Sternotherus depressus

Geographic Range

Flattened musk turtles are Nearctic and are native to northern Alabama. They specifically inhabit the Upper Black Warrior River system which starts at 78 m above sea level then flows into the Gulf of Mexico. Flattened musk turtles range from the northern most section of the William B. Bankhead National Forest to just south of Tuscaloosa, Alabama. The river system has a total area 12,627 square kilometers; their range consists of 200 to 500 square kilometers across the river system.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Flattened musk turtles inhabit freshwater streams with a plentiful snail population. The most suitable size of streams is small to medium (typically third-order, but can tolerate first- or second-order) with rocky substrates. Turtle presence and density is greater in streams that are wider, have deeper pools, and have bedrock as a substrate and overall, more rock coverage. These turtles also use sites with cover – either underwater crevices or woody debris that protect them from predators (Jenkins, 2019). They avoid sites high in silt. Juveniles inhabit shallow waters of about 0.5 meters deep (Dodd Jr., 2008), and adults can inhabit a wider range of depths, up to 1.5 m deep in pools. The streams are found at elevations between 40 and 300 meters above sea level.

These turtle can inhabit impoundments, but not often. Likely, these turtles are from fragmented, isolated populations that existed in the stream reaches before the impoundments were in place.

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• lakes and ponds
• rivers and streams

Physical Description

As their name suggests, flattened musk turtles have a flat appearance from their head to their tail. Their carapace is flattened and lacks a visible keel and is oval-shaped. Iverson (1977) calculated the width of the carapace to be 2.2 times the height of it - or greater. Their plastron is typically yellow, but can look brown with staining over time. Their heads end in a point and are a light gray-green color with black spots. Adult have barbels on their chins.

Juveniles have a slightly more rounded carapace and a visible keel down the median line. Their plastron is typically pink, turning yellow as they age. Juveniles reach sexual maturity at carapace lengths between 60 and 70 mm in length.

Females (carapace length 85 to 120 mm) are slightly larger than males (carapace 75 to 110 mm). Further, female tails are typically shorter and skinnier than that of adult males. Males may also possess spins on the tip of the tail.

Based on a study by Jenkins (2019), 33 adult turtles had an average mass of 83.3g (range 40.0 – 142.9 g).

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

After an incubation period of 2 to 3 months, hatching flattened musk turtles are 28mm on average. Growth rates vary by sex and age. As is typical for reptiles, growth rates are high when turtles are young, then slow after they reach maturity. They exhibit indeterminate growth, though growth rates are negligible after age 30.

Females reach sexual maturity about 6 - 8 years of age while the males reach it about 4 - 6 years of age. Their sizes at sexual maturity are at carapace lengths of ca. 60mm for the males and 70mm for the females.

Other members of the genus have shown to be temperature-sex dependent which determines their sex while in the egg based off the temperature where they grow but it has not been proven in flattened musk turtles (Dodd Jr., 1989).

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

Flattened musk turtles mate between May and July. They will lay their eggs near the stream, choosing sandy areas and avoiding rocky substrate. For males and females to find each other, they use chemical signals and touch. The mating process takes place beneath the water and involves the male getting on top of the female. Males have a spike on their tail that is thought to hold them in place while internal fertilization occurs. The female lays 1 to 3 eggs once every 2 to 3 years. Once males have mated, they leave the females and have no further involvement. Females may mate with multiple males in a season.

• Mating System
• polyandrous polyandrous

Flattened musk turtles are iteroparous, with pairs breeding once a year and females laying 1 to 3 eggs per clutch and 1 to 2 clutches per year. Males and females breed from May through September. While gestation time is unreported their incubation period is thought to be 2 to 3 months. Eggs are reported to weigh an average of 5.5g and attain measures of 32 mm by 16 mm.

Flattened musk turtles’ size at hatching is 20 - 30mm. They are immediately independent at hatching. Males reach the age of sexual maturity around 4 - 6 years old while the females reach that age around 6 - 8 years old.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• oviparous oviparous

Flattened musk turtle males have no parental investment once they have mated with a female. Females have little parental investment outside of making a nest for the eggs in sandy substrates. Once the eggs are laid neither parent has an investment, and leave the eggs and hatchlings to be independent.

• Parental Investment
• no parental involvement

Lifespan/Longevity

The expected and maximum lifespans of flattened musk turtles are unreported, but others in the genus have been shown to live up to 30 years old (Ernst et al., 2009) with one confirmed to have a longevity of 23.9 years in the wild. Dodd (2009) reported that males continue growing until ages of 50 to 60 years and females grow until they are 30 to 40 years. Therefore, maximum longevity could be assumed to be at least 60 years.

Male flattened musk turtles of this species reach maturity between ages 4 and 6 while the females reach maturity between 6 and 8. Their survivorship varies depending on their size and age. The nest, eggs, and early hatchlings are more susceptible to being killed while the adults have a higher chance of surviving.

Behavior

Flattened musk turtles are social and mobile animals. Despite this mobility, they are listed as poor swimmers. They can be found in streams with fallen logs or branches large enough to let them bask in the sun for warmth. They primarily live under rocks and in crevices to avoid predators. They have been found to bury themselves in the sand or mud when needed. They will primarily bottom-feed for molluscs such as snails and clams. Researchers have found larval and adult insects to be prey as well (Dodd, Jr, 2008).

They can be active during any time of day but change their habits with the weather. They have shown to be more active during the nighttime (nocturnal) and at dawn or dusk (crepuscular) once the water becomes warmer. In cooler temperatures, they bask diurnally. They are visibly active and basking from mid-April to mid-October. Their habits during winter are unknown, so they presumably brumate in cooler temperatures.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• sedentary sedentary
• social social

Home Range

Jenkins (2019) measured home ranges for 21 adults, reported in linear river lengths. Their average river length usage was 332 m, (range 22 to 957 m). Most spent >50% of their time in reaches of less than 50 m. These turtles are not known to defend a territory.

Communication and Perception

Flattened musk turtles have very poor hearing and compensate by having a keen sense of smell and a keen color vision. They have the name "musk turtle" due to the musk they secrete for defense. They possess barbels on their chins to better perceive their riverine habitats. They have tactile organs used to detect and eat their prey. These turtles use tactile senses to mate, to find food, to bask, and to dig shallow nests in the sand.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

MArion et al. (1991) reported that the diet of flattened musk turtles was primarily gastropods (snails; 69.5% of diet) and also included clams and mussels (bivalves; 16%). Necropsies revealed stomach contents that also contained insects (1.1%), plant seeds (0.2%), and crayfish (2.0). They will feed on invasive fingernail clams from the genus Corbicula .

• Primary Diet
• carnivore carnivore
  • insectivore insectivore
  • molluscivore molluscivore
• herbivore herbivore
  • granivore granivore
• omnivore omnivore

• Animal Foods
• insects
• mollusks

• Plant Foods
• seeds, grains, and nuts

Predation

Predators of eggs and hatchling flattened musk turtles include North American river otters ( Lontra canadensis ), herons such as great blue herons ( Ardea herodias ) and snowy egrets ( Egretta thula ), and raccoons ( Procyon lotor ).

They avoid these predators by hiding under logs, rocks in the stream. They are also cryptically colored. They also release a musk that helps to deter predators.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Flattened musk turtles regularly eat snails, and other aquatic invertebrates, and occasionally plant seeds. They are eaten by North American river otters ( Lontra canadensis ), herons, and raccoons ( Procyon lotor ).

Apicomplexan blood parasites of the genus Haemogregarina have been found in postmortem studies of flattened musk turtles. They also possess leeches ( Placobdella parasitica ) as ectoparasites, which parasitize them in greater numbers when the habitat is degraded. They also have housed unidentified trematodes. Though several nematodes have been found in congenerics of flattened musk turtles, they have not been confirmed to be found in these turtles.

Economic Importance for Humans: Positive

Flattened musk turtles are sold in the illegal pet trade. Between 2013 and 2019, 640 individuals were taken from the US to foreign countries. Before the Federal Register listed them as threatened in 1987, they were caught for trade purposes and to keep as pets, which continued even after laws prohibited this. Once the laws were place authorities found a website advertising them for sale at $250 Canadian dollars in 2008.

• Positive Impacts
• pet trade pet trade

Economic Importance for Humans: Negative

Flattened musk turtles have no negative economic impacts on humans.

Conservation Status

Flattened musk turtles are listed as "Critically Endangered" on the IUCN Red List. These turtles are listed as "Threatened" on the US Federal List. Flattened musk turtles are listed in Appendix II in CITES, meaning trade is prohibited. There is no special status on the State of Michigan list.

They only inhabit 25-30 percent of their historic range in the Upper Black Warrior River system in Alabama. This is due to the numerous threats the population faces. They were used as pets in the 1980s which caused the state of Alabama to take lawful action. The primary threats today are habitat degradation and pollution. The urban development of the Black Warrior River system has reduced the surface area in which the species can inhabit. This has caused a reduction in water storage that could lead to more severe floods and droughts. The mining and forestry operations ongoing in the river system cause pollutants to not kill the turtles, but their prey as well. Waste from the urban areas surrounding the river system flow into the river and also pollute the water which kill the turtles and their prey. The sedimentation removes any rock crevices the species could use as a place to live or to escape predators and reduces the amount of available prey. Their hybridization with loggerhead musk turtles ( Sternotherus peltifer ) is causing the loggerhead genes to become more prominent in formerly pure flattened musk turtle areas.

Actions being taken to protect this species include: a recovery plan in 1990 that needs to be updated and revised for modern requirements, land and water protection and management for the river system they inhabit, education and awareness for everyone, and laws and policy being made both federally and state level. The recovery plan was implemented in 1990 but has not been updated for the species due to the habitat not being listed as a critical habitat. The US Forest Service implemented a plan in 2022 to reduce the number of wild pigs ( Sus scrofa ) being found in the river system. The pigs are increasing erosion along with hiking and biking trails that need to be maintained for the population downstream. The entirety of the Black Warrior River system is under conservation and restoration actions to preserve the streams and rivers due to flattened musk turtle remnants being found in farther parts of the river system. The species was placed on the US Endangered Species Act in 1987 and has remained on the list without a critical habitat listed. Until 2019, no working groups were every created and no funds have been allocated to complete anything more than surveys for these turtles. In 2019, a working group assembled to begin a stronger, detailed management plan.

The Alabama State Legislation has it listed as protected from any exploitation and intentional impacts since 1984. Research is being done to determine the potential longevity of the species and any fragmentation and any diseases that could threaten the species. Further laws and regulations for mining and water quality are needed to further support the species. There also need to be regulations for the various chicken farms located near the river that leak pollution and chemicals toxic to the species so as to further the species. A captive breeding will be required in the future due to the slow reproductive rate and declining population numbers.

Additional Links

Encyclopedia of Life

Contributors

Matthew Post (author), Radford University, Natalie May (editor), Radford University, Alexander McVicker (editor), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Bour, R. 2008. Global diversity of turtles (Chelonii; Reptilia) in freshwater. Hydrobiologia , 595/1: 593-598.

Collins, J. 1997. Standard common and current scientific names for North American amphibians and reptiles. Herpetological Circular , 25: 17.

Cox, W. 1978. Reproduction, Growth, and Life History Strategies in the Musk Turtle (Sternotherus minor) in North Florida (Master's Thesis) . Birmingham, Alabama: University of Alabama.

Cox, W., K. Marion. 1978. Observations on the female reproductive cycle and associated phenomena in spring-dwelling populations of Sternotherus minor in north Florida (Reptilia: Testudines). Herpetologica , 34/1: 20-33.

Dodd Jr, C. 1988. Disease and population declines in the flattened musk turtle Sternotherus depressus. The American Midland Naturalist , 119/2: 394-401.

Dodd Jr, C. 1990. Effects of habitat fragmentation on a stream-dwelling species, the flattened musk turtle Sternotherus depressus. Biological Conservation , 54: 33-45.

Dodd Jr, C. 1988. Patterns of distribution and seasonal use of the turtle Sternotherus depressus by the leech Placobdella parasitica. Journal of Herpetology , 22/1: 74-81.

Dodd Jr, C. 1989. Secondary sex ratio variation among populations of the flattened musk turtle, Sternotherus depressus. Copeia , 1989/4: 1041-1045.

Dodd Jr, K. 2008. Sternotherus depressus Tinkle and Webb 1955 - Flattened Musk Turtle. Chelonian Research Monographs , 5: 013.1-013.7.

Ernst, C., J. Lovich, A. Larson, D. Eisenhour. 2009. Turtles of the United States and Canada Second Edition . Baltimore, Maryland: John Hopkins University Press.

Fonnesbeck, C., K. Dodd Jr.. 2003. Estimation of Flattened Musk Turtle (Sternotherus depressus) Survival, Recapture, and Recovery Rate during and after a Disease Outbreak. Journal of Herpetology , 37/3: 602-607.

Hickman Jr., C., S. Keen, A. Larson, D. Eisenhour. 2018. Animal Diversity 8th Edition . McGraw Hill Higher Education: McGraw Hill Higher Education.

Iverson, J. 1977. Sternotherus depressus Tinkle and Webb Flattened musk turtle. Catalogue of American Amphibians and Reptiles , 194: 1-2.

Jenkins, A., J. Godwin, D. Warner, D. Steen. 2022. Movement ecology of flattened musk turtles (Sternotherus depressus). Journal of Herpetology , 56/1: 1-7.

Jenkins, A. 2019. Home Range, Activity, Movement, and Habitat Selection of the Flattened Musk Turtle (Sternotherus depressus) in the Bankhead National Forest, Alabama (Master's Thesis) . Auburn, Alabama: Auburn University.

Jenkins, A., J. Godwin. 2024. "Sternotherus depressus" (On-line). The IUCN Red List of Threatened Species 2024: e.T20824A242442967. Accessed May 01, 2025 at https://dx.doi.org/10.2305/IUCN.UK.2024-1.RLTS.T20824A242442967.en .

Marion, K., W. Cox, C. Ernst. 1991. Prey of the flattened musk turtle, Sternotherus depressus. Journal of Herpetology , 25/3: 385-387.

Melancon, S., R. Angus, K. Marion. 2013. Demographic comparisons between reservoir-dwelling and stream-dwelling populations of a threatened turtle (Sternotherus depressus Tinkle and Webb). Southeastern Naturalist , 12/4: 684-691.

Melancon, S., R. Angus, K. Marion. 2011. Growth of the flattened musk turtle, Sternotherus depressus Tinkle and Webb. Southeastern Naturalist , 10/3: 399-408.

Roberts, J., K. Halanych, C. Arias, B. Folt, J. Goessling, S. Bullard. 2017. Emendation and new species of Hapalorhynchus Stunkard, 1922 (Digenea: Schistosomatoidea) from musk turtles (Kinosternidae: Sternotherus) in Alabama and Florida rivers. Parasitology International , 66/6: 748-760.

Scott, P. 2016. Speciation, Hybridization, Lineage Persistence, and Conservation of North American Musk Turtles (Genus: Sternotherus) (PhD Dissertation) . Tuscaloosa, Alabama: The University of Alabama.

Nineteenth meeting of the Conference of the Parties. Convention on International Trade in Endangered Species of Wild Fauna and Flora. CoP19 Prop.XXX. Panama City (Panama): CITES. 2022.

U.S. Fish & Wildlife Service. Flattened Musk Turtle (Sternotherus depressus) 5-Year Review: Summary and Evaluation. None. Jackson, Mississippi: U.S. Fish & Wildlife Service. 2014. Accessed May 01, 2025 at https://ecos.fws.gov/docs/tess/species_nonpublish/2226.pdf .

The Office of the Federal Register. The Federal Register. 52/112. The National Archives and Records Administration: The Office of the Register. 1987.