Sea turtle

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Sea turtles
Temporal range:
Ma
Chelonia mydas is going for the air edit.jpg
A green sea turtle, a species of the sea turtle superfamily
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Clade: Americhelydia
Clade: Panchelonioidea
Superfamily: Chelonioidea
Bauer, 1893[2]
Families
Synonyms[2]

Chelonii - Oppel, 1811
Chlonopteria - Rafinesque, 1814
Cheloniae - Schmid, 1819
Edigitata - Haworth, 1825
Oiacopodae - Wagler, 1828
Pterodactyli - Mayer, 1849

Sea turtles (superfamily Chelonioidea), sometimes called marine turtles,

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 sea turtles.[4]
All six of the sea turtle species present in US waters (all of those listed above except the flatback) are listed as endangered and/or threatened under the Endangered Species Act.[5] The seventh sea turtle species is the flatback, which exists in the waters of Australia, Papua New Guinea and Indonesia.[5] Sea turtles can be separated into the categories of hard-shelled (cheloniid) and leathery-shelled (dermochelyid).[6] There is only one dermochelyid species which is the leatherback sea turtle.[6]

Description

For each of the seven types of sea turtles, females and males are the same size; there is no sexual dimorphism.[7]

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.[8] 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, measuring 2–3 m (6–9 ft) in length, 1–1.5 m (3–5 ft) in width, and weighing up to 700 kg (1500 lb). Other sea turtle species are smaller, being mostly 60–120 cm (2–4 ft) long and proportionally narrower.[9]

The skulls of sea turtles have cheek regions that are enclosed in bone.[10][11] Although this condition appears to resemble that found in the earliest known fossil reptiles (anapsids), it is possible it is a more recently evolved trait in sea turtles, placing them outside the anapsids.[12][10]

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 (khelone).[13] The leatherback sea turtle is the only extant member of the family Dermochelyidae.

Fossil evidence of marine turtles goes back to the

bothremydids, also survived well into the Cenozoic. Other pleurodires are also thought to have lived at sea, such as Araripemys[15] and extinct pelomedusids.[16] 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.[17][18][19] Their closest extant relatives are in fact the snapping turtles (Chelydridae), musk turtles (Kinosternidae), and hickatee (Dermatemyidae) of the Americas, which alongside the sea turtles constitute the clade Americhelydia.[20]

The oldest possible representative of the lineage (

Desmatochelys padillaifrom 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 (Chelonioidea), 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.[21][22] The earliest "true" sea turtle that is known from fossils is Nichollsemys from the Early Cretaceous (Albian) 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.[23]

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.[24][25]

Cladogram

Below is a cladogram showing the phylogenetic relationships of living and extinct sea turtles in the Chelonioidea based on Evers et al. (2019):[26]

Phylogenetic relations of living and extinct chelonioid species
 Panchelonioidea 

Toxochelys

 †

Protostega gigas.jpg

 
Chelonioidea
 

Corsochelys

Erpétologie générale, ou, Histoire naturelle complète des reptiles (Dermochelys coriacea).jpg

Pancheloniidae

Nichollsemys

Allopleuron

Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelonia mydas).jpg

Argillochelys

Procolpochelys

Eochelone

Puppigerus

Ctenochelys

Peritresius

Cabindachelys

An alternate phylogeny was proposed by Castillo-Visa et al. (2022):[23]

Panchelonioidea

Toxochelyidae

Protostegidae

Corsochelys

Chelonioidea
Dermochelyidae

Eosphargis

Dermochelys

Nichollsemys

Leviathanochelys

Allopleuron

Procolpochelys

Argillochelys

Eochelone

Puppigerus

Ctenochelyidae

Cabindachelys

Ctenochelys

Peritresius

Cheloniidae

Natator

Eretmochelys

Chelonia

Lepidochelys kempii

Caretta

Lepidochelys olivacea

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.[27]

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.[28] Once the sea turtle has reached adulthood it moves closer to the shore.[29] Females will come ashore to lay their eggs on sandy beaches during the nesting season.[30]

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 (notably sharks) found in the ocean.[31]

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.[32]

Life cycle

1) Male and female sea turtles age in the ocean and migrate to shallow coastal water. 2) Sea turtles mate in the water near offshore nesting sites. 3) The adult male sea turtles return to the feeding sites in the water. 4) Female sea turtles cycle between mating and nesting. 5) Female sea turtles lay their eggs. 6) When the season is over, female sea turtles return to feeding sites. 7) Baby sea turtles incubate for 60–80 days and hatch. 8) Newly hatched baby sea turtles emerge from nests and travel from the shore to the water. 9) Baby sea turtles mature in the ocean until they are ready to begin the cycle again.

It takes decades for sea turtles to reach sexual maturity. 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.

An olive ridley sea turtle nesting on Escobilla Beach, Oaxaca, Mexico

The 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 40 to 50 centimetres (16 to 20 in) 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.[28] She may also dig decoy nests.[33] The whole process takes 30 to 60 minutes. She then returns to the ocean, leaving the eggs untended.[34]

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 (arrival). With the Kemp's ridley sea turtle this occurs during the day.

Sea turtles have temperature-dependent sex determination, meaning the developing baby sea turtle's sex depends on the temperature it is exposed to.[35][36][37][38][39] Warmer temperatures produce female hatchlings, while cooler temperatures produce male hatchlings.[35][36][37][38][39][40] 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.

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.

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 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.[29][42]

Physiology

Osmoregulation

Sea turtles maintain an internal environment that is

lachrymal gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water.[45]

Leatherback sea turtles face an increased osmotic challenge compared to other species of sea turtle, since their primary prey are jellyfish and other gelatinous plankton, whose fluids have the same concentration of salts as sea water. The much larger lachrymal gland found in leatherback sea turtles may have evolved to cope with the higher intake of salts from their prey. A constant output of concentrated salty tears may be required to balance the input of salts from regular feeding, even considering leatherback sea turtle tears can have a salt ion concentration almost twice that of other species of sea turtle.

Hatchlings depend on drinking sea water immediately upon entering the ocean to replenish water lost during the hatching process. Salt gland functioning begins quickly after hatching, so that the young sea turtles can establish ion and water balance soon after entering the ocean. Survival and physiological performance hinge on immediate and efficient hydration following emergence from the nest.[44]

Thermoregulation

All sea turtles are poikilotherms.[47] However, leatherback sea turtles (family Dermochelyidae) are able to maintain a body temperature 8 °C (14 °F) warmer than the ambient water by thermoregulation through the trait of gigantothermy.[47][48]

Green sea turtles in the relatively cooler Pacific are known to haul themselves out of the water on remote islands to bask in the sun.