Sea turtle

Sea turtles
Temporal range:
Early Cretaceous-Holocene,[1] 110–0 Ma
An olive ridley sea turtle, a species of the sea turtle superfamily
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Clade: Panchelonioidea
Superfamily: Chelonioidea
Bauer, 1893[2]
Type species
Testudo mydas
Linnaeus, 1758
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,[3] are reptiles of the order Testudines. There are seven extant species of sea turtles: the green, loggerhead, Kemp's ridley, olive ridley, hawksbill, flatback, and leatherback.[4]

Taxonomy and evolution

Sea 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 is the only extant member of the family Dermochelyidae.

In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. The reduced volume of a fusiform body means sea turtles can not retract their head, legs, and arms into their shells, like other turtles can.[5]

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 sea turtle that does not have a hard shell. Instead, it bears a mosaic of bony plates beneath its leathery skin. It is the largest sea turtle, measuring 6 to 9 feet (1.8 to 2.7 m) in length at maturity, and 3 to 5 feet (0.91 to 1.52 m) in width, weighing up to 1,500 pounds (680 kg). Other species are smaller, being mostly 2 to 4 feet (0.61 to 1.22 m) and proportionally narrower.[6]

The origin of Sea Turtles goes back to the Late Jurassic (150 Ma) with genera such as Plesiochelys, from Europe. In Africa, the first marine turtle is Angolachelys, from the Turonian of Angola.[7]

Sea turtles constitute a single radiation that became distinct from all other turtles at least 110 million years ago.

Cladogram

Below is a cladogram showing the phylogenetic relationships of living and extinct sea turtles in the Chelonioidea based on Peer and Lee (2005)[8]

Phylogenetic relations of living and extinct chelonioid species
Panchelonioidea

†Toxochelys

†Ctenochelys


Chelonioidea
Pancheloniidae


†Euclastes

†Puppigerus

Cheloniidae



Pandermochelys

†Protostegidae

Dermochelyidae





Distribution & Habitat

Sea turtles can be found in all oceans except for the polar regions. They mostly can be found in coral reefs. Some species travel between oceans. 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.[9]

Sea turtles are generally found in the waters over continental shelves. During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed mats. Green sea turtles in particular are often found in Sargassum mats, in which they find shelter and food.[10] Once the sea turtle has reached adulthood it moves closer to the shore.[11] Females will come ashore to lay their eggs on sandy beaches during the nesting season.[12]

The habitat of a sea turtle has a significant influence on its morphology. Sea turtles are able to grow so large because of the immense size of their habitat: the ocean. The reason that sea turtles are much bigger than land tortoises and freshwater turtles is directly correlated with the vastness of the ocean, and the fact that they travel such far distances.[13] Having more room to live enables more room for growth.

Life cycle

1) Male and female turtles age in the ocean and migrate to shallow coastal water. 2) Turtles mate in the water near offshore nesting sites. 3) The adult male turtles return to the feeding sites in the water. 4) Female turtles cycle between mating and nesting. 5) Females lay their eggs. 6) When the season is over, female turtles return to feeding sites. 7) Baby turtles incubate for 60-80 days and hatch. 8) Newly hatched turtles emerge from nests and travel from the shore to the water. 9) Baby 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 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 beach where they hatched. This can take place every two to four years in maturity.

An Olive ridley 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.[10] The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.[14]

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 turtles this occurs during the day.

Sea turtles have temperature-dependent sex determination, meaning the developing turtle's gender depends on the temperature it is exposed to.[15][16][17][18][19] Warmer temperatures produce female hatchlings, while cooler temperatures produce male hatchlings.[15][16][17][18][19][20] The eggs will incubate for 50–60 days. The eggs in one nest hatch together over a short period of time. The baby 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 Kemps ridley, commonly hatches during the day. Turtle nests that hatch during the day, are more vulnerable to predators, and may encounter more human activity on beach.

Turtle gender 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 turtles are exposed to predators for a shorter amount of time.[21] 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 seaturtles 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, sea turtle young feed in the vicinity of upwelling "fronts".[10] 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.[11][22]

Physiology

Osmoregulation

Sea turtles maintain an internal environment that is hypotonic to the ocean. To maintain hypotonicity they must excrete excess salt ions.[23] Like other marine reptiles, sea turtles rely on a specialized gland to rid the body of excess salt ions, because reptilian kidneys can not produce urine with a higher ion concentration than sea water.[24] All species of sea turtles have a lachrymal salt gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water.[25]

Leatherbacks 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 salt gland found in leatherbacks 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 tears can have a salt ion concentration almost twice that of other species of marine turtle.[26]

Immature Hawaiian green sea turtle in shallow waters

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 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.[24]

Thermoregulation

Most sea turtles (those in family Cheloniidae) are poikilotherms.[27] However the leatherback (family dermochelyidae) are endotherms because they can maintain a body temperature 8 °C warmer than the ambient water.[27]

Green sea turtles in the relatively cooler pacific are known haul themselves out of the water on remote islands to bask in the sun.[28] This behavior has only been observed in a few locations including the Galapagos, Hawaii, Europa Island, and parts of Australia.[28]

A green sea turtle breaks the surface to breathe.

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.[29] Dive duration largely depends on activity. A foraging turtle may typically spend 5 - 40 min under water[29] while a sleeping sea turtle can remain under water for 4–7 hours.[30][31] Remarkably, sea turtle respiration remains aerobic for the vast majority of voluntary dive time.[29][31] 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.[29]

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.

Ecology

Diet

The loggerhead, Kemp's ridley, olive ridley, hawksbill, flatback, and leatherback sea turtles are omnivorous for 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.[32][33][34][35] However some species specialize on certain prey.

The diet of green turtles changes with age.[36] Juveniles are omnivorous, but as they mature they become exclusively herbivorous.[33][36] This diet shift has an effect on the green turtle's morphology.[37][38] Green sea turtles have a serrated jaw that is used to eat sea grass and algae.[39]

Leatherback turtles feed almost exclusively on jellyfish and helps control jellyfish populations.[40][41]

Hawksbills principally eat sponges, which constitute 70–95% of their diets in the Caribbean.[42]

Relationship with humans

Marine sea turtles are caught worldwide, although it is illegal to hunt most species in many countries.[43][44] A great deal of intentional marine sea turtle harvests worldwide are for food. Many parts of the world have long considered sea turtles to be fine dining. Ancient Chinese texts dating to the fifth century B.C.E. describe sea turtles as exotic delicacies.[45] 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.[46]

"Manner in which Natives of the East Coast strike turtle". Near Cooktown, Australia. From Phillip Parker King's Survey. 1818.

To a much lesser extent, specific species of marine sea turtles are targeted not for their flesh, but for their shells. Tortoiseshell, a traditional decorative ornamental material used in Japan and China, comes from the carapace scutes of the hawksbill sea turtle.[47][48] Ancient Greeks and ancient Romans processed sea turtle scutes (primarily from the hawksbill) for various articles and ornaments used by their elites, such as combs and brushes.[49] The skin of the flippers is prized for use as shoes and assorted leather goods.

The Moche people of ancient Peru worshipped the sea and its animals. They often depicted sea turtles in their art.[50]

Leatherback sea turtles enjoy immunity from the sting of the deadly box jellyfish and regularly eat them, helping keep tropical beaches safe for humans.

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. Since the creation of a sea turtle, ecotourism-based economy, Tortugero annually houses thousands of tourists who visit the protected 22-mile beach that hosts sea turtle walks and nesting grounds.[51][52]

Importance to ecosystems

Sea turtles on a beach in Hawaii

Sea turtles play key roles in two habitat types: oceans and beaches/dunes.

In the oceans, sea turtles, especially green sea turtles, are one of 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 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.[53]

Sea turtles use beaches and the lower dunes to nest and lay their eggs. Beaches and dunes are a fragile habitat that 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. Along a 20-mile stretch of beach on the east coast of Florida sea turtles lay over 150,000 lbs of eggs in the sand. Dune vegetation is able to grow and become stronger with the nutrients from sea turtle nests. Stronger vegetation and root systems help to hold the sand in the dunes and help protect the beach from erosion.[53]

Conservation status and threats

A sea turtle entangled in a net

The IUCN Red List classifies three species of sea turtle as either "Endangered" or "Critically Endangered".[54] An additional three species are classified as "Vulnerable".[54] The flatback is considered as "Data Deficient", meaning that its conservation status is unclear due to lack of data.[54] All species of sea turtle are listed in CITESAppendix I, restricting international trade of sea turtles and sea turtle products.[4][55]

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).[56] The US listing status of the loggerhead is under review as of 2012.[56]

IUCN Redlist United States ESA*
Green Endangered[57] Endangered: populations in Florida and Pacific coast of Mexico populations

Threatened: all other populations[58]

Loggerhead Vulnerable[59] Endangered: NE Atlantic, Mediterranean, N Indian, N Pacific, S Pacific populations

Threatened: NW Atlantic, S Atlantic, SE Indo-Pacific, SW Indian populations[60]

Kemp's ridley Critically Endangered[61] Endangered: all populations[62]
Olive ridley Vulnerable[63] Endangered: Pacific Coast of Mexico population

Threatened: all other populations[64]

Hawksbill Critically Endangered[65] Endangered: all populations[66]
Flatback Data Deficient[67] N/A
Leatherback Vulnerable[68] Endangered: all populations[69]

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

Management

In the Caribbean, researchers are having some success in assisting a comeback.[70] 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 ridleys hatchlings along the Texas coast this year.

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.[71]

Evaluating the progress of conservation programs is difficult, because many sea turtle populations have not been assessed adequately.[72] Most information on sea turtle populations comes from counting nests on beaches, but this doesn’t provide an accurate picture of the whole sea turtle population.[73] 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.[74]

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.[75] 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.[75] The results clearly demonstrate that humans should not manipulate or relocate clutches of that turtle, and impart strong evidence of the detrimental effects that human activity can cause.

Predators and Disease

Most sea turtle mortality happens early in life. Sea turtles usually lay around one hundred eggs at a time, on average only one of the eggs from the nest will survive to adulthood.[76] Nests may be raided by raccoons and foxes. Within minutes of hatching, hatchlings may be eaten by raccoons, foxes, seabirds, as they make their initial run for the ocean.[77] Once in the water they are susceptible to seabirds, large fish and even other turtles.

Adult sea turtles have few predators. Large aquatic carnivores such as sharks and crocodiles are the biggest threat. Reports of terrestrial predators attacking nesting females are not uncommon. Jaguars have been reported to smash into the turtle's shell with its paw, and scoop out the flesh.[78]

Fibropapillomatosis disease causes tumors in sea turtles.

While many of the things that endanger these hatchlings are natural, such as predators including sharks, raccoons, foxes, and seagulls,[77] many new threats to the sea turtle species have recently arrived and increased with the ever-growing presence of humans.[79]

Bycatch

A loggerhead sea turtle escapes a circular fisherman's net via a TED.
Loggerhead sea turtle exits from fishing net through a turtle excluder device (TED)

One of the most significant threats now comes from bycatch due to imprecise fishing methods. Long-lining has been identified as a major cause of accidental sea turtle death.[80][81] There is also black-market demand for tortoiseshell for both decoration and supposed health benefits.[82]

Sea turtles must surface to breathe. Caught in a fisherman's net, they are unable to surface and thus drown. In early 2007, almost a thousand sea turtles were killed inadvertently in the Bay of Bengal over the course of a few months after netting.[83]

However, some relatively inexpensive changes to fishing techniques, such as slightly larger hooks and traps from which sea turtles can escape, can dramatically cut the mortality rate.[84][85] Turtle Excluder Devices (TEDs) have reduced sea turtle bycatch in shrimp nets by 97 percent.

Legal notice posted by nest at Boca Raton, Florida

Beach development

Beach development is another area which threatens sea turtles. Since many sea turtles return to the same beach each time to nest, development can disrupt the cycle. There has been a movement to protect these areas, in some cases by special police. In some areas, such as the east coast of Florida, conservationists dig up sea turtle eggs and relocate them to fenced nurseries to protect them from beach traffic.

Since hatchlings find their way to the ocean by crawling towards the brightest horizon, they can become disoriented on developed stretches of coastline. Lighting restrictions can prevent lights from shining on the beach and confusing hatchlings. Sea turtle-safe lighting uses red or amber LED light, invisible to sea turtles, in place of white light.

Poaching

Another major threat to sea turtles is black-market trade in eggs and meat. This is a problem throughout the world, but especially a concern in China, the Philippines, India, Indonesia and the coastal nations of Latin America. Estimates reach as high as 35,000 sea turtles killed a year in Mexico and the same number in Nicaragua. Conservationists in Mexico and the United States have launched "Don't Eat Sea Turtle" campaigns in order to reduce this trade in sea turtle products. These campaigns have involved figures such as Dorismar, Los Tigres del Norte and Maná. Sea turtles are often consumed during the Catholic season of Lent, even though they are reptiles, not fish. Consequently, conservation organizations have written letters to the Pope asking that he declare sea turtles meat.[86]

Marine debris

Another danger comes from marine debris, especially plastics which may be mistaken for jellyfish, and abandoned fishing nets in which they can become entangled.

Climate change

Climate change may also cause a threat to sea turtles. Since sand temperature at nesting beaches defines the sex of a sea turtle while developing in the egg, there is concern that rising temperatures may produce too many females. However, more research is needed to understand how climate change might affect sea turtle gender distribution and what other possible threats it may pose.[87]

Oil spills

Sea turtles are very vulnerable to oil pollution, both because of their tendency to linger on the water's surface, and because oil can affect them at every stage of their life cycle.[88] Oil can poison the sea turtles upon entering their digestive system.

Rehabilitation

Injured sea turtles are sometimes rescued and rehabilitated by professional organizations, such as Sea Turtles 911 in Hainan, China; The Loggerhead Marinelife Center in Juno Beach, FL; The Turtle Hospital in Marathon, Florida, the Mote Marine Laboratory in Sarasota, Florida; the Marine Mammal Center in Northern California; the Clearwater Marine Aquarium in Clearwater, Florida;[89] and Sea Turtle Inc. in South Padre Island, Texas.[90][91] One such sea turtle, named Nickel for the coin that was found lodged in her throat, lives at the Shedd Aquarium in Chicago.

Symbiosis with barnacles

Sea Turtles are believed to have a commensal relationship with some barnacles, in which the barnacles benefit from growing on 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. 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.[92] 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 barnacle’s settlements. These organisms include mollusks, whales, decapod crustaceans, manatees and several other groups related to these species.[93]

Sea turtle shells are an ideal habitat for adult barnacles for three reasons. Turtles tend to live long lives, around 50 years, so barnacles do not have to worry about host death. Secondly, barnacles are suspension feeders. Sea turtles spend most of their lives swimming and following ocean currents and as water runs along the back of the turtle’s shell it passes over the barnacles, providing an almost constant water flow and influx of food particles. Lastly, the long distances and inter ocean travel these sea turtles swim throughout their lifetime, offers 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.[94]

This relationship however is not truly commensal. While the barnacles are not directly parasitic to their hosts, they have negative effects to the 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.[95]

See also

Additional reading

References

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