Highly directional swimming by scalloped hammerhead sharks, Sphyrna lewini, and subsurface irradiance, temperature, bathymetry, and geomagnetic field

The homing behavior of scalloped hammerhead sharks (Sphyrna lewini) to and fro between Espiritu Santo Seamount and Las Animas Island and the surrounding pelagic environment was studied to reveal their mechanisms of navigation in the oceanic environment. Four sharks were tagged with ultrasonic transmitters and tracked at the former location and one shark at the latter site during July, August, or September between 1981 and 1988. Hammerhead swimming movements were highly oriented: the mean coefficient of concentration (r) for sets of ten consecutive swimming directions recorded during eight homing movements by three hammerhead sharks ranged from 0.885 to 0.996. Drift within a current could not explain this directionality, since highly variable directions were recorded from a transmitter floating at the sea surface after becoming detached from a shark. Forward swimming momentum was an unlikely explanation, since highly directional swimming was maintained for a period of 32 min with only a gradual change in course. To maintain directionality over this period, an environmental property should be necessary for guidance. The hammerheads swam at night, with repeated vertical excursions ranging from 100 to 450 m deep, out of view of either the sea surface or the sea floor. The sharks' vertical diving movements were compared to distributions of spectral irradiance (relative to elasmobranch scotopic and photopic visual sensitivities), temperature, and current-flow directions in the water column. No relationships were evident between these properties and the sharks' oriented swimming movements. Movements of scalloped hammerhead sharks to and from a seamount were compared to topographic features in bathymetry and geomagnetic field leading away from the seamount. Sharks swam repeatedly over fixed geographic paths, and these paths occurred less often along submarine ridges and valleys than maxima and minima in the geomagnetic field. No significant difference existed between the degree of association of points from the sharks' tracks and points from track simulations and ≥20° changes in the slope of the depth record. On the other hand, significantly more points from the sharks' tracks were associated with slope changes in the magnetic intensity record than points from track simulations. A magnetic intensity gradient of 0.037 nanoteslas/m (nT/m) existed at 175 m depth, where a shark swam directionally, and this gradient was three times steeper than that measured at the sea surface and exceeded that recorded at a depth of 200 m. The hammerheads are hypothesized to find the seamount using geomagnetic topotaxis. The shark could be attracted to and move back and forth along ridges and valleys, features in the relief of magnetic field intensities occurring over a geographical area.

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