KAJIURA, S.M.; FORNI, J.B.; SUMMERS, A.P.; University of California, Irvine: Vision and Olfaction in Carcharhinid and Sphyrnid Sharks

The unique head morphology of hammerhead sharks (Chondrichthyes, Sphyrnidae) has spawned a variety of adaptive hypotheses. Two hypotheses which have persisted without empirical evidence are the ‘expanded visual field’ and the ‘stereo-olfaction’ hypotheses. It has been suggested that the location of the eyes at the distal tips of the cephalofoil provide sphyrnid sharks with a larger visual field compared to similar-sized carcharhinid sharks. Comparative morphology and swimming behavior were employed to quantify the maximum visual field sampled by several sphyrnid shark species and a representative carcharhinid. Although the broadly spaced eyes of the sphyrnids may provide a greater lateral visual field under extremely turbid conditions, the cephalofoil head morphology produces a significantly greater blind spot directly anterior to the snout. This is at least partially offset by a greater yaw amplitude in the locomotion of the hammerheads. The stereo-olfaction hypothesis was tested by modeling the water volume that is sampled by sharks with different head morphologies. Although the nostrils of the sphyrnids are more widely spaced, a prenarial groove channels water from much of the anterior edge of the cephalofoil directly into the nostrils effectively decreasing the lateral separation distance. Therefore, the volume of water sampled is increased at the expense of spatial resolution – the hammerhead does sample a wider swath but it gains no advantage in localizing the scent source. Thus, neither hypothesis for the evolution of the sphyrnid cephalofoil is strongly supported.