Morphological, electrophysiological and behavioral investigation of visual acuity of the juvenile loggerhead sea turtle (Caretta caretta).

摘要

A majority of the research on sea turtle vision focuses on the turtle's ability to perceive shapes, colors and brightness cues on land. However, aerial vision is a minor component of the visual ability of sea turtles, potentially used only when surfacing to breathe, while basking, and during female reproductive activities. For my doctoral dissertation, I examined the aquatic visual acuity of juvenile loggerhead sea turtles (Caretta caretta) by investigating the morphology of the eye, the electrophysiology of response to stimuli, and the behavior to specific stimuli. For the morphological experiments, eyes were obtained from loggerheads, euthanized due to injury, and histological techniques were used to identify both cell types as well as regionization of cell densities. For the collection of electrophysiological responses from healthy juvenile loggerheads, testing was performed using a water-filled goggle and subdermal platinum electrodes implanted over the optic nerve and contralateral optic tectum. Stimuli were flickering black and white gratings. Visual evoked potentials (VEPs) were collected using an averaging computer and thresholds were measured from the recordings. Finally, for the psychophysical study, loggerheads were trained, in a 500-gallon tank, to discriminate between a vertical grating and a 50% gray panel using positive reinforcement. Thresholds were then recorded from incrementally smaller gratings. These data, from the three techniques used in this study, suggest that vision does play an integral role in the juvenile loggerhead's perception of its surroundings. Higher concentrations of cone photoreceptors and ganglion cells were found in the dorsal region of the eye, indicating an area centralis. Furthermore, the cone and rod photoreceptor densities throughout the eye suggest that this animal is capable of both visual acuity and sensitivity. Acuity threshold, measured from the VEP recordings, was approximately 5.4 minutes of arc for all turtles tested. Moreover, the psychophysical experiment measured an acuity threshold of 12.9 minutes of arc. The regionalization of acuity in the dorsal hemisphere of the eyes as well as the acuity threshold levels recorded for juvenile loggerhead sea turtle suggests that loggerheads are using vision for foraging, predator avoidance, territory selection and defense, and other basic behaviors in their aquatic surroundings.