Why do mother turtles leave their babies at sea?

Distribution

The superfamily Chelonioidea has a worldwide distribution; sea turtles can be found in all oceans except for the polar regions.[citation needed] Some species travel between oceans. The Flatback turtle is found solely on the northern coast of Australia.

Biology

Air breathers

A Green turtle breaks the surface to breathe.

Sea turtles are almost always submerged in water, and, therefore, have developed an anaerobic system of respiration. Although all sea turtles may breathe, under dire circumstances, they may divert to anaerobic respiration for long periods of time. When oxygen is present, a sea turtle, with a single explosive exhalation and rapid inhalation, can quickly refill their lungs when they surface. Their large lungs have adapted to permit rapid exchange of oxygen and to avoid trapping gases during deep dives. However, turtles must emerge while breeding, given the extra level of activity.

Life history

Green turtle swims above corals at Hawaii

A feeding Green sea turtle, Chelonia mydas

The lifespan of sea turtles has been speculated at 80 years.[citation needed]

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 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 200 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 until it is relatively undetectable visually. The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.[1]

The hatchling's gender depends on the sand temperature. Lighter sands maintain higher temperatures, which decreases incubation time and results in more female hatchlings.

Incubation takes about two months. The eggs in one nest hatch together over a very short period of time. When ready, hatchlings tear their shells apart with their snout and dig through the sand. Once they reach the surface, they instinctively head towards the sea. Only a very small proportion of each hatch (usually .01%) succeed, because local opportunist predators, such as the common seagull, gorge on the new turtles.

The survivors then proceed into the open ocean. 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, turtle young feed in the vicinity of upwelling "fronts".[2] In 2007, Reich determined that green 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 smallernekton before they are recruited into inshore seagrass meadows as obligate herbivores.[3][4]

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

Salt gland

Sea turtles possess a salt excretory gland at the corner of the eye, in the nostrils, or in the tongue, depending on the species; chelonian salt glands are found in the corner of the eyes in leatherback turtles. Due to the iso-osmotic makeup of jellyfish and the other gelatinous prey upon which sea turtles subsist, sea turtle diets are high in salt; chelonian salt gland excretions are almost entirely composed of sodium chloride 1500-1800 mosmoll-1 (Marshall and Cooper, 1988; Nicolson and Lutz, 1989; Reina and Cooper, 2000).

Importance to humans

Moche Sea Turtle. 200 A.D. Larco Museum Collection, Lima, Peru

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

Marine turtles are caught worldwide, although it is illegal to hunt most species in many countries.[5][6] A great deal of intentional marine 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. describe sea turtles as exotic delicacies.[7] Many coastal communities around the world depend on sea turtles as a source of protein, often harvesting several turtles at once and keeping them alive on their backs until needed. Coastal peoples gather turtle eggs for consumption.[8]

Turtles are popular in Mexico as boot material and food.[9]

To a much lesser extent, specific species of marine 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 turtle.[10][11] Ancient Greeks and ancient Romansprocessed turtle scutes (primarily from the hawksbill) for various articles and ornaments used by their elites, such as combs and brushes.[12] The skin of the flippers are 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.[13]

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

Conservation

Legal notice posted by nest atBoca Raton, Florida

All species of sea turtles are listed as threatened or endangered. The leatherback, Kemp's Ridley, and hawksbill turtles are critically endangered.[14][15] The Olive Ridley and green turtles are endangered, and the loggerhead is threatened.[16] The flatback's conservation status is unclear due to lack of data.

One of the most significant threats now comes from bycatch due to imprecise fishing methods. Donnelly points to long-lining as a major cause of accidental sea turtle death.[17] There is also black-market demand for tortoiseshell for both decoration and supposed health benefits.[18]

Turtles must surface to breathe. Caught in a fisherman's net, they are unable to surface and thus suffocate. In early 2007, almost a thousand sea turtles were killed inadvertently in the Bay of Bengalover the course of a few months after netting.[19]

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.[20][21] 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.

Beach development is another area which threatens sea turtles. Since many 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 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. 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 the Philippines, India, Indonesia and the coastal nations of Latin America. Estimates reach as high as 35,000 turtles killed a year in Mexico and the same number inNicaragua. 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 ManĂ¡. 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 turtles meat.

A Green Sea Turtle at rest

Climate change may also cause a threat to sea turtles. Since sand temperature at nesting beaches defines the sex of a 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.[22]

Fibropapillomatosis disease causes tumors in sea turtles.

Injured sea turtles are sometimes rescued and rehabilitated by professional organizations, such as theMote Marine Laboratory in Sarasota, Florida, the Marine Mammal Center in Northern California, the ClearWater Marine Aquarium in Clearwater, Florida,[23] and the Sea Turtle Inc. organization in South Padre Island, Texas.[24] One such 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.[25] 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 onMustang 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 turtles and olive ridleys were threatened.[26]

In Southeast Asia, the Philippines has had several initiatives dealing with the issue of 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 Batanganmarkets. 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 turtle eggs.[27]

Fragile 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, for example, 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 must be kept short to remain healthy, and beds of healthy sea grass are essential breeding and development areas for many species of fish and other marine life. A decline or loss of sea grass beds would damage these populations, triggering a chain reaction and negatively impacting marine and human life.

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[citation needed]. Every year, sea turtles lay countless eggs on beaches. Along one twenty-mile (32 km) stretch of beach in Florida alone, for example, more than 150,000 pounds of eggs are laid each year.