During the summer, leatherback sea turtles (Dermochelys coriacea) swim all along the cool temperate waters of the western and eastern margins of the North Atlantic, foraging for jellyfish. These giants have an odd-looking, non-pigmented pink area on the crown of their heads (pictured below) called the pineal, or simply “pink spot.” Now, researchers are saying that the spot functions like a skylight in their skull, allowing the turtles to sense subtle changes in sunlight.
At the end of summer, the foraging turtles turn south and leave their feeding grounds, which can be thousands of kilometers from where they breed. But how they know when to start their journey back is a bit of a mystery. To investigate how they sense seasonal changes, a team led by John Davenport from University College Cork examined a database of leatherback sightings in waters around Great Britain and Ireland. Then they compared those with historical data for sea surface temperatures and day lengths to see if the levels or periodicity of either environmental triggers would prompt foraging turtles to turn south and leave their feeding grounds at summer’s end.
They found that, in the study area at least, sea surface temperature was too variable and too slow to change to be useful as a trigger. Rather, the shortening of day length as the late summer equilux approaches provides a credible cue to the changing seasons. This cue, which they suspect acts via the pineal, basically tells leatherbacks to leave their foraging areas, whether it’s near Nova Scotia or the British Isles.
Furthermore, by examining four leatherbacks (who were found dead), the team discovered skeletal structures underneath the pink spot in juvenile and adult turtles (below) that are compatible with the idea of a “pineal dosimeter function” for light stimuli. The layers of bone and cartilage underlying the spot is much thinner there than in other areas of the skull, Science explains. The region is so thin, it lets light through to the pineal gland—a region of the brain that helps regulate day and night cycles, as well as seasonal patterns of behavior. The work was published in the Journal of Experimental Marine Biology and Ecology.