Sea turtles (Chelonioidea) are turtles found in all the world's oceans with the exception of the Arctic Ocean,and some species travel between oceans.The Flatback turtle is found solely on the northern coast of Australia.The Leatherback Sea Turtle is the largest,measuring six or seven feet (2 m) in length at maturity,and three to five feet (1 to 1.5 m) in width,weighing up to 1300 pounds (600 kg).Most other species are smaller,being two to four feet in length (0.5 to 1 m) and proportionally less wide.There are seven types of sea turtles:Kemp's Ridley,Flatback,Green,Olive Ridley,Leatherback,Loggerhead,and Hawksbill.
Different species are distinguished by varying anatomical aspects: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 carrying a mosaic of bony plates beneath its leathery skin.
Sea turtles have an extraordinary sense of time and location.They are highly sensitive to the Earth's magnetic field and probably use it to navigate.They live up to 189 years.The fact that most species return to nest at the locations where they were born seems to indicate an imprint of that location's magnetic features.The ridley turtles are especially peculiar because instead of nesting individually like the other species,they come ashore in one mass arrival known as an "arribada" (arrival).With the Kemp's ridley this occurs during the day and on only one beach in the entire world.Their numbers used to range in the thousands but due to the effects of extensive egg poaching and hunting in previous years the numbers are now in the hundreds.
After about 30 years of maturing,adult female sea turtles return to the land to nest,usually on the same beach from which they hatched.This can take place every two to four years in maturity.They make from four to seven nests per nesting season.They dig a hole with their hind flippers and lay from 70 to 190 eggs in it (depending on the species) before covering it up and returning to the ocean.Some of the eggs are unfertilized 'dummy eggs' and the rest contain young turtles.Incubation takes about 2 months.When the eggs hatch,these baby turtles dig their way out and seek the ocean.Only a very small proportion of them (at most 1 in 100) will be successful,as many predators are waiting to eat them.
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