The Leatherback Turtle’s Skylight: A Unique Adaptation for Detecting Ambient Light
Introduction
Animals with poor vision have evolved unique adaptations to see in dark environments. One such adaptation is the leatherback turtle’s skylight, an unusually thin area of bone on the top of its skull. This skylight allows light to reach the turtle’s pineal gland, a structure that regulates sleep and other cyclical activities.
The Pineal Gland and Ambient Light
In most vertebrates, the pineal gland uses ambient light to regulate sleep and other cyclical activities. However, in some species, such as reptiles and amphibians, the pineal gland has evolved into a third eye, complete with a lens and retina. This third eye is used to measure daylight and determine the time of day.
The Leatherback Turtle’s Skylight
The leatherback turtle is the only known animal that has a skylight instead of a third eye. The skylight is located on the top of the turtle’s skull, just beneath a spot of unpigmented skin. This allows light to penetrate directly to the pineal gland.
Equilux and Migration
The leatherback turtle uses its skylight to detect changes in long-wave light. This information allows the turtle to calculate the “equilux,” the day when sunset and sunrise are exactly 12 hours apart. This is a more reliable signal for migration than water temperature or light intensity. Leatherback turtles feeding in the North Atlantic use the equilux to know when to head south each fall.
Other Animals with Photoreceptors
Evolution has equipped many animals with photoreceptors in various parts of their bodies to respond to light. For example, some sea snakes have photoreceptors in their tails to ensure they fully enter caves when hiding. Certain butterflies have light-sensing cells in their male genitalia to prevent ejaculating into the open air. Some corals cycle reproduction based on the amount of blue light during the second full moon of springtime.
Conclusion
The leatherback turtle’s skylight is a remarkable adaptation that allows it to detect ambient light and determine the time of day. This information is crucial for the turtle’s survival, as it uses it to regulate sleep and migration patterns. The discovery of the skylight highlights the diverse and ingenious ways that animals have evolved to perceive and respond to their environment.
