Sea turtle

• 1. Introduction

  ›

  Sea turtles cannot pull their heads and limbs into their shells. Almost every other turtle and tortoise can, but the seven living species of sea turtle have a tapered, fusiform body that closes off that ancient escape route. The trade-off is speed. A streamlined shape cuts friction and drag in the water, so these reptiles glide and swim with an ease their land-bound relatives never manage. They breathe air through lungs, yet a sleeping one can stay submerged for four to seven hours at a stretch. They navigate entire ocean basins, sometimes returning to the exact beach where they hatched. Five of the seven species are listed as threatened with extinction on the IUCN Red List. So what kind of animal trades its armor for the open sea, finds its way home across thousands of miles of featureless water, and glows in the dark under a blue light? The answers run from the salt glands behind their eyes to the barnacles riding on their backs.

• 2. Seven Of A Kind

  ›

  The flatback, green, hawksbill, leatherback, loggerhead, Kemp's ridley, and olive ridley make up the living roster of sea turtles. They split into two camps. Six are hard-shelled cheloniids in the family Cheloniidae. The leatherback stands alone as the only leathery-shelled dermochelyid, the sole extant member of the family Dermochelyidae. That leatherback is also the giant of the group. It reaches 1.4 to more than 1.8 m in length and weighs between 300 and 640 kg. At the other end of the scale sits the Kemp's ridley, the smallest species, as little as 60 cm long. The green turtle, second largest, grows to about 120 cm. Sizing up males against females offers no shortcut, because in all seven species the sexes grow to the same size. The clue is the tail. Adult males carry a long tail with a cloacal opening near the tip, while females have shorter tails with the opening near the base. Hatchlings and sub-adults give nothing away at all, showing no sexual dimorphism a person could read by eye. The superfamily name Chelonioidea and the family name Cheloniidae both trace back to the Ancient Greek word for tortoise, χελώνη.

• 3. Deep Roots

  ›

  Fossil marine turtles reach back to the Late Jurassic, 150 million years ago, in genera such as Plesiochelys from Europe. Africa's earliest marine turtle, Angolachelys, comes from the Turonian of Angola. Yet the modern sea turtles are not the heirs of every sea-going turtle that ever lived. They form a single radiation that became distinct from all other turtles at least 110 million years ago. Their closest living relatives are surprising. The snapping turtles, musk turtles, and the hickatee of the Americas join the sea turtles in a clade called Americhelydia. Desmatochelys padillai from the Early Cretaceous may be the oldest possible member of the lineage leading to modern sea turtles. It was a protostegid, a group that produced some enormous species before dying out at the end of the Cretaceous. The protostegids include the famous giants Archelon and Protostega, but their exact tie to modern sea turtles is still debated. They may be the sister group, or an unrelated line that convergently evolved similar adaptations. The earliest true sea turtle known from fossils is Nichollsemys, from the Early Cretaceous of Canada. In 2022, scientists described Leviathanochelys from Spain, a giant of the Late Cretaceous European seas and a true member of the Chelonioidea.

• 4. The Long Way Home

  ›

  Below the surface, the cues a traveler might rely on fall apart. Light fades fast with depth and refracts as the water moves, celestial markers vanish, and ocean currents push everything off course. To cross open water reliably, migrating sea turtles carry two tools at once: a bicoordinate magnetic map and a magnetic compass sense, a system known as magnetoreception. The map lets a turtle fix its position relative to a goal using both latitude and longitude. It reads more than one magnetic parameter running in opposite directions, such as magnetic field intensity and inclination angle. The compass sense lets it hold a steady magnetic heading. These senses appear to be inherited. Hatchlings swim in the directions that would keep them on course when exposed to the magnetic signatures of points along their species' migratory routes. Natal homing, the return to one's birthplace, is well documented here. Genetic testing across nesting sites shows that magnetic field is a more reliable predictor of genetic similarity than the physical distance between beaches. Nesting sites have even been recorded drifting as the magnetic field's isolines shift. Three theories compete to explain how turtles learn the natal site: inherited magnetic information, socially facilitated migration, and geomagnetic imprinting. Experiments that relocated turtle populations before hatching lend some support to imprinting, though the exact mechanism remains unknown.

• 5. Born In The Sand

  ›

  The nesting female hauls herself onto the beach almost always at night, finds suitable sand, and digs a circular hole 40 to 50 cm deep with her hind flippers. Into it she lays a clutch of soft-shelled eggs, anywhere from 50 to 350 depending on the species. Then she refills the nest, smooths and re-sculpts the surface, and camouflages it with vegetation until it is nearly invisible. She may even dig decoy nests. The whole performance takes 30 to 60 minutes, after which she returns to the sea and leaves the eggs untended. A female may lay 1 to 8 clutches in a single season. Most species nest individually, but ridley sea turtles arrive together in a mass event called an arribada, meaning arrival. The temperature of the sand decides the sex of the young, a system called temperature-dependent sex determination. Warmer temperatures yield females, cooler ones yield males. After incubating for 50 to 60 days, the eggs in a nest hatch together over a short window. The babies break the shell, dig up through the sand, and crawl to the water. Most species hatch at night, but the Kemp's ridley commonly hatches by day, leaving it more exposed to predators and beach activity. Size matters from the first minute. Larger hatchlings move faster, spend less time exposed, and survive better, a pressure that has pushed the lineage toward large body sizes over evolutionary time. In 1987, Carr found that young green and loggerhead turtles spend much of their pelagic lives in floating sargassum mats, sheltered and fed. In 2007, Reich determined that green turtle hatchlings spend their first three to five years in pelagic waters, feeding on zooplankton and smaller nekton before settling into inshore seagrass meadows as obligate herbivores.

• 6. Salt, Heat, And Breath

  ›

  A sea turtle's body stays hypotonic to the ocean around it, which means it must constantly shed excess salt. Reptilian kidneys cannot produce urine saltier than sea water, so every species relies on a lachrymal gland in the orbital cavity that weeps tears more concentrated than the sea itself. The leatherback faces the hardest version of this problem. Its main prey are jellyfish and other gelatinous plankton, whose fluids are as salty as sea water. Its much larger lachrymal gland may have evolved for that load, producing tears with a salt concentration almost twice that of other species. Hatchlings start drinking sea water the moment they enter the ocean to replace water lost during hatching, and their salt glands switch on quickly so they can balance ions and water right away. Temperature is its own challenge. All sea turtles are poikilotherms, but the leatherback can hold a body temperature 8 C-change warmer than the surrounding water through gigantothermy. Green turtles in the cooler Pacific take the opposite approach, hauling out onto remote islands to bask in the sun, a behavior seen only in a few places including the Galapagos, Hawaii, Europa Island, and parts of Australia. Then there is breathing. A foraging turtle may spend 5 to 40 minutes underwater, while a sleeping one can stay down for 4 to 7 hours, with respiration remaining aerobic for the vast majority of a voluntary dive. Force a turtle under, as a trawl net does, and its endurance collapses, making it far more likely to drown. When it surfaces it can refill its large lungs with a single explosive exhalation and rapid inhalation. Cold water below 45 to 50 F brings a separate danger called cold-stunning, which floats the turtle helplessly to the surface and leaves it unable to swim.

• 7. What They Eat

  ›

  The loggerhead, Kemp's ridley, olive ridley, and hawksbill stay omnivorous their whole lives, eating a sweeping menu of decapods, seagrasses, seaweed, sponges, mollusks, cnidarians, echinoderms, worms, and fish. Some species, though, specialize. The green sea turtle changes course as it ages: omnivorous as a juvenile, exclusively herbivorous once mature, with a serrated jaw built for sea grass and algae. The leatherback feeds almost entirely on jellyfish and helps keep their populations in check. The hawksbill is a sponge specialist, with sponges making up 70 to 95 percent of its diet in the Caribbean. The loggerhead is the opportunist, a flexible predator of slow-moving animals that will take terrestrial insects like ants, planthoppers, and beetles alongside sea creatures and plants. Its core diet leans on gelatinous medusae and ctenophores and on crustaceans, especially crabs. These diets reshaped the animals themselves. The limbs of sea turtles first evolved for locomotion, then more recently took on a feeding role, used to hold, swipe, and forage food so the turtle can eat more efficiently.

• 8. Living With People

  ›

  Ancient Chinese texts from the 5th century B.C.E. describe sea turtles as exotic delicacies, and the appetite never really faded. In England during the 1700s, sea turtles were eaten as a delicacy to near extinction, often as turtle soup. Coastal communities have long kept captured turtles alive on their backs until needed and gathered their eggs for food. Shells drew their own trade. Tortoiseshell, prized in Japan and China, comes from the carapace scutes of the hawksbill, and ancient Greeks and Romans worked those same scutes into combs, brushes, and ornaments for their elites. The Moche people of ancient Peru worshipped the sea and painted sea turtles into their art. J. R. R. Tolkien's poem Fastitocalon echoes a second-century Latin tale in the Physiologus of the Aspidochelone, a turtle so large that sailors land on its back, light a fire, and drown when it dives. The turning point came at Tortuguero, Costa Rica, considered the founding location of sea turtle conservation. In the 1960s, demand for meat, shells, and eggs was rapidly killing the once-abundant turtles nesting there. The Caribbean Conservation Corporation worked with villagers to make ecotourism a permanent replacement for hunting. Today Tortuguero hosts thousands of tourists each year along a protected 22 mi beach, where guided turtle walks give locals a financial stake in protecting the animals, and the guides now defend them from poaching. The wider picture stays precarious. The IUCN Red List rates two species as critically endangered and three as vulnerable, and all species sit in CITES Appendix I, which restricts international trade. Bycatch from imprecise fishing, with long-lining a major cause, kills turtles accidentally. Light pollution misleads hatchlings, the black-market trade in eggs and meat persists, plastic debris gets mistaken for jellyfish, oil pollution can harm turtles at every life stage, and warming sand may skew the sex ratio toward too many females. Even the passengers leave a mark. Barnacles ride along in a relationship that is not truly commensal, adding weight and drag that raise the energy a turtle needs to swim and hunt, the burden growing with every shell fixed to its back.