Sea turtle

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 (4.6 to 5.9 ft) in length and weighing between 300 and 640 kg (661 to 1,411 lbs). Other sea turtle species are smaller, ranging from as little as 60 cm (2 ft) long in the case of the Kemp's ridley, which is the smallest sea turtle species, to 120 cm (3.9 ft) 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 (anapsids), 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: (''''). The leatherback sea turtle is the only extant member of the family Dermochelyidae. Fossil evidence of marine turtles goes back to the Late Jurassic (150 million years ago) 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 (side-necked) 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 (Chelydridae), musk turtles (Kinosternidae), and hickatee (Dermatemyidae) of the Americas, which alongside the sea turtles constitute the clade Americhelydia. The oldest possible representative of the lineage (Panchelonioidea) 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 (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. 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. 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. (2019): An alternate phylogeny was proposed by Castillo-Visa et al. (2022): }}|style=font-size:85%;line-height:85%|label1=Panchelonioidea}} == 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. 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. 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. , 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. 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 (arrival). 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. 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. == Physiology ==

Osmoregulation Sea turtles maintain an internal environment that is hypotonic to the ocean. To maintain hypotonicity they must excrete excess salt ions. Like other marine reptiles, sea turtles rely on a specialized gland to rid the body of excess salt, because reptilian kidneys cannot produce urine with a higher ion concentration than sea water. All species of sea turtles have a lachrymal gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water. 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. in the Northwestern Hawaiian Islands. 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. However, leatherback sea turtles (family Dermochelyidae) are able to maintain a body temperature warmer than the ambient water by thermoregulation through the trait of gigantothermy. 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. This behavior has only been observed in a few locations, including the Galapagos, Hawaii, Europa Island, and parts of Australia. while a sleeping sea turtle can remain underwater for 4–7 hours. Remarkably, sea turtle respiration remains aerobic for the vast majority of voluntary dive time. Fluorescence Gruber and Sparks (2015) have observed the first fluorescence in a marine tetrapod (four-limbed vertebrates). Sea turtles are the first biofluorescent reptile found in the wild. According to Gruber and Sparks (2015), fluorescence is observed in an increasing number of marine creatures (cnidarians, ctenophores, annelids, arthropods, and chordates) and is now also considered to be widespread in cartilaginous and ray-finned fishes. Sensory modalities Navigation Below the surface, the sensory cues available for navigation change dramatically. Light availability decreases quickly with depth, and is refracted by the movement of water when present, celestial cues are often obscured, and ocean currents cause continuous drift. Passive drifting within major current systems, such as those in the North Atlantic Gyre, can result in ejection well outside of the temperature tolerance range of a given species, causing heat stress, hypothermia, or death. Specific migratory routes have been shown to vary between individuals, making the possession of both a magnetic map and compass sense advantageous for sea turtles. A magnetic compass sense allows sea turtles to determine and maintain a specific magnetic heading or orientation. Natal homing behavior is well described in sea turtles, and genetic testing of turtle populations at different nesting sites has shown that magnetic field is a more reliable indicator of genetic similarity than physical distance between sites. Additionally, nesting sites have been recorded to "drift" along with isoline shifts in the magnetic field. Magnetoreception is thought to be the primary navigation tool used by nesting sea turtles in returning to natal beaches. There are three major theories explaining natal site learning: inherited magnetic information, socially facilitated migration, and geomagnetic imprinting. Some support has been found for geomagnetic imprinting, including successful experiments transplanting populations of sea turtles by relocating them prior to hatching, but the exact mechanism is still not known. == Ecology ==

Diet The loggerhead, Kemp's ridley, olive ridley, and hawksbill sea turtles are omnivorous their entire life. Omnivorous turtles may eat a wide variety of plant and animal life including decapods, seagrasses, seaweed, sponges, mollusks, cnidarians, Echinoderms, worms and fish. However, some species specialize on certain prey. The diet of green sea turtles changes with age. Juveniles are omnivorous, but as they mature they become exclusively herbivorous. Green sea turtles have a serrated jaw that is used to eat sea grass and algae. Leatherback sea turtles feed almost exclusively on jellyfish and help control jellyfish populations. Hawksbill sea turtles principally eat sponges, which constitute 70–95% of their diets in the Caribbean. Loggerhead turtles are regarded as flexible and predators of slow-moving animals. They eat a broad variety of things, including terrestrial insects like ants, planthoppers, and beetles, as well as sea animals and plants. This species' primary diet consists of gelatinous creatures (medusae and ctenophores) and crustaceans, particularly crabs. Sargassum, barnacles, gastropods, anemones, salps, and pelagic coelenterates have also been found in numerous studies to be loggerhead turtles' primary food sources. Larynx mechanisms There was little information regarding the sea turtle's larynx. Sea turtles, like other turtle species, lack an epiglottis to cover the larynx entrance. Key findings from an experiment reveal the following in regards to the larynx morphology: a close apposition between the linguolaryngeal cleft's smooth mucosal walls and the laryngeal folds, a dorsal part of the glottis, the glottal mucosa attached to the arytenoid cartilage, and the way the hyoid sling is arranged and the relationship between the compressor laryngis muscle and cricoid cartilage. The glottal opening and closing mechanisms have been examined. During the opening stage, two abductor artytenoideae muscles swing arytenoid cartilages and the glottis walls. As a result, the glottis profile is transformed from a slit to a triangle. In the closing stage, the tongue is drawn posteriorly due to the close apposition of the glottis walls and linguolaryngeal cleft walls and hyoglossal sling contractions. Relationship with humans Sea turtles are caught worldwide, although it is illegal to hunt most species in many countries. A great deal of intentional sea turtle harvests worldwide are for food. Many parts of the world have long considered sea turtles to be fine dining. In England during the 1700s, sea turtles were consumed as a delicacy to near extinction, often as turtle soup. Ancient Chinese texts dating to the 5th century B.C.E. describe sea turtles as exotic delicacies. Many coastal communities around the world depend on sea turtles as a source of protein, often harvesting several sea turtles at once and keeping them alive on their backs until needed. Coastal peoples gather sea turtle eggs for consumption. , Australia. From Phillip Parker King's Survey. 1818. To a much lesser extent, some species are targeted for their shells. Tortoiseshell, a traditional decorative ornamental material used in Japan and China, comes from the carapace scutes of the hawksbill sea turtle. Ancient Greeks and ancient Romans processed sea turtle scutes (primarily from the hawksbill sea turtle) for various articles and ornaments used by their elites, such as combs and brushes. The skin of the flippers is prized for use as shoes and assorted leather goods. In various West African countries, sea turtles are harvested for traditional medicinal use. The Moche people of ancient Peru worshipped the sea and its animals. They often depicted sea turtles in their art. J. R. R. Tolkien's poem "Fastitocalon" echoes a second-century Latin tale in the Physiologus of the Aspidochelone ("round-shielded turtle"); it is so large that sailors mistakenly land and light a fire on its back, and are drowned when it dives. Beach towns, such as Tortuguero, Costa Rica, have transitioned from a tourism industry that made profits from selling sea turtle meat and shells to an ecotourism-based economy. Tortuguero is considered to be the founding location of sea turtle conservation. In the 1960s the cultural demand for sea turtle meat, shells, and eggs was quickly killing the once-abundant sea turtle populations that nested on the beach. The Caribbean Conservation Corporation began working with villagers to promote ecotourism as a permanent substitute to sea turtle hunting. Sea turtle nesting grounds became sustainable. Tourists love to come and visit the nesting grounds, although it causes a lot of stress to the sea turtles because all of the eggs can get damaged or harmed. Since the creation of a sea turtle ecotourism-based economy, Tortuguero annually houses thousands of tourists who visit the protected beach that hosts sea turtle walks and nesting grounds. Walks to observe the nesting sea turtles require a certified guide and this controls and minimizes disturbance of the beaches. It also gives the locals a financial interest in conservation and the guides now defend the sea turtles from threats such as poaching; efforts in Costa Rica's Pacific Coast are facilitated by a nonprofit organization, Sea Turtles Forever. Thousands of people are involved in sea turtle walks, and substantial revenues accrue from the fees paid for the privilege. In other parts of the world where sea turtle breeding sites are threatened by human activity, volunteers often patrol beaches as a part of conservation activities, which may include relocating sea turtle eggs to hatcheries, or assisting hatching sea turtles in reaching the ocean. Locations in which such efforts exist include the east coast of India, São Tomé and Príncipe, Sham Wan in Hong Kong, and the coast of Florida. Importance to ecosystems Sea turtles play key roles in two habitat types: oceans and beaches/dunes. In the oceans, sea turtles, especially green sea turtles, are among the very few creatures (manatees are another) that eat sea grass. Sea grass needs to be constantly cut short to help it grow across the sea floor. Sea turtle grazing helps maintain the health of the sea grass beds. Sea grass beds provide breeding and developmental grounds for numerous marine animals. Without them, 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. Sea turtles use beaches and sand dunes as to lay their eggs. Such coastal environments are nutrient-poor and depend 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 and therefore protecting these nesting habitats for sea turtles, forming a positive feedback loop. Sea turtles also maintain a symbiotic relationship with yellow tang, in which the fish will eat algae growing on the shell of a sea turtle. Conservation status and threats The IUCN Red List classifies two species of sea turtle as "critically endangered" and several others at lower levels of threat. An additional three species are classified as "vulnerable". Conservation efforts attempt to address the multiple threats to their populations, but evaluation of the success of these efforts has been limited. Counting nests on beaches not provide an accurate picture of whole populations. More detailed information on birth rates and mortality is needed. The presence of humans has increased the threats. Death by bycatch due to imprecise fishing methods is a major threat; long-lining is a major cause of accidental sea turtle deaths. Light pollution from beach development is a threat to newly-hatched sea turtles. Another major threat is the black-market trade in eggs and meat. Another danger to sea turtles comes from marine debris, especially plastics that they can mistake for jellyfish. Climate change may pose a threat, since rising temperatures may change the sex ratio, resulting in too many females. Oil pollution can affect turtles at every stage of their life cycle. Symbiosis with barnacles Sea turtles are believed to have a commensal relationship with some barnacles, in which the barnacles benefit from growing on sea turtles without harming them. 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 and 10 years. However, estimates of age for a common sea turtle barnacle species, Chelonibia testudinaria, suggest that this species lives for at least 21 months, with individuals older than this uncommon. Chelonibia barnacles have also been used to distinguish between the foraging areas of sea turtle hosts. By analyzing stable isotope ratios in barnacle shell material, scientist can identify differences in the water (temperature and salinity) that different hosts have been swimming through and thus differentiate between the home areas of host sea turtles. A favorite settlement for barnacle larvae is the shell or skin around the neck of sea 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 have an obligatory commensal relationship with specific animals, which makes finding a suitable location harder. Around 29 species of "turtle barnacles" have been recorded. However, it is not solely on sea turtles that barnacles can be found; other organisms also serve as a barnacle's settlements. These organisms include mollusks, whales, decapod crustaceans, manatees and several other groups related to these species. Sea turtle shells are an ideal habitat for adult barnacles for three reasons. Sea turtles tend to live long lives, greater than 70 years, so barnacles do not have to worry about host death. However, mortality in sea turtle barnacles is often driven by their host shedding the scutes on which the barnacle is attached, rather than the death of the sea turtle itself. This relationship, however, is not truly commensal. While the barnacles are not directly parasitic to their hosts, they have negative effects to the sea turtles on which they choose to reside. The barnacles add extra weight and drag to the sea turtle, increasing the energy it needs for swimming and affecting its ability to capture prey, with the effect increasing with the quantity of barnacles affixed to its back. ==See also==