Differences in visual perception are correlated with variation in sea-finding behaviour between hatchling leatherback, Dermochelys coriacea, and loggerhead, Caretta caretta, marine turtles.

After completing embryonic development, marine turtle hatchlings emerge from their subsurface nest, generally at night, and crawl to the ocean ('sea finding'). That response depends upon the ability of the turtles to discriminate between the brighter seaward versus a dimmer landward horizon, followed by a positive phototaxis. While the crawls of most marine turtle hatchlings are well oriented and straight, those of leatherback hatchlings are sometimes interrupted by bouts of circling. We conducted experiments comparing the orientation and crawling behaviour of leatherbacks to those of loggerhead hatchlings to determine why those differences occur. The two species did not differ in the light spectra attracting the hatchlings, but leatherback thresholds for detection and for intensity discrimination were significantly higher than those of loggerheads. At the nesting beach, loggerheads under full (bright) or new (darker) moon conditions crawled straight to the ocean; circling rarely occurred. Leatherback crawls under a full moon were indistinguishable from those of loggerheads, but during new moon trials, when horizon brightness differences approached leatherback intensity discrimination thresholds, circling increased significantly. We conclude that circling is probably used by leatherbacks to reinforce orientation decisions when horizon cues become more difficult to discern. Circling could be costly as it lengthens the crawl and increases exposure to terrestrial predators. We hypothesize that those costs persist because other visual adaptations affecting sensitivity enhance the ability of leatherbacks to detect prey, mates or favourable habitats in an open ocean environment. • We compared leatherback and loggerhead hatchlings' phototaxis responses. • Leatherbacks were less sensitive to wavelengths of 340–580 nm than loggerheads. • Under full moon, both species crawled directly from the nest to the sea. • Under new moon, leatherbacks were less accurate; loggerheads remained unchanged. • These responses may reflect sea-finding orientation–foraging efficiency trade-offs. [ABSTRACT FROM AUTHOR]