By Eileen Dowd Stukel
Originally published in the July/August 1993 edition of the SD Conservation Digest.
Over 200 million years ago, amphibians evolved into a more advanced life form - the reptile. Several adaptations gave reptiles the advantage they needed to dominate the earth during the Mesozoic Era. Drier climates weren't a problem for a creature that laid moisture-retaining, hard-shelled eggs. Unlike an amphibian egg, a reptile egg contains several embryonic membranes, as well as a yolk sac, an additional nutrient store. A reptile's scaly skin has few surface glands, meaning less water loss.
Spiny Softshell - note tubercles on the front edge of the carapace and the black markings on the carapace
One reptile group, the turtles, aren't believed to have changed much since their first appearance. A turtle's shell is its most prominent feature, and is made of two main parts, the upper shell, or carapace, and the lower shell, called the plastron.
One turtle group, the softshells, is adapted for an aquatic lifestyle. The shell has less height than that of a land turtle, and is covered by a flexible leathery skin. This adapted shell enables the softshell to burrow in sand, where it appears to be a lifeless mound. Nostrils at the end of a long tubular snout allow a softshell to float inconspicuously below the water's surface with only its snout tip exposed. Webbed feet aid the softshell's propulsion through the water. The vulnerability of a soft shell is offset by the strong jaws and quick movements of this group of 3-clawed turtles. A bit of Southern folklore attests to the tenacity of these creatures. Some believe that a softshell won't release its grip until a thunder clap sounds.
South Dakota's aquatic ecosystems host two softshell turtle species - the smooth softshell and the spiny softshell. The smooth softshell has a round, smooth-textured carapace. Softshell measurements are typically given as carapace length. Female smooth softshells range in length from 6 to 14 inches. The smaller males range from 4 to 7 inches. The only distinguishing mark on this turtle's head and limbs is the black-bordered white line running from the snout onto the neck.
Although fast on both land and water, the smooth softshell rarely ventures far from water, except to nest on sunny sandbars or other open areas having a moist substrate. Females usually become sexually active after reaching 8 inches in length, males at about 5 inches. The female digs a nest 6 to 9 inches deep and lays 3 to 33 fertilized eggs, which are covered with sand. After 9 to 11 weeks, young hatch from their thick shells and make their way to the closest lake or river. Older females may lay as many as 3 clutches of eggs each nesting season.
This carnivorous turtle captures prey by ambush from a concealed position or by active pursuit through the water. Although most commonly found in free-flowing reaches of the Missouri River in southeastern South Dakota, the smooth softshell also inhabits some West River tributaries and several larger lakes.
The spiny softshell is so-named for the spiny projections on the forward edge of its carapace, the edge behind the turtle's head. In contrast to the smooth texture of the smooth softshell, the spiny's carapace has a rougher, sandpapery texture. Also differing from the smooth, the head and legs of the spiny softshell have dark spots and streaks. Both sexes have scattered spots, called ocelli, on the carapace, although males may have a more centralized spotting pattern than females.
Spiny Softshell- note distinctive patterns on head and legs
Females range from 6 to 17 inches in carapace length, males from 5 to 7 inches. Sexual activity is thought to begin when females are greater than 11 inches long and in males with a length of at least 5 inches. This turtle nests in June and July, when the female digs a nest 4 to 10 inches deep, where she deposits up to 32 eggs. The specific incubation period is unknown. Young may hatch from August through October. In the northern reaches of its range, the spiny softshell will hibernate underwater, beneath several inches of mud.
Found in a wider variety of habitats than the smooth, the spiny softshell may inhabit swift-flowing rivers, oxbow wetlands, lakes or impoundments. In South Dakota, this species is primarily found in the Missouri River and its major tributaries. This turtle may commonly be seen basking on floating debris, rocks or logs, sometimes in large groups. An active predator, the spiny softshell seeks prey by swimming and searching under water, or by probing its snout into vegetation and under rocks.
What are the threats to the smooth and spiny softshells in South Dakota? The loss and degradation of our natural river systems has hurt these species, particularly during the nesting season. Because of the lack of good nesting habitat, softshells are forced to concentrate nests in a few areas. The clichÃ© cautioning against putting all your eggs in one basket rings true, since these nest concentrations are extremely vulnerable to destruction and predation, particularly by coyotes and people. The fluctuating water levels of the Missouri River reservoirs have caused bank erosion, leaving shelves of potential nesting habitat that are inaccessible to softshells.
What can you do to help these interesting creatures? If you catch a softshell while fishing, safely release it as quickly as possible. Report any softshell sightings to the South Dakota Natural Heritage Program. Because of the lack of information on nesting habits and success in our state, we are especially interested in learning of locations of small softshells and of nesting areas. Only with your help can we learn more about our rare species and translate knowledge into conservation.