Do swimming loggerhead sea turtles (Caretta caretta L.) use light cues for offshore orientation?

Sea turtle hatchlings emerge from underground nests on oceanic beaches, crawl to the ocean and swim out to sea. The primary cue used to find the ocean from the nest is the brighter oceanic horizon. We conducted laboratory tests to determine if similar visual cues also guide hatchlings while they swim away from land. The stimuli employed were shapes (horizontal bars, circles) and artificial “horizons”;. Responses elicited by these stimuli while turtles crawled and swam within an orientation tank were compared.

All hatchlings, whether crawling or swimming, oriented toward shapes. Crawling turtles oriented toward a brighter horizon but swimming turtles did not, even though they could detect it. We conclude that responses evoked by shapes were due to “light trapping”; effects. Responses by swimming hatchlings to more natural horizons suggest that these cues are no longer attractive. Thus it is unlikely that such photic stimuli are of primary importance in guiding hatchling movements offshore.

Key words:

• Sea turtles
• orientation
• migration
• light trapping effects