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Flattened Musk Turtle

Sarin Tiatragul, Auburn University
The flattened musk turtle (Sternotherus depressus) is a small species of aquatic turtle that is found only in Alabama. They occur exclusively above the fall line in the upper Black Warrior River Basin and are Alabama's only endemic (meaning found only there) reptile species. The turtle's limited range and specific habitat requirements make the species especially vulnerable to habitat degradation and loss. Flattened musk turtles are listed as "critically endangered" on the International Union for Conservation of Nature (IUCN) Red List and as a "threatened" under the U.S. Endangered Species Act since 1987.
Flattened musk turtles belong to the Kinosternidae family, which contains 25 species. These species, many of which are referred to as "mud" and "musk" turtles, are characterized by their highly domed upper shell, or carapace. The flattened musk turtle is in the Sternotherus (meaning loosely in Greek, "hinged sternum animal") genus, commonly referred to as "musk turtles," as they are all capable of releasing foul-smelling musk from glands located along the sides of their bodies. The species name, depressus, derives from their unique carapace, which is significantly more flattened than in other species of musk turtles.
These turtles inhabit larger streams in the area above the fall line of the Black Warrior River Basin, which is characterized by substrate that includes bedrock, slab rock, and sand with plenty of cover, especially rock crevices. It is in this isolated and unique habitat that the flattened musk turtle evolved over thousands of years. The turtle's compressed shell and flattened carapace is an adaptation that allows them to wedge themselves into small rock crevices to hide from predators, reduce drag, and prevent being washed away during flooding events.
Adults range between 2.4 and 4.7 inches (6-12 centimeters) and may live upwards of 40 years. Males reach sexual maturity between four and six years, whereas females reach maturity between six and eight years. Males can be distinguished by their long thick spine-tipped tail, which is thought to be used as an anchor to stabilize the body during mating. Carapace coloration varies from yellowish brown to dark brown with some spots and streaks. Juveniles are usually lighter yellow in color with a keeled carapace that flattens as they age.
Breeding occurs in the spring and nesting season stretches from May to September. Females deposit eggs in shallow nests on sandy slopes near streams. Females usually lay one or two clutches of two eggs per year. The incubation period ranges from 45 to 122 days, depending on temperature and other environmental factors. Like many other species of turtles, musk turtles are known to exhibit temperature-dependent sex determination, meaning that eggs incubated at temperatures of 77-86 degrees Fahrenheit (25-30 degrees Celsius) yield predominantly males and higher temperatures produce females.
Flattened musk turtles prey largely on snails and non-native Asian fingernail clams (genus Corbicula). Juveniles, however, tend to consume fewer mollusks and more soft-bodied aquatic invertebrates compared with adults. Because these preys are vital to their survival, flattened musk turtles are often found in areas with high abundances of invertebrate prey. Turtles and turtle eggs are valued food items for various carnivorous mammals that live near the rivers, especially raccoons. River otters are noted predators of many turtle species at all life stages in the Black Warrior River Basin. Juveniles are likely preyed upon by water snakes, bullfrogs, and predatory fish. In many localities, leeches are found parasitizing a high proportion of individual turtles but do not appear to have a major effect on their health and are an indicator of good stream health.
Flattened musk turtle populations are fragmented and declining owing to habitat loss and degradation of water quality from sedimentation of streams caused largely by strip coal-mining operations in addition to poor management during some forestry and development operations. Sedimentation clogs streams and fills up the rock crevices that flattened musk turtles rely upon. Additionally, chemical runoff from abandoned and active coal mines dramatically reduces populations of mollusks and invertebrates, the principal food source for the flattened musk turtle. Analyses of recent and historic surveys for the flattened musk turtle show that around 50 percent of known populations have been lost in the past four decades and the species is now extirpated from 70 to 90 percent of its historical range. Many of the remaining populations are considered "relict populations," meaning that they consist largely of older animals and few younger ones. It is estimated that less than seven percent of the historic range of the flattened musk turtle currently consists of populations that are sustainable into the future. Fragmented small populations and low reproductive success in disturbed areas have increased their vulnerability to being wiped out by natural disaster events and diseases. If efforts are not soon made to address the ongoing degradation of habitat in the Black Warrior River Basin, Alabama's only endemic reptile species, along with many other unique aquatic species, will be forever lost to extinction.

Additional Resources

Dodd, C. Kenneth. "Effects of Habitat Fragmentation on a Stream-Dwelling Species, the Flattened Musk Turtle, Sternotherus depressus." Biological Conservation 54, no. 1 (1990): 33?45.
Dodd, C. Kenneth, Kevin M. Enge, and James N. Stuart. "Aspects of the Biology of the Flattened Musk Turtle, Sternotherus Depressus in Northern Alabama." Bulletin of the Florida State Museum, Biological Sciences 34 (1988): 1-64.
Ernst H. Carl., William A. Cox, and Ken R. Marion. "The Distribution and Status of the Flattened Musk Turtle, Sternotherus depressus (Testudines: Kinosternidae)." Tulane Studies in Zoology and Botany 27 (September 1989):1?20.
Published:  December 8, 2017   |   Last updated:  December 8, 2017