The subspecies of T. spinifer occur in all fresh-water habitats. In the southern part of the geographic range, which overlaps that of T. ferox, T. s. asper occurs principally in running-water habitats. T. s. pallidus and T. s. asper are tolerant of brackish-water habitats. T. s. guadalupensis, known at present only from rivers and streams, occurs principally in river systems that drain the Edward's Plateau of southcentral Texas. T. ater is confined to crystal-clear ponds in central Coahuila.
The subspecies muticus occurs in large rivers and streams throughout its geographic range, but is known from lakes and impoundments principally in the southern part of its range (the northernmost record is from Reelfoot Lake, Obion County, Tennessee); there is only one record of muticus from a small, shallow, headwater creek (Reno County, Kansas), and only one from a lentic habitat (Meredosia Bay, Illinois) in the northern part of its range. T. muticus calvatus is known at present only from rivers and streams.
The seemingly greater restriction of muticus to running-water habitats suggests less vagility than in spinifer (Netting, 1944:86).
Size and coloration are adaptations to habitat. Soft-shelled turtles of large size are best adapted to mesic, essentially continuous swampy or marshy habitats, whereas small size is an adaptation to less continuous, semi-isolated habitats. A turtle of the maximum size attained by ferox in the habitat of emoryi would, in a general way, probably be more conspicuous and exposed to its enemies, both in the aquatic environment and during overland excursions; perhaps the kind and amount of food would be insufficient. In any event, small size is correlated with the more arid habitats of the southwest, and large size with mesic ones in the southeast. T. ferox, the largest species, and the smallest population of T. spinifer (resembling muticus) both occur in the southernmost part of the range of the genus. This situation does not support the corollary of Bergmann's Rule, that pertains to some groups of terrestrial reptiles, in which those subspecies occurring farther north, or in cooler climates during their season of activity, tend to be smaller.
Within the species spinifer, the emoryi group of subspecies are pallid having whitish dots on the carapace and lack extensive black pigmentation; these features seem to confer protective coloration on the inhabitants of arid, essentially sandy or muddy habitats having sluggish, turbid waters, whereas the more contrasting patterns of the spinifer group of subspecies eastward seem more suited to existence in clearer, swifter waters.
The occurrence of the two clines, spinifer-hartwegi and pallidus-guadalupensis, in the species spinifer are notable in that the former occurs mostly in one large continuous drainage system, that of the Mississippi, and shows no sharp break in the one character distinguishing the two subspecies whereas populations composing the pallidus-guadalupensis cline are separated into several river drainages, and show a relatively sharp break in several characters at the Brazos-Colorado river divide. This situation seemingly supports the thesis that clines are maintained by some sort of parallel gradient in ecological or geological conditions. It is notable that streams draining the Edward's Plateau (inhabited by guadalupensis) differ in quantity (more) and quality (especially CO_{3}—, Ca++, and Mg++ ions) of their solutes, and probably pH (higher) from those farther east (Hubbs, 1957:102). The gross difference in habitats mentioned above (sandy, turbid, sluggish streams in the west vs. clear, swift streams in the east) may affect the differentiation recognizable in the spinifer-hartwegi cline.
Daily and Seasonal Activity
Diurnal Habits
Softshells bask on débris in the water or on banks close to the water; basking presumably raises the bodily temperature. In general in the southeastern and southwestern United States, I have seen softshells basking only rarely but once saw six at one time close together on logs in Bowie Creek, Hattiesburg, Mississippi (species undetermined). Surface (1908:122) saw spinifer in rows on rocks or logs in tributaries of the Ohio River. Duellman and Schwartz (1958:271-72) stated that ferox basks on banks or beds of aquatic vegetation. Deckert (1918:31) mentioned large ferox "sunning in shallow water at edge of pond." Minton (1944:447) wrote that muticus and spinifer sun on steep mud banks (Wabash River). Cahn (1937:180) stated that muticus (in Illinois) basks on banks at the water's edge but seldom on logs, and suggests that muticus is less prone to leave the water than spinifer. According to Carr (1952:438), muticus never basks on logs or rocks. In Ohio, Conant (1951:159) mentioned spinifer as occasionally basking upon a log or rock, or sometimes on steep clay banks of streams. On banks, quick escape is facilitated by directing the head toward the water, thus eliminating the time that it would take to turn around on land (Conant, loc. cit.; Newman, 1906:129). Evermann and Clark (1920:593) mentioned spinifer as basking on sandy or grassy shores, and large boulders. Muller (1921:181) wrote that muticus basks four to ten feet from the water's edge on gently sloping sand and mud shores of small islands in the Mississippi River (near Fairport, Iowa). Muller stated that basking usually occurs in the morning, up until 2 p. m., and that beaches with a northern exposure were preferred; he observed 37 turtles within a 50-foot stretch of beach. In captivity, hatchlings bask on wire-mesh supports.
I have frequently observed softshells floating at the surface of the water, a habit previously mentioned by Surface (1908:122) and Pope (1949:305, 311). Individuals of Pseudemys and, to a lesser extent, Graptemys also float at the surface; those kinds of turtles and softshells at least, often appear at the surface of the water, seemingly as a result of an inquisitiveness, following repeated disturbances that cause submergence.