Seafinding revisited: how hatchling marine turtles respond to natural lighting at a nesting beach

Hatchling marine turtles emerge at night from underground nests on oceanic beaches and then use visual cues to crawl from the nest site to the sea ("seafinding"). However, the light wavelengths (λ's) used to accomplish this orientation have not been thoroughly documented, nor do we understand why some λ's are favored over others. We measured nocturnal radiance on the horizon at 20 nm intervals between 340 and 600 nm at two nesting beach sites and then, under laboratory conditions, determined the lowest intensities of those λ's that induced green turtle and loggerhead hatchlings to crawl toward each light source (a low positive "phototaxis threshold"). Both species were similarly sensitive and were attracted to all λ's. Radiance measures at all λ's were greater toward the seaward horizon than toward the landward horizon, providing an important orientation cue regardless of variation in lunar illumination. Previous studies document that both species detect λ's longer than those that are most attractive. We hypothesize that seafinding is a specialized response mediated by cones that are sensitive to the shorter λ's (to minimize the effects of dark noise) but such as rods, are especially sensitive to low levels of nocturnal illumination.