Feeding kinematics of hatchling swellsharks, Cephaloscyllium ventriosum (Scyliorhinidae): the importance of predator size

Abstract

Capture, manipulation, and transport of prey were quantified from high-speed video of hatchling swellsharks, Cephaloscyllium ventriosum. Kinematic variables were contrasted with those of 1 yr-old swellsharks. Hatchling prey-captures were ram-dominated, while 1 yr-old prey-capture events had a detectable suction component. Timing differences between kinematic patterns of the age groups were not detected. Significant differences in displacement maxima of kinematic variables between the two age groups during feeding were detected, but were consistent with the expectations of isometry; they doubled in accordance with a doubling in shark length. A scaling analysis confirmed that swellsharks grow isometrically. A simple model of the head during prey capture confirmed that buccal expansion scaled isometrically between age groups. Thus, this study suggests that hatchlings generally perform the suite of movements necessary for suction generation within the buccal cavity during feeding. A suction component to the strike, however, was generally not detected by the “ram:suction index”. It appears that although it is probably generated within the buccal cavity, suction has little effect on the prey item and makes a minimal contribution to prey capture. Suction may be ineffective due to the highly active nature of the hatchlings. During a strike, a hatchling's forward locomotion may be sufficient to overwhelm any suction produced by the expanding buccal cavity; thus, the swimming shark effectively “scoops” the prey up in its open mouth (i.e. ram feeding) before the prey can be entrained in the flow of water entering the mouth (i.e. suction feeding). It is also likely that the hatchling sharks are sufficiently small to render any suction generated ineffective. Even though the sharks scale isometrically, the sheer size of the 1 yr-old sharks allows a greater amount of force to be generated, that will ultimately draw the prey to the open mouth. Thus, there are absolute consequences of size for feeding behaviors.