Electroreception in carcharhinid and sphyrnid sharks (Sphyrna lewini, Carcharhinus plumbeus)

The evolution of the unique head morphology of hammerhead sharks (Family Sphyrnidae) has been the subject of much speculation. The dorso-ventrally compressed and laterally expanded pre-branchial head forms a cephalofoil that is an unmistakable diagnostic feature of the sphyrnid sharks. Various hypotheses have been advanced to explain the adaptive significance of this peculiar head morphology but few have been empirically tested. This study tests one specific hypothesis, the enhanced electrosensory hypothesis, to determine if the sphyrnid head morphology confers an electrosensory advantage compared to the pointed-snout head morphology typical of their close relatives, the carcharhinid sharks. Scalloped hammerhead sharks, Sphyrna lewini have a greater head width than similar sized sandbar sharks, Carcharhinus plumbeus, and also have a greater number of electrosensory pores (3068.3 ± 26.7SE vs. 2317.3 ± 26.3SE). The greater number of electrosensory pores are distributed over a greater surface area thus providing the hammerhead cephalofoil with a pore density (pores cm−2) equivalent to that of similar sized sandbar sharks. Despite gross differences in head morphology, the general distribution pattern of pores is conserved on both species. Both species orient to and bite at prey-simulating dipole electric fields in the seawater. The minimum electric field intensities at which the sharks initiate an orientation toward the dipole (i.e., response threshold) are similar for both species (S. lewini: median = 25.2 nV cm−1, minimum <1nV cm−1; C. plumbeus: median = 30.3 nV cm−1, minimum <1nV cm−1). Therefore, the hammerheads do not demonstrate greater sensitivity to electric fields than the sandbar sharks. The smaller cross sectional area of the trunk of the hammerheads enables them to exhibit a greater degree of flexibility that is manifest in a greater repertoire of orientation pathways compared to the sandbar sharks. The greater head width of the hammerheads enables them to sample a greater area with equivalent resolution to the smaller area sampled by the head of similar sized sandbar sharks. Therefore, although the hammerheads are not more sensitive than the sandbar sharks to prey-simulating dipole electric fields, their wider head does provide them with a greater probability of prey encounter. Thus, the enhanced electrosensory hypothesis is supported as an explanation of the adaptive significance of the sphyrnid cephalofoil.