Leatherback turtle

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.

Sea turtle nests were compared to determine the effects of nest depth on hatchling anaerobic metabolism in Juno Beach, Florida, USA. In situ nests of 3 species (Caretta caretta, Chelonia mydas and Dermochelys coriacea) were compared. Relocated loggerhead nests were studied under an experimental regime. Nest temperatures and oxygen concentrations were monitored. On the night of first emergence, blood samples were taken from hatchlings resting at the nest chamber bottom and sand surface, and digging to the sand surface. Samples were analyzed for lactate concentrations. Blood lactate levels were high in hatchlings actively digging and low for those resting. Lactate levels differed among species and nest depths. Within in situ nests, actively digging green turtle hatchlings had the highest lactate, followed by loggerhead hatchlings and leatherbacks (lowest). Loggerhead hatchlings digging from deeper relocated nests had higher lactate than those digging from shallower depths.

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.

Leatherback sea turtles (Dermochelys coriacea) feed exclusively on gelatinous prey. Hatchlings are solitary and must possess a predisposition to respond to prey. In laboratory experiments, I studied the responses of nineteen leatherback hatchlings to visual (jellyfish model and shapes: circle, square, diamond) and chemical (homogenates of three prey) stimuli presented alone or as paired (visual + chemical) treatments once daily. When presented alone visual stimuli resembling jellyfish outlines elicited stronger feeding responses (changes in locomotion and orientation) than those not resembling jellyfish. Chemical stimuli alone induced a rheotaxis, but responses evoked by some homogenates were stronger than responses to others. Paired stimuli evoked stronger orientation and more consistent increases in swimming (flipper stroke) rate, indicating additive effects. Results suggest that both stimuli elicit food searching behavior and when they begin to forage, hatchlings already possess predispositions to respond to an adaptive array of prey shapes and odors.

The reproductive success of leatherback turtles (Dermochelys coriacea) is typically the lowest of the seven sea turtle species. Why this vital rate is decreased has remained unanswered for nearly a century. Recently, detailed postmortem examination of leatherback hatchlings identified muscular pathologies that suggested possible selenium deficiency. High bodily burdens of mercury compounds are associated with selenium depletion. Selenium is a necessary detoxifying nutrient that itself can be toxic at elevated concentrations. Mercury compounds are toxicants with no known biological function. High bodily concentrations of mercury can be detrimental to marine organismal health, reproduction and survival, both directly and indirectly through inducing selenium depletion. The goals of this dissertation are to evaluate several related hypotheses to explain low leatherback nest success. ... Because leatherbacks take in high volumes of prey, high tissue concentrations of mercury and selenium can result. This study provides the first evidence that chemical contaminants may explain low reproductive success in leatherback sea turtles.