Sea turtle

Sea turtles
Temporal range:
Early Cretaceous-Holocene,[1] 110–0Ma
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 species of sea turtles. They are the leatherback sea turtle, green sea turtle, loggerhead sea turtle, Kemp's ridley sea turtle, hawksbill sea turtle, flatback sea turtle and olive ridley sea turtle. Four of the species have been identified as "endangered" or "critically endangered" with another two being classed as "vulnerable".

Taxonomy and evolution

Sea turtles, along with other turtles and tortoises, are part of the order Testudines.

The seven living species of sea turtles are: leatherback sea turtle, green sea turtle, loggerhead sea turtle, Kemp's ridley sea turtle, hawksbill sea turtle, flatback sea turtle and olive ridley sea turtle.[4] All species except the leatherback are in the family Cheloniidae. The leatherback belongs to the family Dermochelyidae and is its only member.

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.[5]

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)[6]

Phylogenetic relations of living and extinct chelonioid species
Panchelonioidea

†Toxochelys

†Ctenochelys


Chelonioidea
Pancheloniidae


†Euclastes

†Puppigerus

Cheloniidae



Pandermochelys

†Protostegidae

Dermochelyidae





Distribution

An Olive ridley turtle nesting on Escobilla Beach, Oaxaca, Mexico

The superfamily Chelonioidea has a world-wide distribution; sea turtles can be found in all oceans except for the polar regions. Some species travel between oceans. The flatback sea turtle is found solely on the northern coast of Australia. Kemp's ridley sea turtle is found solely in the Gulf of Mexico and along the East Coast of the United States.[7]

Behavior and ecology

Habitat

Sea turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore again.[5] During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed beds. Green sea turtles in particular are often found in Sargassum beds, a brown seaweed in which they find shelter and food.[8] Once the sea turtle has reached adulthood it moves closer to the shore.[9] Females will come ashore to lay their eggs on sandy beaches during the nesting season.[5]

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.[10] Having more room to live enables more room for growth.

A green sea turtle breaks the surface to breathe.

Respiration

Sea turtles are almost always submerged, and, therefore, have an anaerobic system of energy metabolism. Although all sea turtles breathe air, under dire circumstances they may divert to anaerobic metabolism for long periods of time. 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.

Life cycle

It takes decades for sea turtles to reach sexual maturity. After mating at sea, adult female sea turtles return to land to nest at night. 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. They make from one to eight nests per season. The mature nesting female hauls herself onto the beach, nearly always at night, and finds suitable sand on 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 a clutch of soft-shelled eggs one by one until she has deposited around 50 to 350 eggs, depending on the species. Some species have been reported to lay 250 eggs, such as the hawksbill. After laying, she re-fills the nest with sand, re-sculpting and smoothing the surface, and then camouflaging the nest with grasses until it is relatively undetectable visually.[8] The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.[11]

Turtle gender depends on sand temperature while the egg is incubating.

The hatchling's gender depends on the sand temperature.[12][13][14][15][16] Lighter sands maintain higher temperatures, which decreases incubation time and results in more female hatchlings.

It takes several decades for adult sea turtles to reach sexual maturity. The mature turtles migrate, sometimes for thousands of miles, to reach breeding sites. Male and female turtles mate in the water, and the males return to deep sea to feed. For several weeks, female sea turtles alternate between mating in the water and laying their eggs on land. Before laying her eggs, a female turtle will dig a hole in the sand with her hind flippers. She covers it with sand and returns to the ocean. About two months pass for the eggs to incubate under the sand. Afterwards, the eggs hatch, generally at night to avoid predation, and the hatchlings crawl to the water. They then swim out to sea to begin their own cycle of maturing and reproducing. Sea turtles can continue this cycle until they are 80 years old.

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, making between 3 and 4 nests a per season. 5. Females lay their eggs, often between 100 to 120 at a time. 6. When the season is over, female turtles return to feeding sites. 7. Baby turtles mature for 60-80 days and hatch. 8. Newly hatched turtles emerge from nests and travel from the shore to the water, usually at night. 9. Baby turtles mature in the ocean until they are ready to begin the cycle again.

Incubation takes about 50–60 days. The eggs in one nest hatch together over a very short period of time. When ready, hatchlings tear their egg shells apart with their caruncle, a special egg tooth, to help break free of the egg shell and dig through the sand. Again, this usually takes place at night. Once they reach the surface, they instinctively head towards the sea.

Most species of Sea Turtles will hatch during the night hours, but the Kemp Ridley Sea Turtle, will commonly hatch during the day. This has caused the Kemp Ridley to be the most endangered species of all sea turtles. Turtle nests that hatch during the day, are more prone to predators like birds, crab, sea birds, raccoon, ants and other animals on the beach. They also encounter more human activity on beaches after hatching and can run into human obstacles such as beach chairs, umbrellas, sand castles, as well as dogs and people on the beaches themselves. 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.[17] The fact that there is size dependent predation on chelonians has led to the evolutionary development of large body sizes.

The hatchlings then proceed into the ocean, where a variety of marine predators await them. In 1987, Carr discovered that the young of Chelonia mydas and Caretta caretta spent a great deal of their pelagic lives in floating sargassum beds, where there are thick mats of unanchored seaweed. Within these beds, they found ample shelter and food. In the absence of sargassum beds, sea turtle young feed in the vicinity of upwelling "fronts".[8] 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.[9][18]

Instead of nesting individually like the other species, Ridley sea turtles come ashore en masse, known as an "arribada" (arrival). With the Kemp's ridley sea turtles this occurs during the day.

Diet

Sea turtles feed on a wide range of animals and plants. They are mostly omnivorous in their adult life, except the green sea turtle which is herbivorous, changing from a carnivorous diet when young. Adult green sea turtles are herbivores. The jaw is serrated to help the turtle easily chew their primary food source -- seagrasses and algae.

Juvenile green sea turtles are omnivores. They eat a wide variety of plant and animal life, including insects, crustaceans, seagrasses and worms. This diet shift has an effect on the green turtle's morphology. The turtles switch from an omnivorous diet of planktonic material as juveniles to an herbivorous diet of algae and seagrass material as adults, and there is a concurrent transformation of skull morphology.[19] This reflects how the morphological characteristics of turtles are correlated with the foods they consume. Some species specialise on certain prey; sea sponges are the principal food of hawksbill sea turtles, constituting 70–95% of their diets in the Caribbean.[20] Leatherback turtles feed almost exclusively on jellyfish and help control jellyfish populations.[21][22]

Aside from sponges, hawksbills also feed on algae and cnidarians (including the Portuguese man o' war), comb jellies and other jellyfish and sea anemones.[23] Green sea turtles are commonly found in seagrass meadows closer inshore as herbivorous grazers. The leatherback turtle eats a variety of organisms such as seagrass, marine invertebrates including molluscs, jellyfish and shrimp and also fishes. It also consumes of soft coral, sea cucumbers and other soft-bodied creatures.[24] The loggerhead sea turtle is omnivorous, feeding mainly on bottom-dwelling invertebrates, such as gastropods, bivalves, and decapods. The loggerhead has a greater list of known prey than any other sea turtle. Other food items include sponges, corals, sea pens, polychaete worms, sea anemones, cephalopods, barnacles, brachiopods, isopods, insects, bryozoans, sea urchins, sand dollars, sea cucumbers, starfish, fish (eggs, juveniles, and adults), hatchling turtles (including members of its own species), algae, and vascular plants.[25] During migration through the open sea, loggerheads eat jellyfish, floating molluscs, floating egg clusters, squid, and flying fish.[26] The Kemp's ridley turtle feeds on molluscs, crustaceans, jellyfish, algae or seaweed, and sea urchins. The olive ridley turtle is predominantly carnivorous, especially in immature stages of the life cycle. Animal prey consists of protochordates or invertebrates, which can be caught in shallow marine waters or estuarine habitats. Common prey items include jellyfish, tunicates, sea urchins, bryozoans, bivalves, snails, shrimp, crabs, rock lobsters, and sipunculid worms.[27] Aside from jellyfish, leatherbacks also feed on other soft-bodied organisms, such as tunicates and cephalopods.[28]

Salt gland

Marine vertebrates maintain a balance of dissolved solutes and water in the body fluids by excreting excess salt ions.[29] 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.[30] 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.[31]

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.[32]

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.[30]

Commensalism with barnacles

Immature Hawaiian green sea turtle in shallow waters

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 to 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.[33] 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.[34]

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.[35]

There are a few speculations however at the idea that this relationship is truly commensal. The barnacles are not parasitic to their hosts but have been found to have negative effects to the turtles on which they choose to reside. These effects however seem to depend on the quantity of barnacles affixed to its back. The barnacles add extra weight to the sea turtle, potentially increasing the energy it needs for swimming and affecting its ability to capture prey.[36]

Relationship with humans

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

Marine sea turtles are caught worldwide, although it is illegal to hunt most species in many countries.[37][38] 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.[39] 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.[40]

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.[41][42] 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.[43] 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.[44]

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.[45][46]

Importance to ecosystems

Sea turtles on a beach in Hawaii

Sea turtles play key roles in two ecosystem types that are critical to them as well as to humans——oceans and beaches/dunes. In the oceans, sea turtles, especially green sea turtles, are one of very few creatures (manatees are another) that eat the sea grass that grows on the sea floor. Sea grass needs to be constantly cut short to help it grow across the sea floor. Sea turtles act as grazing animals that cut the grass short and help maintain the health of the sea grass beds. Sea grass beds provide breeding and developmental grounds for numerous species of fish, shellfish and crustaceans. 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.[47]

Beaches and dunes form a fragile ecosystem that depends 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. Sea turtles use beaches and the lower dunes to nest and lay their eggs. Sea turtles lay around 100 eggs in a nest and make between 3 and 7 nests during the summer nesting season. 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 presence of nutrients from sea turtle eggs, unhatched nests, eggs and trapped hatchlings. As the dune vegetation grows stronger and healthier, the health of the entire beach/dune ecosystem becomes better. Stronger vegetation and root systems help to hold the sand in the dunes and help protect the beach from erosion.[47]

Conservation status and threats

A sea turtle entangled in a net

Of the seven species of sea turtles,[48][49] four are listed on the IUCN Red List of Endangered Species as either "endangered" or "critically endangered".[50] Globally, the Kemp's ridley, and hawksbill sea turtles are listed as "Critically Endangered", the loggerhead and green as "Endangered", the olive ridley and leatherback as "Vulnerable" and the flatback as "Data Deficient", meaning that its conservation status is unclear due to lack of data.[51] Additionally, all populations of sea turtles that occur in United States waters are listed as threatened or endangered by the United States government, with the leatherback, Kemp's ridley, green, hawksbill, and the Mexican nesting population of the olive ridley all listed as "Endangered", and the loggerhead and non-Mexican populations of olive ridley as "Threatened".[52] The US listing status of the loggerhead is under review as of 2012.[52] Although 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.[53] While many of the things that endanger these hatchlings are natural, such as predators including sharks, raccoons, foxes, and seagulls,[54] many new threats to the sea turtle species have recently arrived and increased with the ever-growing presence of humans.[55] It was originally believed that nest relocation could be a useful conservation technique for sea turtles. In a certain study on sea turtle conservation, researchers examined the effects of nest relocation on the endangered giant South American turtle, Podocnemis expansa. They discovered that turtle clutches that were transplanted to a new location had higher mortality rates and more morphological abnormalities compared to non transplanted clutches.[56] The results clearly demonstrate that humans should not manipulate or relocate turtle clutches, and impart strong evidence of the detrimental effects that anthropogenic activity can cause.

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.[57][58] There is also black-market demand for tortoiseshell for both decoration and supposed health benefits.[59]

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.[60]

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.[61][62] Turtle Excluder Devices (TEDs) have reduced sea turtle bycatch in shrimp nets by 97 percent. Another danger comes from marine debris, especially from abandoned fishing nets in which they can become entangled.

Legal notice posted by nest at Boca Raton, Florida

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.

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.

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.[63]

Other species threaten sea turtles too. Fibropapillomatosis disease causes tumors in sea turtles. While capable of cracking turtle shells, the jaguar may simply smash into the shell with its paw and scoop out the flesh.[64] When attacking sea turtles as they try to nest on beaches, the jaguar will bite at the head, often beheading the prey, before dragging it off to eat.[65]

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)

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,[66] and the Sea Turtle Inc. organization in South Padre Island, Texas.[67][68] One such sea turtle, named Nickel for the coin that was found lodged in her throat, lives at the Shedd Aquarium in Chicago.

In the Caribbean, researchers are having some success in assisting a comeback.[69] 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.

Also in 2007, the U.S. Fish and Wildlife Service and the National Marine Fisheries Service issued a determination that the leatherback, the hawksbill and the Kemp's Ridley populations were endangered while that of green sea turtles and olive ridleys were threatened.[70]

In Southeast Asia, the Philippines has had several initiatives dealing with the issue of sea turtle conservation. In 2007, the province of Batangas in the Philippines declared the catching and eating of Pawikans illegal. However, the law seems to have had little effect as Pawikan 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]

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.[75] Oil can poison the sea turtles upon entering their digestive system.

See also

Additional reading

References

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