Seafinding by the green turtle, Chelonia mydas: the orientation response is tuned to the lighting environment at the nesting beach

Hatchling marine turtles use visual cues to orient from their nest to the sea at
night. However, the wavelengths of light that carry this information have not been
properly documented, nor do we understand why they are favored. I measured
wavelength irradiance at 20 nm intervals between 340 – 600 nm at a dark nesting beach
and then, in the laboratory, determined the thresholds of the hatchlings for each λ that
evoked a positive phototaxis. In this study, I show that green turtle hatchlings are (i) most
sensitive to the shorter (360 – 480 nm) light wavelengths. Those light energies (ii)
dominated the available natural lighting at the nesting beach. They also (iii) presented a
steep gradient in irradiance between a landward and seaward view, an important cue for
orientation. I attribute the phototactic responses to “stimulus filtering”, the outcome of
natural selection that optimizes behavioral responses (seafinding) according to their
function, as well as when and where they occur.