Sea Turtles | Smithsonian Ocean

Senses

Sensory adaptations help sea turtles survive in the ocean environment.

VISION

Sea turtles spend most of their lives submerged in an aquatic environment, but they also can spend significant amounts of time at the surface of the water because they require air to breathe. In addition, new hatchlings and nesting females spend time, however brief, on the beach. Therefore, sea turtles have eyes adapted to see in both water and air. However, their eyes are best adapted for seeing well under the waves.

Humans have a curved cornea that refracts light and helps to see in the air. Corneas lose all ability to refract light when submerged in water, which causes the unfocused view we see when opening eyes while underwater. Underwater, fish have a cornea merely as a protective barrier. All the refractive power of a fish's eye is contained in their lens. Animals such as turtles that spend time in both aquatic and air environments have eyes that combine these two eye types. Sea turtles have flat corneas and a nearly spherical lens adapted to their mostly aquatic lifestyle. Consequently, they are a little nearsighted on land but perfectly suited to seeing under the ocean surface.

Sea turtles also spend time at a variety of depths in the ocean with very different light levels—the deeper the depth, the dimmer the light becomes. Surprisingly, sea turtles seem to have eyes that are specialized for bright light rather than the dim deep water.

In the eye, cells called rods detect movement in dim light conditions and cells called cones detect colored light. Animals that see well in the dark have more rods than cones. Another trait that equips animals to see well in dim light are large eyes with large pupils.

Sea turtles have fairly evenly distributed rods and cones and their eyes are small in comparison to their body size. These traits equip sea turtles to see best in bright light. So, how do sea turtles see when they feed at depth or at night? The answer may lie in their ability to see bioluminescence. The pupil of a sea turtle’s eye is sufficiently big enough to detect the point-source light of bioluminescent prey. This glow is all they need to locate their meal.

A study in 2012 showed that juvenile loggerhead and green sea turtles have the photoreceptor cells that are required to see in color, but not many behavioral studies have been done to test whether they can actually distinguish the colors they see.

HEARING

Sea turtle ears are much smaller than ours and though not visible on the outside of their head, their ears allow them to hear vibrations in the water. Their ears are covered by a protective layer of skin called the tympanum. Underneath the tympanum is a layer of fat unique to sea turtles; freshwater turtles do not have this feature. Scientists think this layer may act similarly to the way toothed whales’ fatty tissues can channel low-frequency sounds into the inner ear. A 2014 study showed that both hatchling and juvenile loggerhead sea turtles hear and respond to low-frequency sounds. This observation suggests that, like some whales, sea turtles are low-frequency specialists. More research needs to be done to further understand how and what sea turtles are hearing.

TOUCH

The bony shell of a sea turtle can actually sense when something touches it. A series of thin nerves run along the shell and detect pressure changes associated with touch. The nerves do not contain pain receptors so scientists do not think sea turtles feel pain when touched on the shell, but these thin nerves could transmit information to other parts of the sea turtle’s body. In addition, researchers and scuba divers have observed sea turtles respond to the stimulation of being touched on the skin of their head and their flippers.

SMELL

Sea turtles possess an acute sense of smell. It is thought that this well-developed sense of smell, along with vision, plays a role in a sea turtles’ ability to locate prey. Being able to smell prey in murky or dark water would be an invaluable asset to a hungry sea turtle. Smell is also believed to contribute to how female sea turtles find their way back to their home beach to nest (see “Natal Homing” section below).

MAGNETIC SENSE

Sea turtles also have a sense that enables them to detect and utilize the magnetic field of the Earth as a guide, although scientists do not fully understand how their magnetic sensitivity works. In one study, baby loggerhead sea turtles were placed in a large pool of water in a controlled laboratory setting. Researchers varied the magnetic orientation of the pool to observe the hatchlings behavior. When the sea turtles were exposed to simulated magnetic conditions that placed them either north or south of their normal migratory pathway, they reacted by swimming in the direction that put them back on their migratory course. This remarkable magnetic sense provides sea turtles directional information, the same way humans use a magnetic compass, as well as an accurate sense of their location and position on the Earth, helping them orient as they travel vast distances. Magnetic sense functions as a way for sea turtles to orient themselves and navigate to where they want to go.

NATAL HOMING

Female sea turtles migrate thousands of miles, sometimes to the same beach where they were born, in order to nest. This phenomenon is called natal homing. There are two main hypotheses for how sea turtles are able to do this. The first is that sea turtles use Earth's magnetic field to distinguish their global position by latitude and longitude. Navigating by this internal compass would enable a sea turtle to return to the specific location it remembers from when she first entered the water as a newly hatched baby turtle. Another possibility is that sea turtles can imprint or distinctly remember certain characteristics of their home beach, such as a distinctive smell. Scientists think that sea turtles use a combination of these two techniques to find their way back to their birth beach. The use of the magnetic field would get them to the right area of the globe, while the imprinting would help them identify the exact beach.