Montana Field Guide

In the spiny softshell the shell is flattened (pancake-like), with flexible edges and covered with leathery skin; small conical tubercles or "spines" are present on the front edge of the carapace above the neck. The snout is tubular, with a ridge along the inner margin of each nostril, which allows this turtle to remain beneath the surface with just the snout exposed. In mature males, the carapace is like sandpaper, and marked with small dark spots or circles. The tail is thick and long, with the vent well beyond the rear edge of the carapace. In mature females, the carapace is not notably like sandpaper, is more generally mottled or marked with blotches, the tubercles at the front edge of the carapace are more prominent than in males, and the tail is relatively short.

Juveniles have characteristics that are female-like, except the carapace coloration, which is male-like. In hatchlings, the carapace is olive to tan, with small dark circles, spots, or dashes, and a yellowish margin bordered by a black line. The eggs are hard and white, smooth, thick-shelled, and about 24 to 32 millimeters in diameter. Adult females can reach 52 centimeters in carapace length, but much less in adult males (which average about 10 centimeters shorter); hatchlings are about 3 to 4 centimeters in carapace length.

The spiny softshell differs from other Montana turtles by having a flattened and leathery shell that is soft and lacks horny plates, and a pointed snout with tubular nostrils. The smooth softshell (A. muticus), which occurs in the Missouri River in southern North Dakota, differs by lacking the ridge on the inner margin of each tubular nostril and the absence of tubercles or spines along the front edge of the carapace.

No specific information is available for Montana. In Vermont, softshells migrated about 3 kilometers between riverine wintering sites and river mouth nesting sites near Lake Champlain; migratory movements were most extensive in spring and late summer (Graham and Graham 1997). Annual home range size in Arkansas was 784 to 2,310 meters of stream length for males and 683 to 2,145 meters for females (see Hammerson 1999).

Habitat use by spiny softshells in Montana is probably similar to elsewhere in the range, but studies are lacking and there is little qualitative information available. They occupy larger rivers and tributaries. Both sexes have been observed basking together on partially submerged logs in backwater sites of slow-moving water, and on sandy or muddy riverbanks (P. Hendricks personal observation).

Generally, the spiny softshell is primarily a riverine species, occupying large rivers and river impoundments, but also occurs in lakes, ponds along rivers, pools along intermittent streams, bayous, irrigation canals, and oxbows. It usually is found in areas with open sandy or mud banks, a soft bottom, and submerged brush and other debris. Spiny softshells bask on shores or on partially submerged logs. They burrow into the bottoms of permanent water bodies, either shallow or relatively deep (0.5 to 7.0 meters), where they spend winter. Eggs are laid in nests dug in open areas in sand, gravel, or soft soil near water (Baxter and Stone 1985, Ernst et al. 1994, Hammerson 1999, Stebbins 2003).

The food habits in Montana have not been studied.

Generally, spiny softshells forage in the water, often in shallows with vegetation. They are considered to be generalist carnivores, and usually feed on the bottom. Major foods are crayfish, aquatic insects (of at least seven orders), and fishes, but mollusks, worms, isopods, amphibians, carrion, and vegetation also are eaten (Ernst et al. 1994, Hammerson 1999). The diet in an Iowa study was about 25% insects, 36.5% fish as carrion, 5.8% small fish as live prey, and 55% crayfish, with plant material in 61% of the stomachs sampled; this breakdown of categories appears representative for other states (Ernst et al. 1995). Prey may be chased, ambushed, or flushed and pursued.

Animals are active from April to October (usually May to September) in Kentucky and Colorado (Ernst et al. 1994, Hammerson 1999). Water temperatures of 12 degrees C. appear to determine when animals enter or emerge from hibernation in Vermont (Graham and Graham 1997). Adults emerge earlier from hibernation, and remain active longer into the fall, than juveniles. The period of activity in Montana is poorly documented, with records from early June to late July (Hendricks and Reichel 1996, Hendricks 1999). Egg predators include striped and spotted skunks, raccoons, red foxes, and probably coyotes; young turtles are captured and eaten by predatory fish, wading birds, and muskrats (Ernst et al. 1994, Hammerson 1999). Some individuals are caught by anglers using live or dead bait, and then killed. No information on predators is available from Montana, but some adults are incidentally captured and killed by anglers.

No specific information is available for Montana, but data from other locations indicate that eggs are laid mostly in the second half of May and in June (most areas), mainly in the first half of June in southern Ontario and mid-June to early July in the far north. In Colorado evidence indicates nesting is from late May to early July, with June as the norm. In the more arid Great Plains and Rocky Mountain states, nesting activities may be stimulated by spring and early summer rains (Ernst et al. 1994, Hammerson 1999); most nesting occurs in either the morning or evening. Nests are bowl-shaped, with a narrower opening descending to a larger egg chamber; depths are usually from 7 to 18 centimeters, but may be up to 26 centimeters or more.

Clutch size averages 20 to 40 eggs, but may be as few as 6 or as high as 109 (Ernst et al. 1994, Hammerson 1999). A single clutch is produced, with most mature females nesting each year, although some may skip a year. Hatchlings emerge in 55 to 125 days in late August to early October (mainly September).

Females are sexually mature in about 8 years in Iowa, 10 to 20 years in Ontario (later in north than in south), and 11 to 16 years in southeastern Michigan (Congdon et al. 1994). Also in Ontario, the mean age of first nesting was estimated at 17 to 19 years (Galbraith et al. 1989); size at maturity is about 26 centimeters carapace length in Colorado (Hammerson 1999). In Ontario, the mean age of nesting females was estimated at 33 to 40 years (Brooks et al. 1988, Galbraith and Brooks 1989). Total reproductive failure (nest loss) is common. In Michigan, nest survivorship over 17 years ranged from 0 to 64% and averaged 23% (Congdon et al. 1994). In Ontario, growth rate and reproductive output increased with habitat productivity (Brown et al. 1994). In Michigan, minimum reproductive frequency was less than annual (0.85) (Congdon et al. 1994). The longevity record for the snapping turtle is nearly 39 years (a captive animal); estimated ages are commonly 20 to 30 years in wild populations (Ernst et al. 1994).

Montana populations of the spiny softshell are poorly understood, making management of them more difficult. It is apparent that the construction of dams and large reservoirs on rivers (e.g. Fort Peck Dam and Reservoir) is detrimental to population continuity, effectively creating smaller isolated populations. Impacts of other habitat disturbances are not clear. Studies of nesting success, population structure, dispersal, and population size need to be conducted throughout the range of both Montana sub-populations (Missouri River and Yellowstone River). Routine surveys for softshells in appropriate habitats could be made a standard part of the field duties of agency fishery biologists. Records should be maintained of the incidental "take" by anglers, who should be encouraged to report any captured spiny softshell; killed animals should be examined by agency fishery or wildlife biologists if possible so that data on sex, size, and food habits can be gathered and a base of information on the biology of this species developed. Nesting sites need to be identified and protected from disturbance by human activities.