Voyage of the Lonely Turtle Photo Essay: Turtle Anatomy
The head is retractable within the shell for most terrestrial turtles and tortoises but not for the sea turtle. A sea turtle's head remains out at all times.
Sea turtles do not have teeth but their mouths are sharp and beak-like for crushing and tearing food. Depending on the species, sea turtles may be carnivorous, herbivorous, or omnivorous. Their jaws are adapted for their diet. A loggerhead's diet consists primarily of crabs, mollusks, shrimps, jellyfish, so their jaws are adapted for crushing and grinding. Whereas, green sea turtles, the only adult herbivorous sea turtles have finely serrated jaws adapted for a vegetarian diet of sea grasses and algae.
Sea turtles are near-sighted which suits them well for underwater living but doesn't provide sharp eyesight above the water. Living in saltwater has demanded that sea turtles develop a way to rid their bodies of the salts they accumulate from living in and drinking seawater. Just behind each eye is a salt gland which helps sea turtles maintain a healthy water balance by shedding large "tears" of excess salt. The gland also excretes fluids that help to keep a female turtle's eyes moist while they are on land nesting.
Sea turtle ears are located inside of the turtles' heads, so that the turtles are more aerodynamic in the water, and have the capability of detecting low frequency sounds and vibrations.
Sea turtles have a very powerful sense of smell. It has been theorized that this sense of smell helps guide female sea turtles back to a natal beach for nesting.
A sea turtle shell is lighter and more streamlined than that of land turtles but it is considered the most highly developed protective armor of any vertebrate species to have ever lived. Sea turtles have both an internal and external skeleton. The external skeleton is a bony outer shell that offers substantial protection from predators. The shell covers both the dorsal (back) and ventral (belly) surfaces. Covering the dorsal surface is the carapace. This portion of the shell is composed of bone and contains the fused, broadened ribs. The carapace is covered with large scale-like structures made of keratin called scutes. The species of a turtle can be classified based on the number of each type of scute. The ventral portion of the shell is known as the plastron. The plastron is actually made up of four bones in the sea turtle, with one bone, the entoplastron, often used to differentiate among species of sea turtles. Between the carapace and plastron there are openings for the sea turtle's head, tail, and flippers. Hard-shelled plates called lateral bridges connect the carapace and plastron. The internal skeleton anchors the turtles' muscles. The spine is fused to the carapace in all species except the leatherback.
Muscles and Blood
A sea turtles' muscle and blood are part of what makes its body so well-adapted to life in the ocean. These tissues are able to store oxygen in large quantities, allowing sea turtles to remain underwater for long periods of time. Because sea turtles can keep higher concentrations of carbon dioxide in their blood than most other air-breathing animals, they are able use their oxygen very efficiently. When sleeping or resting, which usually occurs at night, adult sea turtles can remain underwater for more than six hours without breathing.
The long digits in the front and rear limbs of the turtle fuse together to form flippers. Turtle flippers are sensitive to touch. A turtle's front flippers propel its body through the water, forming a figure-8 pattern. The rear flippers provide direction and stability to the turtle's movement. It's the flippers that make sea turtles such efficient and graceful swimmers. Besides swimming, rear flippers have a second function in female sea turtles. They use them to dig out holes when nesting.
The tail is one of the few features of a sea turtle's body that marks a physical differences between males and females. A female sea turtle's tail typically does not extend beyond the hind flippers, while the tail of a males does.