Abstract
To ascertain how scalloped hammerhead sharks make nightly migrations to their feeding grounds as many as 20 km from their daytime abode, a seamount, a sensor was developed that measured irradiance intensity within the spectral range and sensitivity of the vision of the species. Could the sharks guide their movements by sensing the polarity of irradiation energy radiated from the sun or moon that penetrated into the oceanic depths? Two sensory receptors, cones and rods, are present in the retina of sharks to enable them to see both during daytime and nighttime. The peak sensitivity of the cones is red-shifted due to the presence of these wavelengths during the former period, while their response is linear under the range of the high light levels also present at this time; the peak sensitivity of rods is blue-shifted due to the presence of these wavelengths during dawn, dusk, and nighttime and is linear over the complementary range of low light levels. Spectral response curves for these two receptors were determined for sharks, and an attempt was made to match those of the sensors to the shark’s wavelength perception. The first sensor was matched to the photopic range using a photocell covered with a red-shifted gel filter; the second was matched to the scotopic range using a blue-shifted gel filter.
Highlights
Vision is of less importance to animal navigation in the marine environment because of its dependence upon irradiance, which is absorbed and scattered rapidly in sea water
Could the sharks guide their movements by sensing the polarity of irradiation radiated from the sun or moon that penetrates more into the oceanic depths? To find an answer to that question, scalloped hammerhead sharks were tracked by boat while carrying an ultrasonic transmitter outfitted with sensors of swimming depth and irradiance, the latter matched to their photopic and scotopic visual sensitivities
Even if the sharks were unable to see an object through the surface of the water, it was improbable that even polarized light, emitted from a celestial object, could provide the directional information necessary for the scalloped hammerhead to swim in a straight line at considerable depths
Summary
Vision is of less importance to animal navigation in the marine environment because of its dependence upon irradiance, which is absorbed and scattered rapidly in sea water. The shoreward movements of parrot fishes were less directional when their vision was blocked with opaque cups than when not blocked by transparent cups and when the sun was obscured by clouds [2]. It is not known whether fishes might use vision to orient in deeper water, when swimming not far from the surface. The depths at which a scalloped hammerhead was swimming as well as the irradiance levels it experienced were telemetered back to an on-board receiver that automatically decoded the pulse-interval modulated signals into measurements of swimming depth and irradiance [3,4] In this manner, it was possible to find out whether the hammerhead might be orienting using underwater irradiance
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