Sea turtles are some of the most mysterious yet enduring animals on earth. As with all turtles, sea turtles are reptiles. They are covered with protective scales, breathe air into a set of lungs and reproduce by laying amniotic eggs, eggs with fluid filled sacs and soft but tough calcareous shells that protect embryos from dehydration.

 

Reptiles evolved from small land animals known as the cotylosaurs, which lived about 250 million years ago. Evolving from the cotylosaurs, the ancestors of the turtles appeared during the Triassic period, a little over 200 million years ago. The “first” true turtle, Proganochelys

quenstedi, appeared in the late Triassic period and shared many traits common to turtles today including a fully developed shell and a turtle-like skull and beak. Proganochelys also had some primitive features not found in modern turtles: small teeth, a clavicle (collarbone) and a simple ear. This primitive turtle could not pull its head into its shell, which left it vulnerable to predators.

 

As turtles evolved to withdraw their heads into their shells for protection, they became divided into two main groups: the side-necked turtles and the arch-necked turtles. Modern side-necked turtles are only found in the Southern Hemisphere and withdraw their heads sideways under their shell. The arch-necked turtles are the dominant group of turtles today and retract their heads in an s-shaped curve. These turtles represent the box and aquatic turtles, as well as the tortoises.

 

Not until late in the Jurassic period did sea turtles appear. The largest, Archelon ischyros, was a possible ancestor to the Leatherback and lived over 65 million years ago. Archelon reached lengths of about 13 feet. Fossil records indicate that many other sea turtles, including species living today, such as the Green and Loggerhead sea turtles, appeared during this time as well.

Despite their later evolution, sea turtles share some characteristics with primitive turtles. They cannot retract their heads or limbs into their shells, although they developed from the arch-necked turtles.

 

While the ancestors have gone extinct, there are about 249 species of turtles living today: 180 freshwater turtles, about 62 land turtles and tortoises and seven sea turtles.

 

                                                                                      Turtle Parts

 

The Body

Most notably, sea turtles have shells. The shell encloses and protects the soft, inner body parts. From the shell, the head, flippers and tail protrude. The shell has two parts: the top, carapace, and the bottom, plastron. They are connected to each other on the sides by the bridge. The carapaces’ bone plates that comprise the under layer are fused with the vertebrae and ribs of the turtle so that the turtle cannot leave its shell. The bone plates that make up the top of the carapace are covered with a protective keratin outer layer, much like our fingernails. This keratinous outer covering is called a scute. Through all of this armor, the sea turtle’s shell may seem indestructible, but it is actually quite sensitive. Beneath the thin, keratin scutes is living tissue with sensitive nerve endings.

 

The sea turtle’s appendages are paddle-like flippers (two large front flippers and two smaller rear flippers) and a tail. Sea turtles use their back flippers as rudders to maneuver while swimming.

In adult sea turtles, the tail length is used to determine sex. Male characteristics include a longer tail and larger claws on the front flippers, which are used to grasp the female during mating.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Head

The sea turtle’s skull is solid and does not contain teeth. Turtles have horny beaks. The beak is adapted to the diet of the turtle: sharp for cutting and slicing and flat for crushing. Sea turtle beaks are species specific and so is their diet. A Loggerhead sea turtle’s beak is very strong with powerful jaw muscles that enable it to crush crabs and other shellfish, while Leatherbacks have a beak with a special notch to grab and pierce soft jellyfish.

 

Green sea turtles have a very finely serrated beak that enables them to cut sea grasses and seaweed for food. The turtle’s tongue is a large broad muscle anchored securely in the turtle’s mouth and cannot be extended. The turtle swallows food whole or in large chunks without chewing.  They will also use their beaks to test and explore new objects. Food is located by means of an acute sense of smell. Sea turtles detect scents in the water by opening their mouths slightly and drawing in water through the nares. The water is not swallowed but is immediately expelled through the mouth. A sea turtle’s sense of smell is so acute that some scientists believe it might help an adult turtle locate the beach where it hatched.

 

Sea turtles do not have externally visible ears, but they can hear low frequency sounds and vibrations underwater. Sea turtle eyes are well developed to see underwater, but they only provide nearsighted vision in the air. Large eyelids protect a sea turtle’s eyes from predators.

 

 

                  Survival at Sea: Sea Turtle Adaptations

 

Sea turtles have many adaptations that enable them to live in the seas throughout the world. As seaworthy as they are, sea turtles still have a strong tie to land and coastal habitats. 

 

 

 

 

 

 

 

 

Sea turtle bodies are hydrodynamic, meaning that they can move swiftly through the water with minimal drag or resistance. Contributing to their streamlined design is the compact, flattened shell that they cannot retract into for safety. They also have flippers, which act like big paddles or oars, enabling the turtles to move through the water more like fish rather than lumbering reptiles.

 

Turtles use camouflage to hide from predators and avoid detection by prey. To help blend in with the ocean bottom, a sea turtle’s carapace is usually dark or a combination of colors. The plastron is a lighter color, from a creamy white to a yellowish brown. This prevents them from being seen by prey living on the bottom looking skyward. This type of camouflage is called countershading.

 

Sea turtles are ectothermic or “cold blooded,” which means that they cannot regulate their internal body temperature. Their metabolism is dependent on the surrounding environment. Being ectothermic enables them to lower their metabolic rate, so they need less oxygen and can stay submerged for longer periods of time. Being dependent on the temperature of the environment has its disadvantages. Most sea turtles are limited in range to tropical and temperate waters. If they reach cold waters, they can become cold-stunned and will become stranded or die. Leatherbacks can venture farther and longer into colder waters, as their large size compared to their body surface area allows for a more efficient storage of heat. A layer of fat under the carapace insulates the turtle’s insides from the cool water, and the oils in its body store heat. Diving deeper than other sea turtles, Leatherbacks reach depths of over 3,000 feet as they hunt for jellyfish. They have a higher number of red blood cells and higher concentrations of myoglobin in their muscles, enabling them to carry more oxygen and stay submerged for longer periods of time.

 

Despite their adaptations for life at sea, sea turtles must breathe air with lungs, like humans do.

They can hold their breath for 30 minutes or several hours while in a resting state, but they must eventually surface to take a breath of air. Because they do not need a constant supply of oxygen-rich water in order to respire, sea turtles can return to land, although this usually only happens when females are nesting. Female sea turtles come ashore to lay their eggs in nest cavities they dig in the sand. It is here, on land, where all sea turtles begin life before crawling to the water.

 

Every living organism needs water to live. If we were to drink seawater and not freshwater, it would dehydrate us and lead to death.  Sea turtles do not have this problem. They obtain water from their diet, and they can also metabolize saltwater. Specialized glands in their bodies concentrate and excrete the salt and retain the water. Folklore describes a sea turtle mother crying while she lays her eggs. Though nesting females look very much as though they are shedding tears, the drops are actually salt secretions from glands near their eyes. This adaptation also helps keep sand out of their eyes as they nest.

 

Identifying Sea Turtles