Apalone mutica (LeSueur, 1827)
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Basis for Listing
The smooth softshell turtle inhabits large rivers of the central United States, preferring large unpolluted rivers with sandy substrate. The midland smooth softshell (Apalone mutica mutica) is the subspecies recognized in Minnesota. Records of this subspecies are limited to the lower St. Croix River, portions of the Minnesota River, and the Mississippi River below the Twin Cities. Smooth softshell turtle populations have declined in recent years due to river channelization, siltation, and water pollution. This species' ability to extract oxygen from water (Cahn 1937) may make it particularly vulnerable to water pollution. In addition, sandy beaches utilized as nesting habitat by this species can be degraded by humans who recreate in such areas. In particular, waste left behind by humans can attract turtle egg predators, such as raccoons (Procyon lotor) and skunks. Nesting activities can also be disrupted by human activities. Finally, the market for commercially harvested softshell turtles has recently experienced rapid growth. All of these factors have led to the species' status as a special concern species in Minnesota.
Unlike most turtles, the smooth softshell turtle has a smooth, leatherlike, flexible carapace and a long, tubular snout. Females are larger than males, with a carapace length of 16.5-35.6 cm (6.5-14 in.) compared to 11.4-17.8 cm (4.5-7 in.) in males (Conant and Collins 1991). Females and males also differ in color. The carapace of adult females is tan or brown with irregular dark brown blotches, while the carapace of males and juveniles is a brown or grayish color with scattered, small dark brown dots or dashes (Oldfield and Moriarty 1994). Smooth softshell turtles may be easily confused with the spiny softshell turtle (Apalone spinifera), as the differences between the two species are subtle. Spiny softshell turtles have a rough carapace with spines or bumps along the front edge, while smooth softshell turtles have a smooth carapace which lacks spines on the front edge. Also, the spiny softshell has a lateral projection on the nasal septum that extends into the nasal opening, while the smooth softshell does not (Oldfield and Moriarty 1994). Additionally, the smooth softshell's white eye stripe is more distinctive than the spiny softshell's, and their white chin and throat are unmarked, compared to the splotchy chin and throat of the spiny softshell. When handled, spiny softshells can be quite aggressive compared to smooth softshells (Oldfield and Moriarty 1994).
Smooth softshell turtles prefer large unpolluted rivers, with sandy substrates. They can also be found in lakes, impoundments, and shallow bogs (Ernst and Barbour 1972). In Minnesota, the smooth softshell turtle has been found in large rivers with currents which are moderate to fast (Oldfield and Moriarty 1994). They prefer water with sand or mud bottoms, without rocky areas or dense vegetation (Pope 1946; Oldfield and Moriarty 1994).
Biology / Life History
Smooth softshells hibernate underwater buried under the substrate (Plummer and Shirer 1975). They emerge in the spring and breed in Minnesota from May to June (Oldfield and Moriarty 1994). Male smooth softshells mature in their fourth year, while females mature when they are 7-9 years old (Ernst and Barbour 1972; Ernst et al. 1994). Females nest from June to early July on sandbars and riverbanks with full exposure to sun (Vogt 1981). Females seldom use beaches which are entirely brushed (Vose 1964). They lay 1-33 eggs which are 20-24 mm (0.8-0.9 in.) in diameter and have thick, brittle white shells (Pope 1946; Ernst et al. 1994). Eggs which become submerged from extended rises in water level due to rain or human regulation, do not survive (Plummer 1976). The eggs require 8-12 weeks of incubation, after which time the hatchlings tear out of their shells using their front legs and egg tooth (Oldfield and Moriarty 1994). The main predators of smooth softshell turtle eggs are skunks and raccoons, while the main predators of hatchlings are fish, turtles, snakes, wading birds, and some mammals (Ernst et al. 1994). Adult smooth softshell turtles in Minnesota have few predators, other than humans (Ernst et al. 1994).
Conservation / Management
Several threats impact the health of smooth softshell turtle populations. Among them are habitat degredation, pollution, human activities at nesting sites, and overharvest of this slow maturing species. Land clearing and wetland drainage for agriculture, pollution by sewage, and construction of locks and dams have seriously harmed or eliminated populations of smooth softshell turtles (Moll and Moll 2000). Smooth softshell populations can also be damaged by increased runoff and silt caused by changes in the habitat structure near rivers (Moll and Moll 2000). Because of this, waterways and lands that contain important turtle populations should be protected from human disturbance and pollution (Ernst et al. 1994) through enforcement of best management practices. Areas upstream from turtle populations should be protected, as well, as turtle habitats are subject to degradation and pollution from upstream activities (Moll and Moll 2000). For smooth softshell turtles to survive, protected areas need to contain basking and nesting habitats, sufficient prey, hibernation habitat, and movement corridors. Minimizing human disturbance at sandy beaches where smooth softshell turtles nest would benefit this species. Human activities can disturb or destroy nests, and debris left behind attracts nest predators. Additionally, wave action from boats can result in erosion of nest sites and exposure of eggs. Nesting females are easily frightened and may stop nesting if disturbed (Oldfield and Moriarty 1994).
Conservation Efforts in Minnesota
The Minnesota DNR funded a study of turtles in the Weaver Bottoms area of the Upper Mississippi River in 2001 to assess management and conservation concerns (Pappas et al. 2001). The authors indicated that more research was needed, but noted that smooth softshell turtles were rare and probably impacted by commercial fishing and turtle harvest (Pappas et al. 2001). Since then, the Minnesota DNR revised Minnesota's commercial turtle harvest laws, and smooth softshell turtles are no longer a harvestable species.
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Jessen, T. 1998a. A survey of the herpetofauna of the Minnesota River Valley from Granite Falls to Ortonville: July 2 to December 3, 1998. 19 pp.
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Jessen, T. 1999. A report on the search for Blanding's and Smooth Softshell Turtles in south central Minnesota: April 19 to August 26, 1999. 15 pp.
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Oldfield, B., and J. J. Moriarty. 1994. Amphibians and reptiles native to Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 237 pp.
Pappas, M. J., J. Congdon, and A. Pappas. 2001. Weaver Bottoms 2001 turtle survey: management and conservation concerns. Report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources and the United States Fish and Wildlife Service. Unpaged.
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Vogt, R. C. 1981. Natural history of amphibians and reptiles of Wisconsin. Milwaukee Public Museum, Milwaukee, Wisconsin. 205 pp.
Vose, R. N. 1964. Nesting habits of the Soft-shelled Turtles (Trionyx sp.). Proceedings of the Minnesota Academy of Science 31(2):122-124.