Spectral sensitivity

Hatchling marine turtles exhibit a positive phototaxis by crawling toward the lowest and brightest horizon when they emerge from nests on the beach at night, which should lead them to the ocean (“seafinding”). Previous research with cheloniid (loggerhead and green turtle) hatchlings demonstrated that the perceptual spectral sensitivities are well below the light available on the beach regardless of lunar phase. The goal of this research was to determine the perceptual spectral sensitivities of leatherback hatchlings, the most distantly related of all extant sea turtle species. This study revealed that, like cheloniids, leatherbacks are most sensitive to shorter wavelengths (< 500 nm). However, leatherbacks were 10 – 100x less sensitive than cheloniids at all tested wavelengths. This difference in sensitivity corresponds with increased crawl duration and circling behavior under new moon conditions when light levels are lowest and the difference in radiance between the landward and seaward direction is small.

Species that are ontogenetic migrators have early life stages (juveniles) that live
shallower in the water column than the adults and therefore experience a brighter
environment than the adults. This work provides evidence that juveniles and adults of the
ontogenetically migrating crustacean species Gnathophausia ingens, Oplophorus
gracilirostris, and Systellaspis debilis have evolved visual adaptations to their respective
environments. The juveniles use apposition optics that provide greater resolution,
whereas the adults use superposition optics that maximize sensitivity. These animals also
have regional specializations to aid in viewing a light field that is brighter above than
below, such as accessory screening pigments located dorsally and superposition type
optics ventrally. The non-ontogenetic migrators Notostomus elegans and Notostomus
gibbosus possess superposition optics as both juveniles and adults, implying that the
changes seen in ontogenetic migrators are indeed visual adaptations.

The pelagic longline fishery is responsible for significant mortality to sea turtles as a
result of foul hooking, entanglement in the lines, and internal injury after consuming the
baited hook. Bait, gear and lights (used to attract the target fishes to the baits at night) are
three variables that could also attract sea turtles to the lines. This study tests the role of
the lights in attracting leatherback (Dermochelys coriacea) turtles and compares their
behavior to the loggerhead (Caretta carelta), shown in previous studies to orient toward
both lightsticks and battery powered LEDs used in the fishery. The same lights were
used in experiments done on leatherbacks reared at Florida Atlantic University's Marine
Laboratory. The leatherbacks were exposed to the lights at night when they were
between 5 and 42 days old. The results show that leatherbacks, unlike loggerheads, either do not orient toward the lights or orient away from them at an angle that enabled
the turtles to keep the light in their peripheral field cf view. Thus, the capture of
leatherbacks in longlines is probably a consequence of other factors (such as attraction to
the odor of the baits, or to natural prey located near the Iines) that need to be investigated
through future research. The results also show that efforts to reduce the incidental capture
and injury of marine turtles in longlines must be based upon a firm understanding of the
similarities, as well as the differences, between turtle species.

In the ocean, lighting varies with habitat; the eye's spectral sensitivity must vary with visual ecology. Green turtles are the only sea turtle whose spectral sensitivity has been studied. Loggerheads and leatherbacks see visible light between 340 and 700 nm. However, the wavelengths detected with the greatest sensitivity by both species are those best transmitted at the specific depths where food, mates and predators are likely to be encountered. Both species have trichromatic vision, but the species differ in the concentration and peak sensitivity of each visual pigment resulting in either a broadly tuned (loggerhead) or finely tuned (leatherback) spectral sensitivity. Spectral sensitivity of leatherbacks overlaps both bioluminescence of prey, and light available in clear, deep, oceanic waters.