What do we know about sea turtles (Chelonioidea)?

Unlike land tortoises, of which there are many species, there are only seven species of sea turtle, and they are all classified as endangered (some as critically endangered) according to the IUCN (International Union for Conservation of Nature). They are found in all the world’s seas, except at the poles. They live in the pelagic zone of the ocean (in the first few metres below the surface).

They are reptiles known as ectotherms, which means that their body heat is not produced by their bodies but comes from the outside. In sea turtles, males and females are roughly the same size and show very little sexual dimorphism, which means it is very difficult to tell them apart just by looking at them.

Illustrations of the seven species of sea turtles, both adult and juvenile 

Sea turtles are land-dwelling animals that returned to live in the ocean during the course of their evolution, acquiring in the process all the characteristics necessary for marine life 1,2. This return to the ocean involved numerous physical and physiological changes for sea turtles. Some particularly well-preserved fossils (which still bear traces of soft tissue) have enabled us to learn more about the evolution of their skin and scales 3,4. One juvenile fossil in particular has even provided insights into their metabolism 5! From a very early stage, young sea turtles had to make their way across the beaches and dash towards the sea 6.

The oldest sea turtle fossils ever found date back approximately 150 to 110 million years ago (from the Cretaceous period) 2,7. Fossils of sea turtles have been found in many parts of the world and across all geological periods since 8. A rare specimen of one of the earliest sea turtles was found to be pregnant! It contained eggs which revealed that there had been a change in the structure of sea turtle eggs over the course of evolution. These evolved from hard shells (like those of today’s tortoises) to soft shells 9. The earliest sea turtles had elongated fingers and consequently large flippers, and were of considerable size. The largest sea turtle ever found, Archelon, measured approximately 4 m and had an estimated weight of around 2 tonnes. Four families appear to have survived the Cretaceous–Palaeogene extinction event (the major biological crisis that caused the extinction of the dinosaurs), two of which became extinct a little later, during the Eocene (around 40 million years ago) and the Oligocene (around 30 million years ago). Today, only two major families of sea turtles remain: the Cheloniidae and the Dermochelyidae (of which the leatherback turtle is the sole representative).

Sea turtles have lungs and must return to the surface to breathe. Perfectly adapted to marine life, they make long dives to feed and whilst they sleep. Most sea turtles dive for around 30 minutes whilst feeding, but can remain underwater for several hours whilst sleeping. The records are held by the leatherback turtle, which is capable of diving for 80 minutes and to a depth of up to 1,300 metres to feed.

A collection of fun facts

- Their eyesight is highly developed and their eyes are protected by three eyelids.

- Some turtles are fluorescent! Indeed, chance observations of hawksbill turtles have shown a response to blue UV light, in the form of fluorescence 10,11.

- Turtles communicate through vocalisations, and they do so from the very moment the hatchlings are tiny! These vocalisations are thought to begin whilst they are still inside the nest, particularly at the moment of hatching, which would help to synchronise the emergence from the egg and the emergence of all the hatchlings from the nest 12,13. These vocalisations are then thought to be used for intra- and inter-species communication 14.

- They possess salt glands to rid their bodies of excess salt, a consequence of living in salt water. This is known as osmoregulation. As soon as they hatch, the hatchlings need to rehydrate and drink seawater. Their salt glands quickly kick into action to counteract the excess salt in their bodies.

Reproduction

Females return to lay their eggs on the beaches where they were born, usually at night to avoid excessive exposure to the sun. In some turtles, such as green turtles, for example, a clear preference for a certain type of beach for nesting is observed (beach elevation, type of sand, beach size) 15. Most species, such as loggerhead and green turtles, lay eggs only every 2 to 3 years, but may lay up to 6 clutches per season (one every 2 weeks). Others, such as the Kemp’s ridley turtle, engage in synchronised nesting, known as an ‘arribada’, during which all the turtles emerge from the water at the same time to lay their eggs. The nesting season varies from one location to another and from one turtle population to another 16.

They come out of the water and dig a hole on the beach, into which they lay their eggs before covering them with sand and returning to the ocean. Egg-laying takes between 30 minutes and 1 hour, and each nest contains between 70 and 200 eggs, depending on the species. The eggs are enclosed in a soft shell and are about 4.5 cm in diameter. These eggs are sometimes preyed upon by humans, but also by small mammals such as dogs and mongooses.

The eggs develop in the nest for around two months before hatching (the duration varies depending on the species and the temperature). The sex of the young is determined by the incubation temperature. Low temperatures (below 28°C) produce males, whilst higher temperatures (above 32°C) produce females. Intermediate temperatures produce both males and females in varying proportions. An incubation temperature of around 30°C results in a balanced sex ratio (the relative proportion of males and females). These values depend not only on the species but also on microclimates 17,18.

As a result of climate change, nests are exposed to increasingly high temperatures, fluctuating humidity levels and human-induced environmental threats, posing serious challenges to the long-term survival of turtles 19–23. Indeed, an increasing proportion of females is being observed within populations 24, one of the most worrying effects of climate change due to its short-term consequences 25,26. These climate changes are also altering the nesting sites and breeding seasons of sea turtles 27–30. The incubation period is also affected by changes in temperature, sometimes causing deformities in hatchlings 31. A number of factors must be taken into account when studying the population dynamics of sea turtles 32.

All the hatchlings emerge at the same time and immediately make their way toward the ocean. They are then prey to birds and certain lizards. Only a very small proportion of the hatchlings (less than 1/1,000) survive to adulthood. If all goes well, they could live past 150 years! The hatchlings seem to be drawn to the glint of the sun, moon, and stars on the waves. Hatching tends to occur at night to avoid the intense heat of the sand and the presence of too many predators. Increasing urbanization and light pollution appear to disrupt their sense of direction, which remains a major threat to loggerhead hatchlings. It is essential to take these factors into account in urban planning to reduce the risk of hatchlings getting lost 33–44.

Once they reach the water, the baby turtles head out to sea and feed while drifting with the ocean currents. They are sometimes found in clumps of floating seaweed known as Sargassum. They then choose an area where they will stay for a while, until they reach adulthood. The males remain there and spend their lives in the open ocean, while the females begin a new migration from their home range to their nesting grounds. These behaviours may vary slightly depending on the species. Rare observations of (fertile) hybrids between two species of sea turtles (the hawksbill turtle and the loggerhead turtle) have been recorded 45. 

Ecology

-          Eating habits

All sea turtles are omnivores, but some have more or less pronounced preferences for certain types of food. Some turtles prefer plants, while others prefer a carnivorous diet consisting of jellyfish or small fish. Still others, such as the green sea turtle, change their diet over the course of their lives, shifting from a predominantly carnivorous diet when young to a predominantly herbivorous diet as adults 46.

Turtles play a crucial role in the ecological balance of their environment. For example, the leatherback turtle, which is capable of consuming its own weight in jellyfish daily to meet its high energy needs 47, contributes significantly to the regulation of jellyfish populations. The geographic and temporal distribution of jellyfish populations can even be used to predict where and when leatherback turtle populations will be present 48. Since jellyfish are major predators of baby fish, the presence of turtles promotes the development of many fish species.

The feeding habits of sea turtles also influence their vulnerability to marine plastic debris 49,50. For example, green turtles (which are omnivorous to herbivorous) ingest more debris than loggerhead turtles (which are primarily carnivorous). Large amounts of debris are found in their feces as well as in the contents of their intestines 49,51. This man-made debris has numerous impacts on marine life; it often leads to intestinal blockages, reduced food intake 52, and the absorption of toxic compounds 53,54. Microplastics have even been found in turtle eggs 55!

-          Habitat and migrations

Most sea turtles are long-distance migrants. Their migrations have recently been studied using Argos transmitters, devices that track their location via a satellite network (e.g., Dunbar et al., 2025; Marsac et al., 2026, 56, 57). A second widely used research method involves capturing individuals, tagging them, releasing them, and recapturing them later to conduct a census of individuals in specific geographic areas 58. The genetics of turtle populations is also studied to better understand population diversity, interpopulation exchange, and movement patterns 57,59–61. Only certain turtle species remain more sedentary. This is the case for certain groups of green sea turtles, which appear to remain within a more restricted area than their counterparts, with much shorter migrations 62. In all cases, understanding the behaviours of each sea turtle species and its migratory routes is essential for working toward their protection 63–67.

They travel through various locations to feed, grow, reproduce, and lay eggs. It is now thought that they navigate using the Earth’s magnetic field, beginning as soon as they leave the nest 68–72. It is thought that the magnetic signature of these locations enables them to return to lay their eggs on the beach where they were born 73,74. Winds and currents also serve as reference points to help them navigate 75. They appear to follow major migratory routes and travel through nearly all the world’s seas during their lifetimes, but seem to prefer equatorial regions when it comes to nesting. Only the loggerhead turtle nests in the Mediterranean (from early June to late August). The leatherback turtle, on the other hand, is the only species found as far north as Scandinavia. The leatherback has been observed following very specific temperature and salinity levels 76.

Sea turtles are sometimes seen accompanied by remoras, which rid them of parasites 77 (see my article about remoras). Many plant and animal species travel across the ocean by attaching themselves to the turtles’ shells.

-          Predation and vulnerability

Adult sea turtles are preyed upon by certain sharks (great white, tiger, and bull sharks), but their greatest predator remains humans. Whether through poaching (at sea and on beaches during nesting), accidental entanglement in nets and trawls, pollution (chemical and plastic)78–80, or the destruction of their natural habitat (particularly the large seagrass beds where they feed and the beaches where they come to lay their eggs). Even the most widespread species of sea turtles are now threatened by human activities 81. For example, leatherback turtles, which are found across vast regions, suffer a large number of injuries related to human activities (entanglement in fishing lines and nets, hooks, and boat propellers) 82. They are now critically endangered, and population declines are being observed at nesting sites 83.

Sea turtles are also affected by viruses 84 and bacteria. Certain bacteria threaten hatchlings and their healthy development as soon as the eggs are laid 85. Climate and environmental changes are likely to alter not only the development of these viruses and bacteria, but also the resistance of the turtles’ immune systems to them 84.

Sea turtles are severely affected by climate change. They suffer from shifts in temperature and seasons, changes in precipitation and humidity levels (as discussed earlier), as well as changes in ocean currents and winds, which contribute to a higher number of turtles washing ashore on beaches 86.

The presence of plastic waste in the ocean poses a significant threat to sea turtles, whether through pollution of their environment or the ingestion of this plastic waste (which is often mistaken for food) 49,51–54,87–89.

Some species, such as the hawksbill turtle, have been subjected to massive exploitation for the beauty of their scales and shells. Today, they are struggling to rebuild their populations and are critically endangered 90,91. 

-          Protection

Since all species of sea turtles are endangered, they are among the species prioritized for conservation and marine protected areas. However, recent studies show that very few concrete measures are being implemented within these marine protected areas, and that much remains to be done to protect them. We are learning more and more about their feeding, breeding, and migration areas, many of which are located in zones that are currently heavily used for fishing. An important approach would be to prioritize the protection of these areas 92. Increased monitoring of individuals of each species is essential to implement effective measures for their protection 76,93.

How to help them?

If you see any signs of nesting (an adult turtle coming ashore to lay eggs, tracks in the sand, signs of nests, or hatchlings), do not approach under any circumstances. Keep a distance of several meters. In mainland France—primarily along the Mediterranean coast—immediately contact the coordinator of the RTMMF (French Mediterranean Sea Turtle Network) at +33 6 16 86 26 86.

Best practices:

- Stay at least 10 m away

- Turn off all lights

- Do not use a flash

- Do not touch the turtles or their eggs

- Keep noise to a minimum

- Follow the instructions of authorized personnel

The RTMMF operates within the framework of the national scientific program “Observatoire des tortues marines” (OTM), coordinated by PatriNat (OFB-MNHN-CNRS-IRD). Response efforts in the event of nesting are coordinated by Émergence in close collaboration with the Mediterranean Coast Delegation of the French Office for Biodiversity and the National Museum of Natural History.

You can also contact one of the organizations that protect them.

-          A (non-exhaustive) list of the various organizations working to preserve them and offering numerous educational programs:

-          Émergence (France métropolitaine) : https://association-emergence.fr/

-          Kélonia (Saint-Leu, La Réunion) : https://museesreunion.fr/kelonia/

-          Cest-Med (Grau-du-Roi, France métropolitaine, Méditerranée) : https://www.cestmed.org/

-          CESTM (La Rochelle, France métropolitaine, Côte Atlantique) : https://www.aquarium-larochelle.com/comprendre/centre-etudes-et-soins-tortues-marines/

Te mana o te moana (Moorea, Polynésie Française) : https://www.temanaotemoana.org/ 

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