Sea turtles became distinct from all other turtles at least 110 million years ago. After analyzing genomes of green sea turtle, results indicated the close relationship of the turtles to the bird-crocodilian lineage from which they plit 267-248 million years ago (Upper Permian to Triassic Period). All species of turtles belong to the Testudines order. All species except the Leatherback belongs to the Cheloniidae family. Each species is individually distinguishable by the presence or absence of prefrontal scales on the head, the number and shape of scutes (keratin-based segment of the shell) on the carapace, and the type of inframarginal scutes on the plastron.
The Leatherback is the only sea turtle that does not have a hard shell. It is the only living species in the genus Dermochelys. The Leatherback bears a mosaic of bony plates beneath its leathery skin, and does not have a bony carapace. Instead of scutes, this turtle has thick leathery skin with embedded miniscule osteoderms. They are unique developmentally among reptiles in that their scales lack beta keratin. Leatherbacks are the largest sea turtle, with their shells reaching as long as 7 feet and weighing over 1,000 pounds at maturity. They weigh 1.6 oz when freshly hatched.
The Hawksbill is the only extant species in the genus Eretmochelys. There exists an Atlantic and Pacific subspecies. They possess a flattened body shape, protective carapace, and flipper-like arms adapted for swimming in the open ocean. Hawksbill sand tracks are asymmetrical because they crawl on land with an alternating gait. comparatively, Green and Leatherback sea turtles crawl symmetrically. The common name "Hawksbill" comes from their sharp, curving beak. They have a shell that changes color depending on the water temperature. Hawksbill sea turtle flesh can become toxic due to its consumption of venomous cnidarians. Evolutionary adaptation against predators. Even though Hawksbill is spongivorous, scientists believe they evolved from carnivorous ancestors. 
The Green sea turtle is the only species in genus Chelonia, is native to tropical and subtropical seas around the world. Its closest relative is hawksbill turtle, but green turtle’s snout is shorter and beak is unhooked. Front appendages have only a single claw, hawksbill has two claws on each flipper. Sheath of upper jaw possesses a denticulated edge, lower jaw has stronger, serrated and defined denticulation. The Green sea turtle carapace has various color patterns that change over time. The "Green" common name comes from the greenish color of turtles’ fat found in layer between their inner organs and their shell.
Countershading, also known as Thayer’s Law, is a pattern of animal coloration that aids in camouflage. All sea turtles' undersides are lightly colored, and the dorsal surface is dark grey and/or black. Natural selection favored this form of camouflage as it reduces the ease of detection by predators and prey by counterbalancing the effects of self-shadowing. When light falls on an object it makes it appear solid, acting as a visual cue which makes the object easier to detect. Evolutionary theories regarding why countershading evolved include the benefits of self-shadow concealment which results in improved background matching when viewed from below. Countershading is observed in a wide range of marine animals other than sea turtles, like the grey reef shark.
The Green sea turtle adults inhabit shallow lagoons throughout tropical and subtropical oceans worldwide. Two major subpopulations are in the Atlantic Ocean and Eastern Pacific. Hawksbill sea turtles live part of their life in open ocean, but they spend more time in lagoons, rocky substrate areas, and coral reefs of the Indian, Pacific and Atlantic Oceans. Hawksbills forest and nest in mangrove estuaries specifically in the eastern Pacific. This species is most associated with warm tropical waters.
There are small genetic differences between North Carolina’s and Florida’s turtle populations, but each population is genetically distinct with their own set of nesting and feeding grounds. If the babies hatch on a beach in southeastern America, they are capable of swimming into the current that catapults them across the Atlantic Ocean to the Azores and Madera Islands of Portugal. The Gyre migration describes the circular current that takes many years to do a full cycle around the Atlantic Ocean. North Carolina turtles return to the same coast where they hatched.
Behavior and Ecology
Sea turtle species live in similar habitats; however, the time spent in these habitats varies by species. They generally inhabit fairly shallow waters inside reefs, bays, and inlets due to the abundance of marine grass and algae. Nesting beach locations are also important aspects of sea turtle habitat. Nesting requires open beaches with minimal disturbance in order for eggs to incubate and hatch safely.
There are three types of life cycles associated with sea turtles. They are:
- Entirely neritic
- Neritic and oceanic
- Entirely oceanic
A neritic life cycle means the turtle will spend all stages of life from juvenile to adult near the shore in the habitat described above. The Australian Flatback Sea Turtle are the only species that exhibits this lifestyle. Entirely oceanic means that the turtle's entire life is spent in the open ocean except when nesting occurs. This life cycle is specific to Leatherback turtles. With a mixed life cycle, turtles will spend the hatchling and transitional stages near shore. They will then spend their early juvenile stage in the open ocean, returning to the shore for the late juvenile and adult stages. This is the most common life cycle and is seen in Loggerhead, Hawksbill, and Green Sea Turtles.
Nesting seasons vary between species. Occurring nocturnally, nesting will take place at 2, 3, or 4 intervals. Females can lay as many as 9 clutches of 75-200 eggs in one nesting season.  The female will come onto the beach at night and dig a nest for her eggs. She will then lay them and cover them up, at which point she will move back to the ocean. After being laid, eggs incubate from 45-75 days. This incubation time is one of the most vulnerable times in the turtles life cycle because they are susceptible to large fluctuations in temperature as well as many predators who can dig up the eggs and eat them. Temperature ranges not only affect the incubation period of the eggs, it also affects the sex of the hatchlings. Warmer sand temperature produce clutches that are predominantly female. Green sea turtles exhibit high nesting fidelity, and regularly return to the same nesting beaches year after year. 
Most sea turtles have a very simple omnivorous diet, feeding almost exclusively on sea grass and marine algae. Hatchlings will eat small animals as well in order to support the large amount of energy required for their initial phase of life. In addition, Leatherback Sea Turtles get their nutrition from eating jellyfish  Green sea turtles are entirely herbivorous.
Offshore migration is a distinct characteristic of sea turtles that puzzled scientists because of the accuracy with which they returned to the coast that they were born. It was believed that baby turtles passively drifted in the Atlantic Ocean current, but we now know that it is no coincidence. Primitive research documented observational evidence that the babies do not seem to be going around the current every two years because they actively drop out of the current to lounge and eat on certain islands. Turtles use positional information from earth’s magnetic field. Turtles use their resources in a specific sequence of events; from visual cues of light, to wave orientation when in the ocean near the shore, to using magnetic orientation when swimming in the open ocean, they get to where they need to be. If they used local environmental cues on the beach, they would not be able to find the current. Hatchlings use wave cues under many circumstances. When they are tiny and have to swim, they are easily led by waves. They have to swim fast because they are living off of the yolk sac that fuels only three days of swimming. They typically feed on small animals in the seaweed once the energy from the yolk sac is depleted. The hatchlings are under pressure to get away from predators. Birds and fish feed on turtles close to land and the hatchlings have no defense against them.
Turtles swimming into oncoming waves brings them away from land. Hatchlings cannot visibly see a wave train to determine which way the waves are moving. Hatchling sea turtles can detect the direction of wave movement by monitoring the sequence of accelerations that occurs under water as waves propagate. When the wave direction changes, the turtles swim in that direction. The direction of the approaching waves predict the average orientation angle of the turtle. An experiment was conducted where a tank had a wave-making machine with turtles capable of swimming into the waves with the lights off, to eliminate "moon light" as a confounding variable. The forces below the surface that produce the wave moves in a column of circles. The turtle feels the force going up, back, down, and forward. Could a turtle distinguish between the two scenarios? A simulation experiment was done in the air, eliminating any other hydrodynamic factors, and scientists were able to tell visually which way the turtle wanted to swim. Suspended in air without touching anything gives the turtles the cue that it must be time to swim, but they will crawl when touching the ground/sand. Turtles possess another behavioral fixed action pattern to swim and breathe periodically. When the hatchling’s left flipper is out, they want to turn left. If it does not want to turn, the turtle tucks both flippers behind it. If waves were coming on their left side, they would try to turn left to face the wave. If the wave was coming on their right side, they would try to turn right.
Under non experimental conditions, turtles pay attention to moonlight while crawling on the beach. Scientists thought that perhaps turtles in nature use light to set their magnetic compasses for the correct course. An experiment created an artificial magnetic field and changed the earth’s magnetic field to see if the turtles pay attention to the switch. A light bulb was used to artificially encourage swimming towards the "moon." The experimenters created a geomagnetic field with light in the east, then the light was turned off, and the turtle adjusted to the geomagnetic field and to the reversed field when the artificial field was turned on. Turtles swim in circles while looking for the appropriate direction to go. Earth’s force field in the north cued turtles to go northeast. The reverse, synthetic magnetic field was going south and the turtles went southwest, ignoring magnetic north. How do they know how to use the magnetic compass to go east? They were oriented to swim east toward the light and become accustomed to that direction of the magnetic field. The artificial beach crawled down a runway to the light and physically picked up and moved in orientation. When crawling east with light, they swam northeast. If they crawled towards west with light, they swam west. If they crawled east with no light, their swimming direction had no significant orientation.
During the hatchling life stage, they crawl to the sea and start the offshore migration off of the Florida coast and head east to the Gulf Stream. Pelagic Juveniles or Post Hatchlings have many years in the North Atlantic Gyre. Coastal Juveniles establish coastal feeding sites, and are able to come back to them when they are displaced, and some participate in seasonal migrations between summer and winter sites. Inclination angles increase as you move from equator to the pole, and can realize how north or south you are related to the hemispheres. An experiment was done to test whether a turtle can detect the inclination angle by seeing if it has a compass sense. Isoclinics are lines of equal magnetic inclination, and the angle gets less farther south and higher farther north. 0 runs through South America and Africa, 60 in North Carolina, and -85 in Antarctica. With a 60 degree inclination angle, turtles went mostly oriented themselves south. This observation coincides with the current system of the ocean. Any turtle that does not stay in the southern part of the current risks getting picked up by the northern current heading towards England and Ireland where it is too cold for them to survive. With a 30 degree inclination angle, turtles swam northeast. This translates to a westbound current, with the turtles swimming North. Navigation by juvenile turtles led scientists to hypothesize the possibility of a magnetic map. Older adults are capable of navigating to specific locations such as feeding sites and nesting areas, but juveniles are not specific when navigating. Scientists concluded that they detect the magnetic field via a geomagnetic positional system (GPS).
Relationships with Humans
|Endangered in Mexico, threatened in all other locations
|Endangered in FL and Mexico, threatened in all other locations
Importance in Ecosystems
Sea turtles are vital to maintaining healthy ecosystems especially in sea grass beds and coral reefs. The increase the productivity and nutrient of sea grass blades when grazing. They also act to decrease the supply of nitrogen. Hawksbill turtles play a special role in managing coral reef diversity by limiting the growth of sponges.
Sea turtles are also important for managing marine food webs. They carry barnacles, algae, and epibionts which provide for food for many marine animals. Some species of fish have diets consisting only of epibionts from turtles. Finally, turtles facilitate the cycling of nutrients through ecosystems. The help to speed up disintegration of shelled marine life and also affect the aeration, compaction, and nutrient distribution of sediment.
Historically, turtles have been desired for both their flesh and their shells.  Turtles now face a host of other threats to their survival. Predation is the single natural threat to turtles. Nests can be attacked by raccoons, ants, and crabs which will eat the eggs before they are even able to hatch. Hatchlings can be snatched up by birds and crabs while making their way from the nest to the ocean. Adult turtles have very few natural predators except for an occasional shark attack.
Fishing methods are a major threat to turtle populations, especially those that spend a lot of time in the open ocean or around fisheries. Turtles end up in nets as bycatch and are simply left to die. North Carolina is the only state in the U.S. that has a federal Incidental Take Permit that allows for the killing of these endangered sea turtles in gill nets.  Fishing methods such as long-lining and shrimp trawling are especially harmful to turtles. Oil spills and marine debris also pose a serious threat to the health and safety of turtles. Fibropapillomatosis is a tumor causing disease that can inhibit the turtle’s ability to swim, eat, and potentially cause other dangerous health problems, and it is linked to environmentally disturbed ocean habitats. Most of these habitats have heavy pollution and marine debris due to high human population density.
Development of beaches has a harmful effect on nesting areas as well as a hatchling's ability to find the ocean. Due to increasing development the beach space available for turtle nesting has been steadily decreasing. Since turtles are habitual in their nesting practices, removing nesting locations is detrimental to reproduction. Lighting from large developments also discourages females from nesting on developed beaches, and can make it difficult for a hatchling to find its way to the ocean, since they follow the bright light of the moon. Erosion and armoring, intentional building up of beaches, have also affected the available space for nesting. Use of turtle nesting habitat for leisure has also significantly decreased, and unfortunately, this discourages females from using that area for a nest. Leisure activities include beach driving, furniture on beaches 
Climate change is also threatening the survival of sea turtles. Changes in sea level lead to less beach area for nesting which in turn leads to nest crowding. Turtles have started nesting farther north due to temperature changes which means that areas currently protected for nesting may not overlap with these new nesting locations. 
New research also shows that sea turtles are likely exposed to potentially harmful levels of chemical compounds in the sea water and in their food sources. Some of the compounds include prescription medications, lubricants, adhesive compounds, heavy metals, herbicides, and pesticides. 
As shown above, turtles are affected by a variety of threats. Conservation of sea turtles is extremely important as they are vital to the ecosystems of which they are a part. However, it is hard to monitor the effectiveness of conservation efforts because only nests are being observed. There is no idea of whole population status due to wide range and limited ability to track populations aside from nesting.
The US Endangered Species Act prohibits hunting of sea turtles and reduces incidental losses from shrimp trawling and development.  Three of the major threats to turtles, hunting, trawling, and development are addressed by this act. In terms of threats due to fishing activities, Turtle Excluder Devices (TEDs) have become extremely valuable to saving turtles who may be stuck in nets as bycatch. TEDs are a grid of bars with an opening at either end of the net. When the net catches larger animals it ejects them through the opening while still catching the smaller animals ie: shrimp.  Click here to see a video from National Geographic of Turtle Excluder Devices in action.
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