Plasticity and constraints on feeding kinematics in anuran larvae

Tadpoles of the majority of anuran species have tiny, anatomically complex mouths. In most species the larval jaws are keratinized sheaths (beaks) overlying infrarostral cartilages. Surrounding the beak is a flexible oral disc and transverse rows of small, keratinized denticles. We used high-speed videography (250, 500 and 1000 frames per second) of Rana catesbeiana tadpoles to observe the kinematics of these mouthparts in feeding and breathing. Tadpoles can protract and retract their jaws as well as make them wider and narrower with each gape cycle. We demonstrate that during air-breathing, movement of the oral disc helps surfacing tadpoles to capture air quickly by preventing water from coming into the mouth. For our feeding study, we observed tadpoles as they grazed on both clean and algal covered glass surfaces. As the jaws close, the lower beak narrows to a greater degree when it encounters resistance. The denticle rows are used to both anchor the mouth and rasp surfaces during feeding. The hyperkinetic mouth parts of tadpoles permit grazing on non-planar surfaces of variable resistance. A trade-off in having such mobile jaws is loss of stability; no generalized tadpoles can generate great forces with their jaws, which would be necessary to subdue and dismember large tough prey. The feeding system of tadpoles is built out of soft tissues (such as cartilage and keratin) that can be shed (the keratinized sheaths) or remodeled (the underlying infrarostral cartilage) quickly, thus facilitating metamorphosis.