Newman (1906:131) described the active pursuit of food: "They crawl or swim along the bottom, thrusting their snouts under stones and into masses of aquatic vegetation, occasionally snapping up a crayfish or larva that they have succeeded in dislodging. They do not tear up their food, but swallow it whole, using the forefeet to assist in forcing it down." Surface (1908:123) suggested that softshells may feed "upon insects which may be found floating on the water," and I have had captives take insects from the surface of the water. Carr (1940:107) also wrote that ferox and numerous gars in the Tamiami Canal, often at the mouths of the tributary ditches, snap at each other furiously as floating bits of food are washed in from the Everglades. Another habit that has been mentioned as an aid in acquiring food (Breckenridge, 1944:186; Conant, 1951:156; Hudson, 1942:101) is burrowing just below the surface in a soft bottom in shallow water, to ambush passing fish, or other food. Presumably all kinds of softshells do this in both shallow and deep water of lakes or rivers having a suitable substrate; spinifer and muticus have been reported to burrow in shallow waters (no observations in deep water) by Agassiz (1857:333), Cahn (1937:180, 189), Conant (1951:159) and Weed (1923:48). Marchand (in Carr, 1952:417-19) noted that ferox burrows in deep water, and mentioned that in areas of bare white sand a group of fish invariably surrounds them, and one can locate buried softshells by observing these particular schools of fish. No mention was made of the turtles attempting to catch the fish. Other associations of soft-shelled turtles and fish have been described. Kirtland (in DeKay, 1842:7) observed several large bass that closely followed large numbers of turtles floating at the surface. Newman (1906:131) reported the observations of fishermen in Lake Maxinkuckee that large-mouth black bass stay not far away from swimming softshells; the same author also mentioned the observations of Jacob Reighard, who suggested that bass may be feeding upon minnows that he noticed following softshells. Seemingly some sort of commensalistic relationship exists whereby fish acquire food that is dislodged by grubbing and scurrying of softshells. Probably food is pursued on occasion from a buried position, but this habit probably is not executed specifically for obtaining food. Newman (op. cit.:129) was of the opinion that burrowing in shallow water is a habit to facilitate "warming up."
Marchand (loc. cit.) also wrote of other notable underwater observations on ferox in Florida. He commented on this turtle's inquisitiveness in deep water and unconcern upon being touched or even upon being handled to some degree. Calf-deep in soft mud, he noted a turtle that "emerged from the mud of the bottom, headed up toward shore, circled, and when about three feet above the bottom dived suddenly and completely disappeared." Marchand wrote that some areas on the bottom (Crystal Springs), which are rooted up by the burrowing of softshells, are bare and soft, and assume a characteristic, easily recognized, appearance.
Cahn (1937:180, 189) stated that the burrowing process consists of "flipping" the loose sand or silt over the back, whereas Conant (1951:159) described the process as a rapid lateral movement of the body. My observations of captives agree essentially with Conant's observations. The initial movement, directed at a slight angle, is principally with the forelegs although complemented by lateral movements of the body. When the turtle is approximately half buried, it makes rapid lateral movements of the body, which completely bury the turtle and orient its body in a horizontal position.
Behavior and Adaptations
Some characteristics of softshells that are often mentioned in the literature are: extreme shyness or wariness, ferociousness as captives, dazzling speed and agility on land and in water, and great dependence on aquatic environment. Certainly they are wary; and this wariness may account, in part, for the scarcity of observations of basking, and statements attesting to their great speed on land. To my mind, their reported ferociousness and savage disposition as captives is overrated; of the many softshells that I have collected, only a few attempted to bite. The extensibility of their long neck does warrant more careful handling than needs to be employed with other species. Holbrook (in Hay, 1892:145) even wrote that they "will sometime leap up and give a loud hiss," and Newman (1906:130) wrote that "they hiss violently and thrust out the head." Wright and Funkhouser (1915:120) reported a captive ferox that "could jump forward practically its own length." I have been bitten by individuals of Kinosternon, Sternothaerus, Pseudemys and Graptemys, and cannot support the contention that softshells are more prone to bite than those species, a view shared by LeConte (in DeKay, 1842:7); many softshells on initial capture will tend to withdraw the head completely for a short time. Newman (loc. cit.) also wrote that recently captured specimens exude a thick, yellow, semi-fluid resembling yolk of an egg from the inguinal glands; the substance, however, is odorless but "undoubtedly homologous with the emission of the inguinal glands of the musk and snapping tortoises." Perhaps there is a difference in aggressiveness associated with geographic location, the age of the turtle or individual temperament.
Smith (1956:159), referring to muticus, wrote that they are the best swimmers of all fresh-water turtles, and perhaps of any turtles. Corresponding statements of other authors attesting to their speed and agility (including spinifer and ferox) in water and on land are based principally on the published comments of Muller (1921:181), who observed that females disturbed while laying eggs "about fifty feet from water … covered the distance faster than a man can run." Cahn (1937:180) also stated that muticus on a "level, unobstructed sand beach … can outrun a man," and (op. cit.:181) can "capture fish with ease"; Cahn supported the latter statement by relating his observation of a muticus that captured a small brook trout in a large tank. Smith (op. cit.:162) wrote that spinifer is "said to overtake bass." Doubtless they are good swimmers and they do scurry rapidly on land.
Published statements relating to the strictly aquatic existence of softshells especially muticus, are based on recognition of "its drastic adaptations to aquatic existence" (Carr, 1952:428); these adaptations presumably include pharyngeal respiration and the marked depression of body form. Pharyngeal respiration was demonstrated for muticus and spinifer (Gage, 1884; Gage and Gage, 1886), and was considered the principal type of aquatic respiration (some dermal and some cloacal) in Trionyx spinifer asper by Dunson (1960). Cloacal bursae (anal respiration) are lacking in trionychids (Smith and James, 1955:88). Accessory pharyngeal respiration is meaningful in light of the information furnished by Agassiz (1857:282-83), who found that Trionyx has a smaller lung capacity (weight of body in ounces/capacity of lungs in cubic inches = 16.9) than do some other genera (Pseudemys, 2.8; Testudo, 2.7; Terrapene, 1.1); corresponding values for more aquatic species were Chelydra, 9.3 and Kinosternon, 16.0. Cahn (1937:181), however, wrote that he has demonstrated pharyngeal respiration in individuals of Pseudemys, Chrysemys and Sternothaerus, and Allen and Neill (1950:13) suggested that it occurs in Macroclemys. More conclusive data are required to detect a positive correlation between small lung capacity, pharyngeal respiration, and degree of restriction to an aquatic habitat.
The depressed, soft-margined carapace of softshells has been mentioned as an adaptation to facilitate burrowing in soft sand or mud, and more suited for concealment than for speed in aquatic locomotion (Carr, 1952:429; Smith, 1956:162). Nielsen (1951:264-65), commented that in various lotic invertebrates, dorsoventral flattening of the body was no commoner than in lentic invertebrates; he wrote that a dorsoventral flattening is a disadvantage to an animal in a strong current and is an adaptation "probably … not to withstand the current directly, but to avoid it by seeking shelter in narrow crevices." Probably another aid to concealment, mentioned by Williams and McDowell (1952:272), is the plastral hinge.
Concealment of softshells is not enhanced by growths of algae on the carapace. Proctor (1958:637-38) reported that the common, epizoöphytic alga of chelonians, Basicladia, has never been reported from Trionyx; the same author recorded a large amount of filamentous algae, principally Stigeoclonium, but the algae could be easily wiped from the turtle, and Vinyard (1955:64) recorded an alga, Dermatophyton radians, attached to the skin of the legs of Trionyx. I noted a small patch of greenish scum growing near the insertion of the neck on a softshell (spinifer from Lake Texoma); cursory examination by Dr. R. H. Thompson, disclosed one of the colonial ciliate protozoans (resembling Opercularia) with enmeshed green or blue-green algae. Evermann and Clark (1920:592) mention a spinifer from Lake Maxinkuckee, Indiana, having a growth of Opercularia, covering the plastron.