Sea Turtles 101 | National Geographic

(Mellow music) - [Narrator] Sea turtles are ancient mariners. Present in all but Earth's coldest oceans, these marine reptiles are well-adapted to a life on the move. (Dramatic music) Sea turtles have existed since the time of the dinosaurs. The earliest known marine turtle lived about 120 million years ago. Named Desmatochelys padillai, it was six feet long and had the characteristic features of modern sea turtles including a carapace, or a top shell, and paddle-like limbs.

Today, there are seven species of sea turtles with the largest being the leatherback. Growing as long as eight feet and weighing up to 2,000 pounds, it is larger than known fossils of its prehistoric ancestor, the padillai. Sea turtles can lay more than 150 eggs at a time. Called the clutch, these large egg deposits help ensure the survival of sea turtle hatchlings. Once a female lays her eggs, she returns to the sea, leaving her hatchlings to fend for themselves.

Fewer than .1% of hatchlings survive to adulthood in part due to numerous predators on the beach. Emerging in large numbers increases the chance that some will survive. (Waves crashing) A sea turtle's sex is determined by temperature. Unlike most vertebrates, a sea turtle's sex is not determined by sex chromosomes. Instead, the temperature of the nest determines the sex of the hatchlings. When temperatures are warm, at about 88 degrees Fahrenheit, most of the hatchlings will be female, but when temperatures are cooler, less than around 82 degrees Fahrenheit, most of the hatchlings will be male.

(Mellow music) Sea turtles use Earth's magnetic fields to navigate. Sea turtles have geomagnetic abilities which serve as an internal GPS. The turtles have particles of magnetite, a magnetic mineral, in their brains. The magnetite likely plays a role in orienting sea turtles to Earth's magnetic poles, similar to a compass. Baby sea turtles imprint on the unique magnetic signature of the beaches where they hatch. This magnetic map can guide them back to the same beaches several years later to lay their own eggs.

(Mellow music) All seven sea turtle species are threatened with extinction. Once abundant, sea turtle populations have dramatically declined in the past two centuries. Fishing is a major threat to sea turtles as they become caught in fishing nets as bycatch, or accidental catch. Illegal harvesting of turtle eggs, trafficking in turtle products, and ocean pollution are also concerns.

However, policies have been enacted to reduce bycatch, protect turtle habitats, and prohibit the killing of sea turtles and their eggs. By continuing this kind of intervention, humans can help stop the decline of sea turtles, helping keep these ancient mariners in the world's oceans. (Waves crashing)