and Distribution, Volume 4 – 2009 and 2010 Changes to the Illinois List of Endangered and Threatened Species edited by Anne Mankowski (2010). The Illinois Endangered Species Protection Board found that despite the smooth softshell being susceptible to capture via trapping, it had not been captured recently in the expected numbers at any site except for a sampled section of the Sangamon River in Menard County. They documented that other sampling efforts on the Middle Fork Vermilion, Wabash and Embarrass rivers had been unsuccessful in capturing turtles where they had been expected to occur. It was acknowledged that sampling could be particularly difficult on large rivers such as the Wabash and Mississippi and thus there was a chance that the smooth softshell had a more robust distribution than was known at the time. They recommended additional surveys to verify this species’ distribution to revisit its listing status in the future. But at the time, in the absence of more data, the smooth softshell turtle was listed as state endangered, citing poor water quality and lack of suitable nesting habitat as issues of concern for this species.

From 2013 to 2015, Mike Dreslik managed the Ecology of the Smooth Softshell in the Kaskaskia River: Implications for Managing Flows in an Altered System (T-87-D1) project funded by the Illinois Department of Natural Resources. The results of that study were published in 2015 by Ross, Dreslik and Bluett. True turtle enthusiasts will be interested in Jason Ross’s 254 page 2016 master’s thesis: Spatial Ecology of the Smooth Softshell Turtle (Apalone mutica) in the Kaskaskia River of Illinois.

The Kaskaskia River flows through parts of the Grand Prairie, Southern Till Plain (Effingham Plains section), Ozark and Lower Mississippi River Bottomlands Natural Divisions. Ross’s study sites were near Cowden and Fayetteville where he trapped and attached radio-transmitters to smooth softshells in an attempt to describe what variables influence their movements (rate and frequency) and home ranges. He highlighted that this is important because the turtles’ movements reflect their foraging and reproductive behaviors and knowing their home ranges provides information about their preferred habitats and how much space they need to survive.

Ross captured 115 individual turtles and radio-tracked 29 (8 Cowden females, 2 Cowden males, 8 Fayetteville females and 11 Fayetteville males). He obtained a total of 2,013 radio locations at the two study sites between June 19, 2013 and December 16, 2014. The aquatic habitat categories he used were bar, pool, bar-pool transition, run and channelized. His results showed that at a large spatial scale, smooth softshells established home ranges in the main river channel near meander bends. Within their home ranges, males preferred bars over all other habitats and females preferred pools. He ran mixed models that indicated the females used deeper waters than the males, and neither sex showed an affinity for deadwood.

While there were clear differences in habitat preference between males and females, Ross reported that differences in movement rate and home range size were less pronounced between the sexes. From his movement analysis he found that the turtles rarely stayed in the same location for multiple days. However, their individual movement rates varied greatly. The average maximum movement rate was 0.5 miles per day for females and a little less than 0.7 miles per day for males. How far up and down river the individual turtles moved also varied greatly. A few stayed in short reaches of the river, only 0.6 to 1.24 miles long, while others moved in stretches of the river of more than 6 miles. The largest movements tended to be higher at Fayetteville than at Cowden.

Overall, Ross found that the majority of smooth softshells in the Kaskaskia River had well defined home ranges and a small minority of them behaved nomadically. He recommended that conservation efforts for this species should focus on reducing the effects of fragmentation, maintaining natural river flow and restoring human-impacted river channels to their natural state.

A thesis (however good) is not typically popular reading material, and diving into research papers isn’t everyone’s cup of tea. But if you are long on curiosity and short on time, a thorough read of Distribution of the Smooth Softshell (Apalone mutica) in Illinois by Bob Bluett and Aaron Gooley (2018) is an engaging read, a solid overview of smooth softshell research and well worth the time.

In this study, nine turtle species were captured in 14 mid- to large rivers in and bordering Illinois from 2007 to 2017. The red-eared slider (Trachemys scripta) was caught in high numbers (3,911), predominately in six of the rivers, and only absent from the trapping effort in the Spoon River. The spiny softshell was less abundant with 731 captures, but they were found in all the rivers except the Mississippi. On the other end of the spectrum, they found only two eastern musk turtles (Sternotherus odoratus), one in the Wabash and one in the Sangamon River. There were 11 captures of northern map turtles (Graptemys geographica), one in the Illinois River, seven in the Rock River, and three in the Saline (North Fork).

Smooth softshells were found in 10 of the 14 sampled rivers. Of the 124 captures, the majority of smooth softshells were found in the Wabash, Kaskaskia and Spoon rivers. They were absent from the Big Muddy, Little Wabash, Saline and Skillet Fork rivers. Bluett and Gooley noted that the trapping method may have failed to fully detect turtles where they were present, and recommended further study, particularly of the Little Wabash River where smooth softshells had been previously documented. Their findings, along with other published studies since 2010, were considered during the status review in 2019 when it was determined that the distribution of the smooth softshell turtle at that time warranted changing their status in Illinois from endangered to threatened.

Endangered and threatened species status reviews are required every five years. Which brings us to the present day. The summary of preliminary listing decisions for the changes to the 2020 Illinois List of Endangered and Threatened Species confirmed at a meeting in November 2024 does not recommend removing the smooth softshell turtle from the threatened list. With diligent efforts to protect water quality and conserve aquatic habitat, hopefully their numbers will remain stable and hopefully grow.

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Laura Kammin is a Natural Resources Specialist with the National Great Rivers Research and Education Center. She formerly held positions at Illinois-Indiana Sea Grant, University of Illinois Extension, Prairie Rivers Network and the Illinois Natural History Survey. She received her master’s degree in wildlife ecology from the University of Illinois, Urbana-Champaign.