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Electroreception in juvenile scalloped hammerhead and sandbar sharks
Department of Zoology and Hawaii Institute of Marine Biology, University of Hawaii at Manoa, PO Box 1346, Kaneohe, HI 96744, USA
* Author for correspondence at present address: Ecology & Evolutionary Biology, 321 Steinhaus Hall, University of California — Irvine, Irvine, CA 92697, USA (e-mail: kajiura{at}uci.edu)
Accepted 21 August 2002
The unique head morphology of sphyrnid sharks might have evolved to enhance electrosensory capabilities. The `enhanced electroreception' hypothesis was tested by comparing the behavioral responses of similarly sized carcharhinid and sphyrnid sharks to prey-simulating electric stimuli. Juvenile scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus oriented to dipole electric fields from the same maximum distance (approximately 30 cm) and thus demonstrated comparable behavioral-response thresholds (<1 nV cm-1). Despite the similarity of response threshold, the orientation pathways and behaviors differed for the two species. Scalloped hammerheads typically demonstrated a pivot orientation in which the edge of the cephalofoil closest to the dipole remained stationary while the shark bent its trunk to orient to the center of the dipole. By contrast, sandbars swam in a broader arc towards the center of the dipole. The different orientation patterns are attributed to the hydrodynamic properties of the cephalofoil, which enables the hammerheads to execute sharp turns at high speed. The greater trunk width of the sandbar sharks prevented them from demonstrating the same degree of flexibility. Therefore, although the sphyrnid head morphology does not appear to confer a greater sensitivity to prey-simulating dipole electric fields, it does provide (1) a greater lateral search area, which may increase the probability of prey encounter, and (2) enhanced maneuverability, which may aid in prey capture.
Key words: ampullae of Lorenzini, Carcharhinidae, elasmobranch, enhanced electroreception hypothesis, Sphyrnidae, Carcharhinus plumbeus, Sphyrna lewini
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