KAJIURA, S.M.; University of California, Irvine: Function of the Hammerhead Shark Cephalofoil

It has been supposed that the evolution of the wing-like cephalofoil head of hammerhead sharks (Chondrichthyes, Sphyrnidae) was driven either by sensory systems or hydrodynamic concern; however there is little experimental evidence in favor of either group of hypotheses. The scalloped hammerhead shark, Sphyrna lewini, and the closely related sandbar shark, Carcharhinus plumbeus, were used to test the ‘enhanced electroreception’ and ‘improved hydrodynamic maneuverability’ hypotheses. The electrosensory hypothesis was tested by comparing the electrosensory pore distribution of the two species as well as behavioral responses to prey-simulating electric fields. The maneuverability hypothesis was tested by quantifying the dorsal surface area of the cephalofoil and pectoral fins during straight line swimming and during execution of a sharp (90�) turns. The cephalofoil is a better electrosensory detector of cryptic prey than a conventional head shape, sweeping a broad area with no loss in sensitivity. In addition, hammerheads can detect cryptic prey 40% further from the swimming path than sandbar sharks. The cephalofoil is maintained in the horizontal plane during straight line swimming and turning indicating that it is not used to bank and roll during a turn, thus falsifying the maneuverability hypothesis. Maintaining the head in a horizontal plane keeps the electrosensory system as close as possible to the substratum for localizing benthic prey even during a turn.