Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines. There are seven species of sea turtles. They are the leatherback sea turtle, green sea turtle, loggerhead sea turtle, Kemp’s ridley sea turtle, hawksbill sea turtle, flatback sea turtle and olive ridley sea turtle. Four of the species have been identified as “endangered” or “critically endangered” with another two being classed as “vulnerable”.
Leatherback sea turtle (Dermochelys coriacea)
Taxonomy and evolution
The origin of Sea Turtles goes back to the Late Jurassic (150 Ma) with genera such as Plesiochelys, from Europe. In Africa, the first marine turtle is Angolachelys, from the Turonian of Angola. The difference between a sea turtle and other turtles is that their legs and arms cannot retract back into their shells.
Sea turtles, along with other turtles and tortoises, are part of the order Testudines.
The seven living species of sea turtles are: leatherback sea turtle, green sea turtle, loggerhead sea turtle, Kemp’s ridley sea turtle,hawksbill sea turtle, flatback sea turtle and olive ridley sea turtle. All species except the leatherback are in the family Cheloniidae. The leatherback belongs to the family Dermochelyidae and is its only extant member.
Loggerhead sea turtle (Caretta caretta)
The species are primarily distinguished by their anatomy: for instance, the prefrontal scales on the head, the number of and shape ofscutes 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, it bears a mosaic of bony plates beneath its leathery skin. It is the largest sea turtle, measuring 6 to 9 feet (1.8 to 2.7 m) in length at maturity, and 3 to 5 feet (0.91 to 1.52 m) in width, weighing up to 1,500 pounds (680 kg). Other species are smaller, being mostly 2 to 4 feet (0.61 to 1.22 m) and proportionally narrower.
Sea turtles constitute a single radiation that became distinct from all other turtles at least 110 million years ago.
An Olive ridley turtle nesting on Escobilla Beach, Oaxaca,Mexico
The superfamily Chelonioidea has a world-wide distribution; sea turtles can be found in all oceans except for the polar regions. Some species travel between oceans. The flatback sea turtle is found solely on the northern coast of Australia. Kemp’s ridley sea turtle is found solely in the Gulf of Mexico and along the East Coast of the United States.
Behavior and ecology
Green sea turtle (Chelonia mydas)
Sea turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore again. During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed beds. Green sea turtles in particular are often found in Sargassum beds, a brown seaweed in which they find shelter and food. Once the sea turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season.
The habitat of a sea turtle has a significant influence on its morphology. Sea turtles are able to grow so large because of the immense size of their habitat: the ocean. The reason that sea turtles are much bigger than land tortoises and freshwater turtles is directly correlated with the vastness of the ocean, and the fact that they travel such far distances. Having more room to live enables more room for growth.
A green sea turtle breaks the surface to breathe.
As sea turtles are air breathing reptiles, they require to surface to breathe. Sea turtles can hold their breath for several hours. A sleeping sea turtle can remain under water for 4–7 hours. Sea turtles are almost always submerged, and, therefore, have an anaerobic system of energy metabolism. Although all sea turtles breathe air, under dire circumstances they may divert to anaerobic metabolism for long periods of time. When surfacing to breathe, a sea turtle can quickly refill its lungs with a single explosive exhalation and rapid inhalation. Their large lungs permit rapid exchange of oxygen and avoid trapping gases during deep dives.
A sea turtle entangled in a net
It takes decades for sea turtles to reach sexual maturity. After mating at sea, adult female sea turtles return to land to nest at night. Different species of sea turtles exhibit various levels of philopatry. In the extreme case, females return to the beach where they hatched. This can take place every two to four years in maturity. They make from one to eight nests per season. The mature nesting female hauls herself onto the beach, nearly always at night, and finds suitable sand on which to create a nest. Using her hind flippers, she digs a circular hole 40 to 50 centimetres (16 to 20 in) deep. After the hole is dug, the female then starts filling the nest with a clutch of soft-shelled eggs one by one until she has deposited around 50 to 350 eggs, depending on the species. Some species have been reported to lay 250 eggs, such as the hawksbill.
After laying, she re-fills the nest with sand, re-sculpting and smoothing the surface, and then camouflaging the nest with grasses until it is relatively undetectable visually. The whole process takes thirty to sixty minutes. She then returns to the ocean, leaving the eggs untended.
Turtle gender depends on sand temperature while the egg is incubating.
The hatchling’s gender depends on the sand temperature. Lighter sands maintain higher temperatures, which decreases incubation time and results in more female hatchlings.
It takes several decades for adult sea turtles to reach sexual maturity. The mature turtles migrate, sometimes for thousands of miles, to reach breeding sites. Male and female turtles mate in the water, and the males return to deep sea to feed. For several weeks, female sea turtles alternate between mating in the water and laying their eggs on land. Before laying her eggs, a female turtle will dig a hole in the sand with her hind flippers. She covers it with sand and returns to the ocean. About two months pass for the eggs to incubate under the sand. Afterwards, the eggs hatch, generally at night to avoid predation, and the hatchlings crawl to the water. They then swim out to sea to begin their own cycle of maturing and reproducing. Sea turtles can continue this cycle until they are 80 years old.
Legal notice posted by nest at Boca Raton, Florida
Most species of Sea Turtles will hatch during the night hours, but the Kemp Ridley Sea Turtle, will commonly hatch during the day. This has caused the Kemp Ridley to be the most endangered species of all sea turtles. Turtle nests that hatch during the day, are more prone to predators like birds, crab, sea birds, raccoon, ants and other animals on the beach. They also encounter more human activity on beaches after hatching and can run into human obstacles such as beach chairs, umbrellas, sand castles, as well as dogs and people on the beaches themselves. Larger hatchlings have a higher probability of survival than smaller individuals, which can be explained by the fact that larger offspring are faster and thus less exposed to predation. Predators can only functionally intake so much; larger individuals are not targeted as often. A study conducted on this topic shows that body size is positively correlated with speed, so larger turtles are exposed to predators for a shorter amount of time. The fact that there is size dependent predation on chelonians has led to the evolutionary development of large body sizes.
Hawksbill sea turtle (Eretmochelys imbricata)
The hatchlings then proceed into the ocean, where a variety of marine predators await them. In 1987, Carr discovered that the young of Chelonia mydas and Caretta caretta spent a great deal of their pelagic lives in floating sargassum beds, where there are thick mats of unanchored seaweed. Within these beds, they found ample shelter and food. In the absence of sargassum beds, sea turtle young feed in the vicinity of upwelling “fronts”. In 2007, Reich determined that green sea turtlehatchlings spend the first three to five years of their lives in pelagic waters. In the open ocean, pre-juveniles of this particular species were found to feed on zooplankton and smaller nekton before they are recruited into inshore seagrass meadows as obligate herbivores.
Instead of nesting individually like the other species, Ridley sea turtles come ashore en masse, known as an “arribada” (arrival). With the Kemp’s ridley sea turtles this occurs during the day.
Sea turtles feed on a wide range of animals and plants. They are mostly omnivorous in their adult life, except the green sea turtle which is herbivorous, changing from a carnivorous diet when young. Adult green sea turtles are herbivores. The jaw is serrated to help the turtle easily chew their primary food source—seagrasses and algae.
Sea turtles on a beach in Hawaii
Sea turtles on a beach in Hawaii
Juvenile green sea turtles are omnivores. They eat a wide variety of plant and animal life, including insects, crustaceans, seagrasses and worms. This diet shift has an effect on the green turtle’s morphology. Some species specialise on certain prey; sea sponges are the principal food of hawksbill sea turtles, constituting 70–95% of their diets in the Caribbean. Leatherback turtles feed almost exclusively on jellyfish and help control jellyfish populations.
Aside from sponges, hawksbills also feed on algae and cnidarians (including the Portuguese man o’ war), comb jellies and other jellyfish and sea anemones. Green sea turtles are commonly found in seagrass meadows closer inshore as herbivorous grazers. The leatherback turtle eats a variety of organisms such as seagrass, marine invertebrates including molluscs, jellyfish and shrimp and also fishes. It also consumes of soft coral, sea cucumbers and other soft-bodied creatures. The loggerhead sea turtle is omnivorous, feeding mainly on bottom-dwelling invertebrates, such as gastropods, bivalves, and decapods. The loggerhead has a greater list of known prey than any other sea turtle.
Other food items include sponges, corals, sea pens, polychaete worms, sea anemones, cephalopods, barnacles, brachiopods,isopods, insects, bryozoans, sea urchins, sand dollars, sea cucumbers, starfish, fish (eggs, juveniles, and adults), hatchling turtles (including members of its own species), algae, and vascular plants. During migration through the open sea, loggerheads eat jellyfish, floating molluscs, floating egg clusters, squid, and flying fish.
Olive ridley sea turtle (Lepidochelys olivacea)
The Kemp’s ridley turtle feeds on molluscs, crustaceans, jellyfish, algae or seaweed, and sea urchins. The olive ridley turtle is predominantly carnivorous, especially in immature stages of the life cycle. Animal prey consists of protochordates or invertebrates, which can be caught in shallow marine waters or estuarinehabitats. Common prey items include jellyfish, tunicates, sea urchins, bryozoans, bivalves, snails, shrimp, crabs, rock lobsters, and sipunculid worms. Aside from jellyfish, leatherbacks also feed on other soft-bodied organisms, such as tunicates and cephalopods.
Marine vertebrates maintain a balance of dissolved solutes and water in the body fluids by excreting excess salt ions. Like other marine reptiles, sea turtles rely on a specialized gland to rid the body of excess salt ions, because reptilian kidneys can not produce urine with a higher ion concentration than sea water. All species of sea turtles have a lachrymal salt gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water.
Leatherbacks face an increased osmotic challenge compared to other species of sea turtle, since their primary prey are jellyfish and other gelatinous plankton, whose fluids have the same concentration of salts as sea water. The much larger lachrymal salt gland found in leatherbacks may have evolved to cope with the higher intake of salts from their prey. A constant output of concentrated salty tears may be required to balance the input of salts from regular feeding, even considering leatherback tears can have a salt ion concentration almost twice that of other species of marine turtle.
Hatchlings depend on drinking sea water immediately upon entering the ocean to replenish water lost during the hatching process. Salt gland functioning begins quickly after hatching, so that the young turtles can establish ion and water balance soon after entering the ocean. Survival and physiological performance hinge on immediate and efficient hydration following emergence from the nest.
Commensalism with barnacles
Immature Hawaiian green sea turtle in shallow waters
Sea Turtles are believed to have a commensal relationship with some barnacles, in which the barnacles benefit from growing on turtles without harming them. Barnacles are small, hard shelled crustaceans found attached to multiple different substrates below or just above the ocean. The adult barnacle is a sessile organism, however in its larval stage it is planktonic and can move about the water column. The larval stage chooses where to settle and ultimately the habitat for its full adult life, which is typically between 5 to 10 years. A favorite settlement for barnacle larvae is the shell or skin around the neck of sea turtles. The larvae glue themselves to the chosen spot, a thin layer of flesh is wrapped around them and a shell is secreted. Many species of barnacles can settle on any substrate, however some species of barnacles have an obligatory commensal relationship with specific animals, which makes finding a suitable location harder. Around 29 species of “turtle barnacles” have been recorded. However it is not solely on sea turtles that barnacles can be found; other organisms also serve as barnacle’s settlements. These organisms include mollusks, whales, decapod crustaceans, manatees and several other groups related to these species.
Flatback sea turtle (Natator depressus)
Sea turtle shells are an ideal habitat for adult barnacles for three reasons. Turtles tend to live long lives, around 50 years, so barnacles do not have to worry about host death. Secondly, barnacles are suspension feeders. Sea turtles spend most of their lives swimming and following ocean currents and as water runs along the back of the turtle’s shell it passes over the barnacles, providing an almost constant water flow and influx of food particles. Lastly, the long distances and inter ocean travel these sea turtles swim throughout their lifetime, offers the perfect mechanism for dispersal of barnacle larvae. Allowing the barnacle species to distribute themselves throughout global waters is a high fitness advantage of this commensalism.
This relationship however is not truly commensal. While the barnacles are not directly parasitic to their hosts, they have negative effects to the turtles on which they choose to reside. The barnacles add extra weight and drag to the sea turtle, increasing the energy it needs for swimming and affecting its ability to capture prey, with the effect increasing with the quantity of barnacles affixed to its back.
Relationship with humans
“Manner in which Natives of the East Coast strike turtle”. Near Cooktown, Australia. From Phillip Parker King’s Survey. 1818.
Marine sea turtles are caught worldwide, although it is illegal to hunt most species in many countries. A great deal of intentional marine sea turtle harvests worldwide are for food. Many parts of the world have long considered sea turtles to be fine dining. Ancient Chinese texts dating to the fifth century B.C.E. describe sea turtles as exotic delicacies. Many coastal communities around the world depend on sea turtles as a source of protein, often harvesting several sea turtles at once and keeping them alive on their backs until needed. Coastal peoples gather sea turtle eggs for consumption.
To a much lesser extent, specific species of marine sea turtles are targeted not for their flesh, but for their shells. Tortoiseshell, a traditional decorative ornamental material used in Japan and China, comes from the carapace scutes of the hawksbill sea turtle.
Ancient Greeks and ancient Romans processed sea turtle scutes (primarily from the hawksbill) for various articles and ornaments used by their elites, such as combs and brushes. The skin of the flippers is prized for use as shoes and assorted leather goods.
The Moche people of ancient Peru worshipped the sea and its animals. They often depicted sea turtles in their art.
Leatherback sea turtles enjoy immunity from the sting of the deadly box jellyfish and regularly eat them, helping keep tropical beaches safe for humans.
Loggerhead sea turtle exits from fishing net through a turtle excluder device
Beach towns, such as Tortuguero, Costa Rica, have transitioned from a tourism industry that made profits from selling sea turtle meat and shells to an ecotourism-based economy. Tortuguero is considered to be the founding location of sea turtle conservation. In the 1960s the cultural demand for sea turtle meat, shells, and eggs was quickly killing the once abundant sea turtle populations that nested on the beach. The Caribbean Conservation Corporation began working with villagers to promote ecotourism as a permanent substitute to sea turtle hunting. Sea turtle nesting grounds became sustainable. Since the creation of a sea turtle, ecotourism-based economy, Tortugero annually houses thousands of tourists who visit the protected 22-mile beach that hosts sea turtle walks and nesting grounds.
Kemp’s ridley sea turtle (Lepidochelys kempii)