Sea turtles

Sea turtles (superfamily Chelonioidea), 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 sea turtles.
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. The seventh sea turtle species is the flatback, which exists in the waters of Australia, Papua New Guinea and Indonesia. Sea turtles can be separated into the categories of hard-shelled (cheloniid) and leathery-shelled ( dermochelyid).Wyneken, J. 2001. The Anatomy of Sea Turtles. U.S Department of Commerce NOAA Technical Memorandum NMFS-SEFSC-470, 1-172 pp. There is only one dermochelyid species which is the leatherback sea turtle.

Description

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

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, 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.

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.

Cladogram

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

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

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 (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

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.

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 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'' (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. 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.

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.

Thermoregulation

All sea turtles are poikilotherms. 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.

Diving physiology

Sea turtles are air-breathing reptiles that have lungs, so they regularly surface to breathe. Sea turtles spend a majority of their time underwater, so they must be able to hold their breath for long periods. Dive duration largely depends on activity. A foraging sea turtle may typically spend 5–40 minutes underwater 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. When a sea turtle is forcibly submerged (e.g. entangled in a trawl net) its diving endurance is substantially reduced, so it is more susceptible to drowning.

When surfacing to breathe, a sea turtle can quickly refill its lungs with a single explosive exhalation and rapid inhalation. Their large lungs permit rapid exchange of oxygen and avoid trapping gases during deep dives.

Cold-stunning is a phenomenon that occurs when sea turtles enter cold ocean water (), which causes the turtles to float to the surface and therefore makes it impossible for them to swim.

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.

The two marine biologists accidentally made the observation in the Solomon Islands on a hawksbill sea turtle, one of the rarest and most endangered sea turtle species in the ocean, during a night dive aimed to film the biofluorescence emitted by small sharks and coral reefs. The role of biofluorescence in marine organisms is often attributed to a strategy for attracting prey or perhaps a way to communicate. It could also serve as a way of defense or camouflage for the sea turtle hiding during night amongst other fluorescent organisms like corals. Fluorescent corals and sea creatures are best observed during night dives with a blue LED light and with a camera equipped with an orange optical filter to capture only the fluorescence light.

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. Most sea turtle species migrate over significant distances to nesting or foraging grounds, some even crossing entire ocean basins. 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. In order to reliably navigate within strong gyre currents in the open ocean, migrating sea turtles possess both a bicoordinate magnetic map and magnetic compass sense, using a form of navigation termed Magnetoreception. 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 bicoordinate magnetic map gives sea turtles the ability to determine their position relative to a goal with both latitudinal and longitudinal information, and requires the detection and interpretation of more than one magnetic parameter going in opposite directions to generate, such as Magnetic field intensity and Inclination angle. A magnetic compass sense allows sea turtles to determine and maintain a specific magnetic heading or orientation. These magnetic senses are thought to be inherited, as hatchling sea turtles swim in directions that would keep them on course when exposed to the magnetic field signatures of various locations along their species' migratory routes.

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. This diet shift has an effect on the green sea turtle's morphology. 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.

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.

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.J. R. R. Tolkien, ''The Letters of J.R.R. Tolkien'', #255 to Mrs Eileen Elgar, 5 March 1964; Humphrey Carpenter and Christopher Tolkien, eds. (Allen & Unwin, 1981; )

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, Tortugero 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 minimises 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.Why Care About Sea Turtles?
Sea Turtle Conservancy.

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 three species of sea turtle as either "endangered" or "critically endangered". An additional three species are classified as "vulnerable". The flatback sea turtle is considered as "data deficient", meaning that its conservation status is unclear due to lack of data. All species of sea turtle are listed in CITES Appendix I, restricting international trade of sea turtles and sea turtle products. However, the usefulness of global assessments for sea turtles has been questioned, particularly due to the presence of distinct genetic stocks and spatially separated regional management units (RMUs). Each RMU is subject to a unique set of threats that generally cross jurisdictional boundaries, resulting in some sub-populations of the same species' showing recovery while others continue to decline. This has triggered the IUCN to conduct threat assessments at the sub-population level for some species recently. These new assessments have highlighted an unexpected mismatch between where conservation relevant science has been conducted on sea turtles, and where there is the greatest need for conservation. For example, as at August 2017, about 69% of studies using stable isotope analysis to understand the foraging distribution of sea turtles have been conducted in RMUs listed as "least concern" by the IUCN.
Additionally, all populations of sea turtles that occur in United States waters are listed as threatened or endangered by the US Endangered Species Act (ESA). The US listing status of the loggerhead sea turtle is under review as of 2012.

^*The ESA manages sea turtles by population and not by species.

Management

In the Caribbean, researchers are having some success in assisting a comeback. In September 2007, Corpus Christi, Texas, wildlife officials found 128 Kemp's ridley sea turtle nests on Texas beaches, a record number, including 81 on North Padre Island (Padre Island National Seashore) and four on Mustang Island. Wildlife officials released 10,594 Kemp's ridley sea turtle hatchlings along the Texas coast in recent years.

The Philippines has had several initiatives dealing with the issue of sea turtle conservation. In 2007, the province of Batangas declared the catching and eating of sea turtles (locally referred to as Pawikans) illegal. However, the law seems to have had little effect as sea turtle eggs are still in demand in Batangan markets. In September 2007, several Chinese poachers were apprehended off the Turtle Islands in the country's southernmost province of Tawi-Tawi. The poachers had collected more than a hundred sea turtles, along with 10,000 sea turtle eggs.

Evaluating the progress of conservation programs is difficult, because many sea turtle populations have not been assessed adequately. Most information on sea turtle populations comes from counting nests on beaches, but this does not provide an accurate picture of the whole sea turtle population. A 2010 United States National Research Council report concluded that more detailed information on sea turtles' life cycles, such as birth rates and mortality, is needed.

Nest relocation may not be a useful conservation technique for sea turtles. In one study on the freshwater Arrau turtle (''Podocnemis expansa'') researchers examined the effects of nest relocation. They discovered that clutches of this freshwater turtle that were transplanted to a new location had higher mortality rates and more morphological abnormalities compared to non-transplanted clutches. However, in a study of loggerhead sea turtles (''Caretta caretta''), Dellert et al. found that relocating nests at risk of inundation increased the success of eggs and hatchlings and decreased the risk of inundation.

Predators and disease

Most sea turtle mortality happens early in life. Sea turtles usually lay around 100 eggs at a time, but on average only one of the eggs from the nest will survive to adulthood. Raccoons, foxes, and seabirds may raid nests or hatchlings may be eaten within minutes of hatching as they make their initial run for the ocean."Natural."
Sea Turtle Foundation. Sea Turtle Foundation, 2010. Web. 8 Dec 2010. Once in the water, they are susceptible to seabirds, large fish and even other sea turtles.

Adult sea turtles have few predators. Large aquatic carnivores such as sharks and crocodiles are their biggest threats; however, reports of terrestrial predators attacking nesting females are not uncommon. Jaguars have been reported to smash into sea turtle shells with their paws, and scoop out the flesh.

Fibropapillomatosis disease causes tumors in sea turtles.

While many of the things that endanger sea turtles are natural predators, increasingly many threats to the sea turtle species have arrived with the ever-growing presence of humans.

Bycatch

One of the most significant and contemporary threats to sea turtles comes from bycatch due to imprecise fishing methods. Long-lining

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