Marine turtles — also called sea turtles — are large reptiles that have lived in Earth’s oceans for more than 100 million years. They survived the asteroid that wiped out the dinosaurs. They navigated the seas before humans existed. And they find their way across thousands of miles of open ocean using Earth’s magnetic field like a built-in compass.

There are seven species of marine turtle alive today: the leatherback, green, loggerhead, hawksbill, Kemp’s ridley, olive ridley, and flatback. Six of the seven are listed as threatened or endangered. The seventh — the flatback — is classified as data deficient, meaning scientists do not yet have enough information to assess it properly.

These animals have been quietly doing the same things for longer than most species have existed. Here is what makes them remarkable.

Key Takeaways
– Marine turtles have existed for over 100 million years, surviving mass extinctions that ended most other species on Earth.
– Female marine turtles return to the exact beach where they hatched to lay their own eggs, navigating thousands of miles using Earth’s magnetic field.
– There are seven species of marine turtle; six are threatened or endangered, primarily due to plastic pollution, fishing nets, and coastal development.
– The temperature of the sand where eggs incubate determines whether hatchlings are male or female — warmer sand produces more females.
– Marine turtles are a keystone species: green turtles maintain seagrass beds, and hawksbill turtles control sponge populations on coral reefs.

Seven Species, One Ancient Lineage

Not all marine turtles are alike. The seven species differ in size, diet, habitat, and behavior — but they share the same basic body plan that has worked for over 100 million years.

Leatherback: The largest of all, reaching up to 7 feet long and weighing up to 2,000 pounds. Instead of a hard shell, it has a flexible, rubbery back covered in a mosaic of small bones beneath leathery skin. Leatherbacks eat almost exclusively jellyfish and can dive deeper than 3,000 feet — deeper than any other reptile on Earth.

Green turtle: Named not for the color of its shell (which is brown or olive) but for the greenish color of its fat. Green turtles are the only marine turtles that eat plants as adults, grazing on seagrass and algae.

Loggerhead: Identified by its large, powerful head and strong jaws, which can crush crabs, conchs, and horseshoe crabs. One of the most widely distributed sea turtle species.

Hawksbill: Named for its narrow, pointed beak, which lets it reach into cracks in coral reefs to pull out sea sponges. Critically endangered due to demand for its distinctive patterned shell.

Kemp’s ridley: The smallest and most endangered marine turtle, found primarily in the Gulf of Mexico. Females nest in massive synchronized events called “arribadas” (Spanish for “arrivals”) where thousands of turtles come ashore on the same beach on the same night.

Olive ridley: The most abundant marine turtle, also known for arribadas. Named for the olive-green color of its heart-shaped shell.

Flatback: Found only in Australian waters. Lays the fewest eggs per nest of any marine turtle but has a higher hatchling survival rate.

The Magnetic Map in Their Heads

One of the most extraordinary things about marine turtles is how they navigate.

When a female sea turtle hatches on a beach, she crawls to the ocean and swims away. Years later — sometimes decades later — she will return to that exact beach to lay her own eggs. Not just the same coastline. The same stretch of beach. Sometimes within a few hundred meters of where she was born.

Scientists call this natal homing, and for a long time it was a complete mystery. How does an animal that has spent 20 or 30 years swimming thousands of miles across open ocean find its way back to one specific beach?

The answer, it turns out, is magnetism.

Marine turtles have magnetite crystals in their brains that allow them to detect Earth’s magnetic field. Every location on Earth has a slightly different magnetic signature — a unique combination of field intensity and inclination angle. Sea turtles imprint on the magnetic signature of their birth beach when they hatch. Decades later, they use that magnetic memory like a GPS coordinate to find their way home.

Researcher Dr. Kenneth Lohmann at the University of North Carolina has spent decades studying this navigation system. In experiments, hatchlings placed in a tank surrounded by magnetic coils consistently swam in the direction that corresponded to a programmed magnetic field — even when they could not see, smell, or hear anything useful. The compass in their head was enough.

For more on how animals (and Earth itself) create and use magnetic fields, our piece on what the atmosphere is touches on the same planetary forces that make this navigation possible.

How Baby Turtles Find the Ocean

Zara was seven years old when her family visited a beach in Costa Rica during sea turtle nesting season. At 4am, she watched a loggerhead turtle crawl out of the surf, dig a hole with her back flippers, and lay more than 100 eggs. Then the turtle covered the nest and returned to the ocean — all in the dark, all in silence.

Fifty-two days later, Zara and her family were back. The eggs hatched overnight. Tiny turtles, each about the size of a matchbox car, scrambled out of the sand and began moving toward the water.

What guides them? Light. Hatchlings instinctively move toward the brightest light on the horizon. On a natural beach with no development nearby, the brightest thing at night is the ocean — light from the moon and stars reflected off the water. The hatchlings move toward it automatically.

The problem: artificial light from hotels, streetlamps, and phone screens is now often brighter than the ocean. Hatchlings on developed coastlines frequently crawl in the wrong direction — toward parking lots, roads, and buildings — and die before reaching the water. It is one of the simplest and most damaging effects of light pollution on wildlife.

Conservation volunteers and wildlife managers now use red-spectrum lights (which turtles cannot detect as easily) near nesting beaches and ask beachgoers to turn off flashlights and phone screens during nesting season.

Why the Temperature of Sand Decides the Sex of Turtles

Marine turtles do not have sex chromosomes the way humans do. A sea turtle’s sex is not determined at fertilization. It is determined by temperature during incubation.

Eggs incubated in cooler sand (below about 82 degrees Fahrenheit) tend to produce males. Eggs in warmer sand (above about 88 degrees Fahrenheit) tend to produce females. Eggs in between produce a mix.

This system has worked for millions of years. But as global temperatures rise, beach sand is getting warmer — and marine turtle populations are shifting toward more and more females. Some beaches in the Great Barrier Reef region of Australia are now producing populations that are more than 99% female, according to research published by scientists studying green turtle nesting sites there.

A population that cannot produce enough males to maintain reproduction is a population in trouble. It is one of the less obvious ways that climate change threatens sea turtles — not through dramatic storms or habitat destruction, but through a quiet shift in sand temperature.

What Marine Turtles Do for the Ocean

Like sea otters in kelp forests, marine turtles play roles in their ecosystems that extend far beyond their own survival.

Green turtles and seagrass: Seagrass meadows are among the most productive ecosystems on Earth — they absorb carbon dioxide, provide nursery habitat for fish, and stabilize the seafloor. Green turtles graze on seagrass, keeping it short and dense. When green turtle populations declined due to hunting, seagrass beds in many areas became overgrown and began to die off. Recovering green turtle populations have been linked to healthier seagrass in several regions.

Hawksbill turtles and coral reefs: Hawksbills eat sea sponges, which grow quickly and can smother coral if left unchecked. By controlling sponge populations, hawksbill turtles help coral reefs stay balanced. Fewer hawksbills means more sponges means less coral.

Leatherbacks and jellyfish: Jellyfish populations around the world are growing, partly due to warming oceans and overfishing of jellyfish predators. Leatherback turtles eat enormous quantities of jellyfish. Without them, jellyfish blooms can overwhelm coastal waters, damaging fishing industries and ecosystems.

Remove any of these turtles from their habitat and something shifts. The effects are not always immediate, but they accumulate.

If this sounds familiar, it is the same principle as the sea otters and kelp forests described in our sea otter article — nature is built on connections, and pulling one thread can unravel more than you expect.

The Threats They Face Today

Marine turtles survived 100 million years of natural challenges. The challenges they face now are mostly human-made.

Plastic pollution: Marine turtles, especially leatherbacks, mistake plastic bags for jellyfish and eat them. Plastic causes internal blockages and is now found in turtles in every ocean on Earth. A turtle found stranded in the UK in 2022 had 64 pieces of plastic in its digestive system.

Fishing nets: Turtles need to breathe air. When they get caught in fishing nets — called bycatch — they drown. Estimates suggest hundreds of thousands of marine turtles are caught as bycatch every year.

Coastal development: Hotels, seawalls, and beach lighting destroy nesting habitat, disorient hatchlings, and reduce the area of beach available for nesting.

Poaching: Despite international protections, marine turtle eggs and adults are still poached in many parts of the world for food, traditional medicine, and the illegal wildlife trade.

Climate change: Rising temperatures skew sex ratios (as described above), bleach coral reefs that hawksbills depend on, erode nesting beaches, and shift jellyfish populations.

The Sea Turtle Conservancy, founded in 1959, is the oldest sea turtle research and conservation organization in the world. NOAA’s sea turtle program tracks US populations and coordinates recovery efforts across federal agencies.

One Turtle’s Remarkable Journey

In 2009, a young loggerhead sea turtle named Yoshi was found stranded and injured off the coast of South Africa. She was taken in by the Two Oceans Aquarium in Cape Town, nursed back to health over 20 years, and released into the ocean in 2017.

Researchers attached a satellite tracker before releasing her. Over the following two years, Yoshi swam more than 22,000 miles across the Indian Ocean — all the way to the coast of Western Australia. It was one of the longest tracked journeys of any sea turtle ever recorded.

Her journey made international news. But what struck researchers most was not the distance. It was the precision. Yoshi navigated thousands of miles of open ocean and arrived at the Western Australian coast — a location she likely knew from before her time in captivity. The magnetic map she had built before her injury was still intact after 20 years in a tank.

Questions Kids Ask About Marine Turtles

How long do marine turtles live?

Most marine turtles are estimated to live between 80 and 100 years, though the exact lifespan is difficult to verify because they are hard to track continuously across decades. Some researchers believe they may live even longer.

How fast can they swim?

Leatherbacks are the fastest marine turtle, capable of swimming up to 22 mph in short bursts. Most other species cruise at around 1 to 4 mph during long migrations and can sprint to 15 mph when needed.

Why do female turtles come to shore but males never do?

Male marine turtles almost never come ashore after hatching. They spend their entire lives at sea. Females only come to shore to nest — and they do so at night, as briefly as possible. Everything about their lifestyle is optimized for the ocean.

Do marine turtles have teeth?

No. Marine turtles have no teeth. Instead, they have strong beaks shaped to suit their diet. Loggerheads have wide, flat beaks for crushing hard-shelled prey. Hawksbills have narrow, pointed beaks for extracting sponges. Leatherbacks have backward-pointing spines inside their mouths to help them hold slippery jellyfish.

Why are they called marine turtles and not sea turtles?

Both names are correct and used interchangeably. Scientists tend to prefer “marine turtle” because it is more precise — “sea” can mean many things, while “marine” specifically refers to saltwater ocean environments. In everyday conversation, “sea turtle” is more common.

Watch: Marine Turtles in the Wild

Search YouTube for “sea turtle nesting Costa Rica” or “leatherback turtle deep dive” for real footage of nesting, hatching, and underwater behavior. The BBC and National Geographic both have excellent documentary segments showing the full nesting and hatching process in real time.

100 Million Years and Still Going

Marine turtles have been swimming Earth’s oceans since before the continents fully separated into the shapes we recognize today. They were here when the dinosaurs were here. They were here after the dinosaurs were gone. They crossed oceans that no human had ever seen and found their way back to beaches using a sense we are still trying to fully understand.

What threatens them now — plastic, fishing nets, warming sand — is recent. Measured against 100 million years, the past century of human impact is a very small fraction of their history. The question is whether it will be the fraction that ends the story.

The good news is that recovery is possible. Kemp’s ridley turtles dropped to fewer than 300 nesting females in the 1980s. Coordinated conservation efforts brought that number back to several thousand. Protected beaches have seen nesting populations rebound. Where the threats are reduced, the turtles come back.

For more on what happens when ancient species face sudden change, our piece on why the dinosaurs disappeared covers the last time Earth lost most of its largest animals — and what came after.