Sea turtle
Sea turtles, sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, Kemp's ridley, and olive ridley. Five of the seven species are listed as threatened with extinction globally on the IUCN Red List of Threatened Species. The remaining two are not considered to be threatened with extinction, one of which, the flatback turtle, is found only in the waters of Australia, Papua New Guinea, and Indonesia.
Sea turtles can be categorized as hard-shelled or leathery-shelled. The only dermochelyid species of sea turtle is the leatherback.
Description
For each of the seven species of sea turtles, females and males are the same size. As adults, it is possible to tell male turtles from female turtles by their long tails with a cloacal opening near the tip. Adult female sea turtles have shorter tails, with a cloacal opening near the base. Hatchling and sub-adult turtles do not exhibit sexual dimorphism; it is not possible to determine their sex by looking at them.In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. This tapering at both ends reduces volume and means that sea turtles cannot retract their head and limbs into their shells for protection, unlike many other turtles and tortoises. However, the streamlined body plan reduces friction and drag in the water and allows sea turtles to swim more easily and swiftly.
The leatherback sea turtle is the largest sea turtle, reaching 1.4 to more than 1.8 m in length and weighing between 300 and 640 kg. Other sea turtle species are smaller, ranging from as little as 60 cm long in the case of the Kemp's ridley, which is the smallest sea turtle species, to 120 cm long in the case of the green turtle, the second largest.
The skulls of sea turtles have cheek regions that are enclosed in bone. Although this condition appears to resemble that found in the earliest known fossil reptiles, it is possible it is a more recently evolved trait in sea turtles, placing them outside the anapsids.
Taxonomy and evolution
Sea turtles, along with other turtles and tortoises, are part of the order Testudines. All species except the leatherback sea turtle are in the family Cheloniidae. The superfamily name Chelonioidea and family name Cheloniidae are based on the Ancient Greek word for tortoise: χελώνη. The leatherback sea turtle is the only extant member of the family Dermochelyidae.Fossil evidence of marine turtles goes back to the Late Jurassic with genera such as Plesiochelys, from Europe. In Africa, the first marine turtle is Angolachelys, from the Turonian of Angola. A lineage of unrelated marine testudines, the pleurodire bothremydids, also survived well into the Cenozoic. Other pleurodires are also thought to have lived at sea, such as Araripemys and extinct pelomedusids. Modern sea turtles are not descended from more than one of the groups of sea-going turtles that have existed in the past; they instead constitute a single radiation that became distinct from all other turtles at least 110 million years ago. Their closest extant relatives are in fact the snapping turtles, musk turtles, and hickatee of the Americas, which alongside the sea turtles constitute the clade Americhelydia.
The oldest possible representative of the lineage leading to modern sea turtles was possibly Desmatochelys padillai from the Early Cretaceous. Desmatochelys was a protostegid, a lineage that would later give rise to some very large species but went extinct at the end of the Cretaceous. Presently thought to be outside the crown group that contains modern sea turtles, the exact relationships of protostegids to modern sea turtles are still debated due to their primitive morphology; they may be the sister group to the Chelonoidea, or an unrelated turtle lineage that convergently evolved similar adaptations. The earliest "true" sea turtle that is known from fossils is Nichollsemys from the Early Cretaceous of Canada. In 2022, the giant fossil species Leviathanochelys was described from Spain. This species inhabited the oceans covering Europe in the Late Cretaceous and rivaled the concurrent giant protostegids such as Archelon and Protostega as one of the largest turtles to ever exist. Unlike the protostegids, which have an uncertain relationship to modern sea turtles, Leviathanochelys is thought to be a true sea turtle of the superfamily Chelonioidea.
Sea turtles' limbs and brains have evolved to adapt to their diets. Their limbs originally evolved for locomotion, but more recently evolved to aid them in feeding. They use their limbs to hold, swipe, and forage their food. This helps them eat more efficiently.
Phylogeny
The phylogenetic relationships of living and extinct sea turtles in the Chelonioidea are based on Evers et al. :An alternate phylogeny was proposed by Castillo-Visa et al. :
Distribution and habitat
Sea turtles can be found in all oceans except for the polar regions. The flatback sea turtle is found solely on the northern coast of Australia. The Kemp's ridley sea turtle is found solely in the Gulf of Mexico and along the East Coast of the United States.Sea turtles are generally found in the waters over continental shelves. During the first three to five years of life, sea turtles spend most of their time in the pelagic zone floating in seaweed mats. Green sea turtles in particular are often found in Sargassum mats, in which they find food, shelter and water. Once the sea 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.
Sea turtles migrate to reach their spawning beaches, which are limited in numbers. Living in the ocean therefore means they usually migrate over large distances. All sea turtles have large body sizes, which is helpful for moving large distances. Large body sizes also offer good protection against the large predators found in the ocean.
In 2020, diminished human activity resulting from the COVID-19 virus caused an increase in sea turtle nesting. Some areas in Thailand saw an abnormally high number of nests, and Florida experienced a similar phenomenon. Less plastic and light pollution could explain these observations.
Life cycle
Sea turtles are thought to reach sexual maturity from about 10−20 years old depending on species and methodology. However, reliable estimates are difficult to ascertain. Mature sea turtles may migrate thousands of miles to reach breeding sites. After mating at sea, adult female sea turtles return to land to lay their eggs. Different species of sea turtles exhibit various levels of philopatry. In the extreme case, females return to the same beach where they hatched. This can take place every two to four years in maturity.File:Turtle golfina escobilla Oaxaca Mexico Claudio Giovenzana 2010.jpg|thumbnail|left|An olive ridley sea turtle nesting on Escobilla Beach, Oaxaca, MexicoThe mature nesting female hauls herself onto the beach, nearly always at night, and finds suitable sand in which to create a nest. Using her hind flippers, she digs a circular hole deep. After the hole is dug, the female then starts filling the nest with her clutch of soft-shelled eggs. Depending on the species, a typical clutch may contain 50–350 eggs. After laying, she re-fills the nest with sand, re-sculpting and smoothing the surface, and then camouflaging the nest with vegetation until it is relatively undetectable visually. She may also dig decoy nests. The whole process takes 30 to 60 minutes. She then returns to the ocean, leaving the eggs untended.
Females may lay 1–8 clutches in a single season. Female sea turtles alternate between mating in the water and laying their eggs on land. Most sea turtle species nest individually. But ridley sea turtles come ashore en masse, known as an arribada. With the Kemp's ridley sea turtle this occurs during the day.
Sea turtles have temperature-dependent sex determination. Warmer temperatures produce female hatchlings, while cooler temperatures produce male hatchlings. The eggs will incubate for 50–60 days. The eggs in one nest hatch together over a short period of time. The baby sea turtles break free of the egg shell, dig through the sand, and crawl into the sea. Most species of sea turtles hatch at night. However, the Kemp's ridley sea turtle commonly hatches during the day. Sea turtle nests that hatch during the day are more vulnerable to predators, and may encounter more human activity on the beach.
Image:YosriTelurPenyu6.jpg|thumb|Sea turtle sex depends on sand temperature while the egg is incubating.
Larger hatchlings have a higher probability of survival than smaller individuals, which can be explained by the fact that larger offspring are faster and thus less exposed to predation. Predators can only functionally intake so much; larger individuals are not targeted as often. A study conducted on this topic shows that body size is positively correlated with speed, so larger baby sea turtles are exposed to predators for a shorter amount of time. The fact that there is size dependent predation on chelonians has led to the evolutionary development of large body sizes.
In 1987, Carr discovered that the young of green and loggerhead sea turtles spent a great deal of their pelagic lives in floating sargassum mats. Within these mats, they found ample shelter and food. In the absence of sargassum, young sea turtles feed in the vicinity of upwelling "fronts". In 2007, Reich determined that green sea 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.