Kinematics of aquatic and terrestrial prey capture in Terrapene carolina, with implications for the evolution of feeding in cryptodire turtles.

Studies of aquatic prey capture in vertebrates have demonstrated remarkable convergence in kinematics between diverse vertebrate taxa. When feeding in water, most vertebrates employ large-amplitude hyoid depression to expand the oral cavity and suck in water along with the prey. In contrast, vertebrates feeding on land exhibit little or no hyoid depression. In this study we compared the kinematics of terrestrial and aquatic prey capture within one species of turtle, Terrapene carolina, in order to determine whether an individual species can modulate the magnitude of hyoid depression between air and water. High-speed video (250 frames per second) showed that hyoid depression was over three times greater in aquatic than in terrestrial feedings, indicating that T. carolina is able to modulate hyoid depression magnitude depending on the medium in which feeding occurs. In addition, we observed medium-dependent modulation of hyoid depression in another turtle, Heosemys grandis, and large-amplitude hyoid depression during aquatic feeding in Kinosternon leucostomum, Platysternon megacephalum, and juvenile Chelydra serpentina. In all of these turtles, hyoid depression produced oral cavity expansion during aquatic feeding, but the earthworm prey were never sucked toward the predators. Prey were captured by neck extension (ram feeding), and we conclude that the function of hyoid depression during aquatic feeding in cryptodire turtles is to prevent the forward motion of the predator from pushing the prey away (compensatory suction). Aquatic feeding is probably the primitive condition for all extant turtles, and thus terrestrial feeding in T. carolina and other turtles is a secondarily derived characteristic. We conclude from this historical pattern that it is not appropriate to use extant turtles in attempts to reconstruct the terrestrial feeding mechanisms of primitive amniotes.