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Electroreception in juvenile scalloped hammerhead and sandbar sharks

Stephen M. Kajiura^* and Kim N. Holland

Department of Zoology and Hawaii Institute of Marine Biology, University of Hawaii at Manoa, PO Box 1346, Kaneohe, HI 96744, USA

View larger version (20K): [in a new window]   Fig. 1. Circuit diagram and electrode array used to study the response of scalloped hammerhead and sandbar sharks to prey-simulating dipole electric fields. During each trial, one of the four electrode pairs (e1-e4) was activated with a weak electric current (6 µA), which generated a dipole electric field around the electrodes. Electrodes were spaced 1 cm apart, and each electrode pair was equidistant from an odor-delivery tube in the center of the plate. The electrodes were spaced symmetrically on the plate, and a 10 cm radius circle was drawn around the center of each electrode pair to serve as a frame of reference for subsequent video analysis. A line drawn on the plate through the dipole axis of each electrode pair was also used in video analysis to reference the orientation angle of the shark with respect to the dipole axis. Batt, internal 9 V battery; Ext, external power supply; Hi, high current pathway through the circuit; Lo, low current pathway through the circuit.

View larger version (133K): [in a new window]   Fig. 2. Experimental apparatus used to study the response of scalloped hammerhead and sandbar sharks to prey-simulating dipole electric fields. One of the four electrode pairs (circles on the acrylic plate) was activated with a weak electric current, which generated a dipole electric field around the electrodes. The response of the sharks was recorded with a video camera mounted onto the end of a sliding track and positioned directly above the center of the electrode array.

View larger version (168K): [in a new window]   Fig. 3. Representative sample of a scalloped hammerhead shark orientation to a dipole electric field. (A) The shark is swimming within frame prior to orientation to the electric field. (B) The shark initiates an orientation to the dipole, and the distance (r) of the shark with respect to the center of the dipole is measured, as well as the angle () with respect to the dipole axis. (C) The shark swims towards the electrodes and (D) bites at the electrodes. After biting, the shark (E) swims away and (F) promptly turns back towards the electrodes. The counter in the lower right of each frame denotes the time in seconds followed by the frame number.

View larger version (82K): [in a new window]   Fig. 4. Quantification of the search area of a scalloped hammerhead shark. (A, B) The positions of the distal extremes of the head were marked on every other frame for a 1 s period. (C) The area that the head had covered in that 1 s period was measured by reference to a known area; a 20 cm diameter circle drawn on the electrode array.

View larger version (11K): [in a new window]   Fig. 5. Histogram of the percentage of orientations throughout the range of orientation distances. Whereas the scalloped hammerhead sharks Sphyrna lewini (A) demonstrated a uniform decrease in number of orientations with increasing distance, the sandbar sharks Carcharhinus plumbeus (B) demonstrated a greater number of orientations from distances between 5 cm and 10 cm.

View larger version (22K): [in a new window]   Fig. 6. Orientation distance plotted against angle with respect to the dipole axis. Both scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus oriented from a significantly greater distance at smaller axis angles where the electric-field intensity is greatest. A model dipole in the corner shows voltage equipotentials around the dipole axis.

View larger version (13K): [in a new window]   Fig. 7. Histogram of the percentage of orientations at electric-field intensities of <1 µV cm-1. Scalloped hammerhead sharks Sphyrna lewini (A) and sandbar sharks Carcharhinus plumbeus (B) demonstrate similar distributions across the entire range of field intensities. Approximately 70% of orientations were initiated to stimuli of <0.1 µV cm-1 for both species, with few orientations requiring a higher field intensity to initiate a response.

View larger version (16K): [in a new window]   Fig. 8. Histogram of the percentage of orientations at electric-field intensities of <0.1 µV cm-1. Scalloped hammerhead sharks Sphyrna lewini (A) and sandbar sharks Carcharhinus plumbeus (B) demonstrate similar distributions across the entire range of field intensities. Both species initiated approximately 35-40% of orientations to stimuli of <0.01 µV cm-1.

View larger version (28K): [in a new window]   Fig. 9. Orientation pathways demonstrated by scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus with frequency of occurrence of each type. Solid red lines indicate voltage equipotentials around the dipole, and black arrows indicate the path taken by the sharks. Current flow is indicated by the colored lines, with highest intensity (red) nearest the center of the dipole and lowest intensity (blue) furthest away. Whereas the scalloped hammerhead sharks demonstrated a greater variety of orientation pathways, the sandbar sharks were unable to exhibit the same repertoire of behaviors owing to their stiffer bodies.

View larger version (13K): [in a new window]   Fig. 10. Histogram of maximum body flexure of scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus. Scalloped hammerhead sharks demonstrated a greater degree of flexibility, with a mean maximum flexure () of 85.9°, whereas the of sandbar sharks was 113.3°.

View larger version (17K): [in a new window]   Fig. 11. Cross-sectional area of the trunk (cm2) of scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus. Sandbar sharks have a significantly greater cross-sectional trunk area than the scalloped hammerhead sharks across a wide range of sizes.