Flattened musk turtle  (Sternotherus depressus)

The flattened musk turtle is classified as Critically Endangered (CR) on the IUCN Red List (1).

As its common name suggests, this small freshwater turtle (3) has a distinctly flattened top shell (carapace), which is slightly serrated at the rear edge in juveniles, but becomes smooth with age (2) (4). The upper shell is yellowish-brown to dark brown with small, black spots and streaks and dark bordered seams, while the lower shell (plastron) is either pink or yellow (2) (5) (6). The head, legs and tail are olive-green with a network of fine black mottling (5) (6). All four feet are webbed and males have long, thick, spine-tipped tails (2) (6).

Restricted to the upper Black Warrior River system of west-central Alabama, U.S. (2) (6).

The flattened musk turtle is found in a variety of rivers, streams and in the shallow upper reaches of reservoirs, but its preferred habitat appears to be large, clear, free-flowing creeks or rivers. It inhabits areas with large flat rocks, moderately sandy bottoms and vegetated shallows, with alternating riffles and pools (2) (6) (4). The turtles wedge themselves in crevices and under large flat rocks, hence their flattened shells (4).

Unlike many aquatic turtles, this shy, secretive species rarely basks in the sunshine when healthy, instead showing a strong tendency towards crepuscular and nocturnal activity, particularly during summer (3). However, sick flattened musk turtles often bask in order to raise their body temperature, a phenomenon known as behavioural fever (4). Snails and clams are preferred food items, particularly the introduced Asian clam, but the diet also includes aquatic insects, arachnids, crayfish and the occasional dead fish (3) (6). Juveniles are apparently more active during the day than adults and rely more heavily on insects, although small snails are also consumed (3).

Males mature at around four to six years of age, females at six to eight (3) (6). Females lay one to two clutches of eggs a year between May and early July (2) (3). Each clutch contains one to four eggs and is deposited in a shallow nest dug in a high sandy bank (3).

Flattened musk turtles are long-lived, with a life span of 20 to 40 years (3). Juveniles are at risk from wading birds and possibly some predatory fish, while raccoons and possibly otters are thought to prey on adults and nests (3) (4). Large common snapping turtles are possible predators of both juveniles and adults (3).

The flattened musk turtle is most seriously threatened by habitat degradation and fragmentation, and remaining populations are further threatened by disease, pollution and siltation of its habitat, and over-collection from the wild for the pet trade (2) (3) (7) (8). Pollution from mining, forestry, agricultural and industrial chemicals and residential sewage is thought to be responsible for the high levels of shell erosion and infections recently observed in this species, as well as reducing numbers of molluscs and other invertebrates on which the species feeds (2). Excessive sediment accumulation (siltation) also appears to be having a dramatic impact on this small turtle, possibly through smothering molluscs and other invertebrate food sources, altering the habitat where this turtle seeks food and cover, and accumulating heavy metals and chemical toxins (2) (3). In the 1970s and 80s, commercial collecting for the pet trade further reduced some populations, with large collections having been recorded and individuals advertised in several places for over 80 U.S. dollars each (2) (3) (4). It is also hard for flattened musk turtle populations to recover from any adverse changes, due to its slowness to mature and low reproductive rate (see ‘Biology’) (2).

A bill passed by the Alabama legislature in May 1984 prohibits the taking of this species from the wild and may be a deterrent to commercial overexploitation in the future (9). The flattened musk turtle is also protected as threatened under the US Endangered Species Act of 1973, as amended. It is therefore a nationally-recognised species of concern and is fully protected by law. In addition, a Federal Recovery Plan has been approved (2) (4).

View information on this species at the UNEP World Conservation Monitoring Centre.