Seaward orientation of hatchling turtles: turning systems in the optic tectum

This paper reports studies on the mechanisms underlying seaward orientation in hatchling turtles. The particular aim was to investigate whether activity in different regions of the retina and associated tectal areas, as assessed by some comparator mechanism, results in turning in different directions. Hatchling green turtles (Chelonia mydas) were tested for sea-finding ability in a natural situation on the beach following lesions of the optic tectum. Asymmetrical bilateral lesions resulted in a number of turtles making circles in the direction of the tectum with the posterior lesion and in other turtles deviated in this direction. Bilaterally lesioned turtles were also slower and less consistent in their sea-finding behaviour. No major disruptions of sea finding were detected in animals with unilateral tectal lesions. By suspending lights in the nasal visual field of unilaterally blindfolded green and leatherback turtles (Dermochelys coriacea) it was possible to produce circling in the direction of the covered eye; in contrast, with the light suspended in the temporal field, turning was always in the direction of the uncovered eye. The results are consistent with the view that sea finding depends on a complex phototropotactic system with stimulation in different parts of a single retina being associated with turning in opposite directions.