Marine turtles are turtles that inhabit all of the world's oceans except the Arctic. Most species of marine turtle are endangered. Marine turtles, along with other turtles and tortoises, are part of the order Testudines.

All species except the leatherback are in the family Cheloniidae. The leatherback belongs to the family Dermochelyidae and is its only member. Marine turtles constitute a single radiation that became distinct from all other turtles at least 110 million years ago.

Family Cheloniidae
        Chelonia mydas, green turtle
        Eretmochelys imbricata, hawksbill turtle
        Natator depressus, flatback
        Caretta caretta, loggerhead
        Lepidochelys kempii, Kemp's ridley
        Lepidochelys olivacea, olive ridley
Family Dermochelyidae
        Dermochelys coriacea, leatherback

The species are primarily distinguished by their anatomy: for instance, the prefrontal scales on the head, the number of and shape of scutes on the carapace, and the type of inframarginal scutes on the plastron. The leatherback is the only marine turtle that does not have a hard shell; instead, it bears a mosaic of bony plates beneath its leathery skin. It is the largest marine turtle, measuring 1.8 to 2.1 m in length at maturity, and 0.91 to 1.5 m in width, weighing up to 590 kg. Other species are smaller, being mostly 0.61 to 1.2 m and proportionally narrower.

Distribution

The superfamily Chelonioidea has a world-wide distribution; marine turtles can be found in all oceans except for the polar regions. Some species travel between oceans. The flatback marine turtle is found solely on the northern coast of Australia.

Behaviour and ecology

Habitat

Marine turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore again. During the first three to five years life marine turtles spend most time in the pelagic zone floating in seaweed beds. Green marine turtles in particular are often found in Sargassum beds, a brown seaweed in which they find shelter and food. Once the marine turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season.

Respiration

Marine turtles are almost always submerged, and, therefore, have developed an anaerobic system of energy metabolism. Although all marine turtles breathe air, under dire circumstances they may divert to anaerobic metabolism for long periods of time. When surfacing to breathe, a marine turtle can quickly refill its lungs with a single explosive exhalation and rapid inhalation. Their large lungs have adapted to permit rapid exchange of oxygen and to avoid trapping gases during deep dives.

Life history

According to SeaWorld Parks & Entertainment, a lifespan of 80 years is feasible for marine turtles. It takes decades for marine turtles to reach sexual maturity. After mating at sea, adult female marine turtles return to land to nest at night. Different species of marine turtles exhibit various levels of philopatry. In the extreme case, females return to the beach where they hatched. This can take place every two to four years in maturity. They make from one to eight nests per season. The mature nesting female hauls herself onto the beach, nearly always at night, and finds suitable sand on which to create a nest. Using her hind flippers, she digs a circular hole 40 to 50 centimetres (16 to 20 in) deep. After the hole is dug, the female then starts filling the nest with a clutch of soft-shelled eggs one by one until she has deposited around 50 to 200 eggs, depending on the species. Some species have been reported to lay 250 eggs, such as the hawksbill. After laying, she re-fills the nest with sand, re-sculpting and smoothing the surface until it is relatively undetectable visually. The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.

The hatchling's gender depends on the sand temperature. Lighter sands maintain higher temperatures, which decreases incubation time and results in more female hatchlings. Incubation takes about two months. The eggs in one nest hatch together over a very short period of time. When ready, hatchlings tear their shells apart with their snout and dig through the sand. Again, this usually takes place at night. Once they reach the surface, they instinctively head towards the sea. If, as happens on rare occasions, hatching takes place during daylight, only a very small proportion of each hatch succeed (usually 1%), because local opportunist predators, such as the common seagull, gorge on the new marine turtles. Thus there is an obvious evolutionary drive to hatch at night, when survival rates on the beach are much higher.

The hatchlings then proceed into the ocean, where a variety of marine predators await them. In 1987, Carr discovered that the young of Chelonia mydas and Caretta caretta spent a great deal of their pelagic lives in floating sargassum beds, where there are thick mats of unanchored seaweed. Within these beds, they found ample shelter and food. In the absence of sargassum beds, marine turtle young feed in the vicinity of upwelling "fronts". In 2007, Reich determined that green marine turtle hatchlings spend the first three to five years of their lives in pelagic waters. In the open ocean, pre-juveniles of this particular species were found to feed on zooplankton and smaller nekton before they are recruited into inshore seagrass meadows as obligate herbivores.

Instead of nesting individually like the other species, Ridley marine turtles come ashore en masse, known as an "arribada" (arrival). With the Kemp's ridley marine turtles this occurs during the day.

Diet

Marine turtles feed on a wide range of animals and plants. They are mostly omnivorous in their adult life, except the green marine turtle which is herbivorous, changing from a carnivorous diet when young. Some species specialise on certain prey; sea sponges are the principal food of hawksbill marine turtles, constituting 70–95% of their diets in the Caribbean. Leatherback turtles feed almost exclusively on jellyfish and help control jellyfish populations.

Salt gland

Marine turtles possess a salt excretory gland at the corner of the eye, in the nostrils, or in the tongue, depending on the species; chelonian salt glands are found in the corner of the eyes in leatherback marine turtles.

Commensalism with barnacles

Barnacles are small, hard shelled crustaceans found attached to multiple different substrates below or just above the ocean. The adult barnacle is a sessile organism, however in its larval stage it is planktonic and can move about the water column. The larval stage chooses where to settle and ultimately the habitat for its full adult life, which is typically between 5 to 10 years. A favorite settlement for barnacle larvae is the shell or skin around the neck of marine turtles. The larvae glue themselves to the chosen spot, a thin layer of flesh is wrapped around them and a shell is secreted. Many species of barnacles can settle on any substrate however some species of barnacles need commensals, which gives the larvae a greater task of finding an organism, which may be scarce at the time. Around 29 species of “turtle barnacles” have been recorded. However it is not solely marine turtles that barnacles can found on, other organisms serve at barnacle’s settlements as well. These organisms include mollusks, whales, decapod crustaceans, manatees or several other related groups to these species.

Marine turtle shells are an ideal habitat for adult barnacles for 3 reasons. Turtles tend to live long lives, around 50 years so barnacles, which can live for up to 10 years, do not have to worry about host death. Secondly, barnacles are suspension feeders. Marine turtles spend most of their lives swimming and following ocean currents, this allows a nearly constant source of food for the barnacles attached. As water runs along the back of the turtle’s shell it passes right over the barnacles, providing an almost constant water flow and influx of food particles to the barnacles. Lastly, the long distances and inter ocean travel these marine turtles swim throughout their lifetime, offers for the perfect mechanism for dispersal of barnacle larvae. Allowing the barnacle species to distribute themselves throughout global waters is a high fitness advantage of this commensalism.

Importance to ecosystems

Marine turtles play key roles in two ecosystem types that are critical to them as well as to humans——oceans and beaches/dunes. In the oceans, marine turtles, especially green marine turtles, are one of very few creatures (manatees are another) that eat the sea grass that grows on the sea floor. Sea grass needs to be constantly cut short to help it grow across the sea floor. Marine turtles act as grazing animals that cut the grass short and help maintain the health of the sea grass beds. Sea grass beds provide breeding and developmental grounds for numerous species of fish, shellfish and crustaceans. Without sea grass beds, many marine species humans harvest would be lost, as would the lower levels of the food chain. The reactions could result in many more marine species eventually becoming endangered or extinct.

Beaches and dunes form a fragile ecosystem that depends on vegetation to protect against erosion. Eggs, hatched or unhatched, and hatchlings that fail to make it into the ocean are nutrient sources for dune vegetation.[citation needed] Marine turtles use beaches and the lower dunes to nest and lay their eggs. Marine turtles lay around 100 eggs in a nest and make between 3 and 7 nests during the summer nesting season. Along a 20-mile stretch of beach on the east coast of Florida marine turtles lay over 150,000 lbs of eggs in the sand. Dune vegetation is able to grow and become stronger with the presence of nutrients from marine turtle eggs, unhatched nests, eggs and trapped hatchlings. As the dune vegetation grows stronger and healthier, the health of the entire beach/dune ecosystem becomes better. Stronger vegetation and root systems help to hold the sand in the dunes and help protect the beach from erosion.

Source: https://en.wikipedia.org/wiki/Sea_turtle