Facts About Sea Turtles

Sea turtle facts - we answer all you turtle questions. Green sea turtle, Maldives. Image.
Green Turtle, Maldives © Lisa Bauer

Sea turtles have been around for a really long time – since before the extinction of the dinosaurs 65 million years ago!  Sea turtles, also known as marine turtles, can be found in the waters of all the planet’s oceans, except in the Arctic.

There were once many species, but now we have only seven left: green, hawksbill, olive ridley, leatherback, loggerhead, kemp ridley and flatback. Sadly, six of the sea turtle species fall under IUCN’s category of being at risk of extinction. The seventh species, the flatback, is not listed due to lack of data and needs more research.

Sea Turtle Facts – Everything You Need To Know About Sea Turtles

Below you will find answers to some of the most frequently asked questions we get about turtles.

Sea Turtles FAQ - The Answers to All Your Sea Turtle Questions

Sea turtles cannot breathe underwater, however they can hold their breath for long periods of time. Sea turtles can hold their breath for several hours depending on their level of activity. For instance, a resting turtle can remain underwater for 4-7 hours whereas a foraging individual may need to surface more frequently.

When turtles hold their breath, their heart rate slows significantly to conserve oxygen –  up to nine minutes may pass between heart beats! Despite this adaptation, a stressed turtle – such as one entangled in ghost net – will deplete oxygen stores rapidly and may drown within minutes if unable to reach the surface.

Underwater image of a green turtle popping its head above water for a breath, Maldives. Image.
A green turtle popping its head above water for a breath, Maldives.
A green turtle resting on a reef, Kenya. Adopt a turtle in Kenya. Image.
A green turtle resting on a reef, Kenya.

References:

  • Long dive interval during hibernation: Hochscheid S, Bentivegna F & Hays GC 2005. First records of dive durations for a hibernating sea turtle. Biology Letters 1: 82-86.
  • Dive duration during activity: Hays GC, Akesson S, Broderick AC, Glen F, Godley BJ, Luschi P, Martin C, Metcalfe JD & Papi F 2001. The diving behaviour of green turtles undertaking oceanic migration to and from Ascension Island: dive durations, dive profiles and depth distribution. Journal of Experimental Biology 204: 4093-4098.
  • Hays GC, Hochscheid S, Broderick AC, Godley BJ & Metcalfe JD 2000. Diving behaviour of green turtles: dive depth, dive duration and activity levels. Marine Ecology Progress Series 208: 297-298.

Yes, sea turtles can drown as they have lungs just like other reptiles and similar to our own lungs. Sea turtles cannot breathe underwater, however they can hold their breath for long periods of time. However, a stressed sea turtle, such as one entangled in a ghost net for example, will deplete oxygen stores rapidly and may drown within minutes if unable to reach the surface.

Sea turtle drownings have been documented when turtles became caught in active fishing nets or ghost gear. Typical signs of drowning include a comatose state, lack of reflexes, water in the lungs and specific tissue alterations in the lung, which are visible on radiographs.

Not all turtles die immediately and while still in the water. Once turtles are comatose, they have about a 50 % chance of recovery.

Sea turtle coming up to breath
Sea turtle coming up for air
Hawksbill and olive ridley turtles entangled in ghost gear, Maldives. Image.
Entanglement in ghost gear can lead to drowning if the turtle is not able to surface to breathe

References:

  • Norton TM 2005. Chelonian emergency and critical care. Seminars in Avian and Exotic Pet Medicine 14 (2): 106-130.
  • Pointer IR and Harris ANM 1996. Incidental capture, direct mortality and delayed mortality of sea turtles in Australia’s Northern Prawn Fishery. Marine Biology 125: 813-825.

Yes, sea turtles can feel it when you touch their shell. Sea turtle shells consist of bones, which are covered by a layer of so-called scutes (plates). These scutes are made of keratin, the same material that human fingernails are made of. There are nerve endings enervating even the bones of the shell. These nerve endings are sensitive to pressure, for example from a touch on the back.

Drawing of green turtle shell showing 4 pairs of lateral scutes, graphic
Green turtles have 4 pairs of lateral scutes.
A green turtle swimming in the blue ocean with four pairs of lateral scutes clearly visible on the shell, image.
A green turtle with the lateral scutes clearly visible.

References:

  • Thomson JS 1932. The Anatomy of the Tortoise. Scientific Proceedings of the Royal Dublin Society.
  • Zangerl R 1969. The turtle shell. In: Gans C and Bellairs A (eds.): The Biology of Reptilia, Vol. 1: 311-319. Academic Press, New York.

No, sea turtles cannot retract their heads into their shells. Their bodies are well adapted to swimming with generally flatter shells as opposed to the high domed shells of tortoises. Sea turtles have the same muscles as other turtles, which allows them to pull back their heads, but there is simply not enough space in the shell to fully retract the head.

Green sea turtle on a reef in Kenya. Sea turtles have a flatter shell than tortoises and cannot retract their head into their shells.
Sea turtles have a flatter shell than tortoises and have no space to retract their head into their shell. Seen here a green sea turtle.
Tortoises have a domed shell that allows them to retract their head into their shell. Image of a Greek spur-thighed tortoise.
Tortoises have a domed shell that allows them to retract their head into their shell. Seen here a Greek spur-thighed tortoise.

References:

  • Wyneken J 2001. The Anatomy of Sea Turtles: Part II. U.S. Department of Commerce NOAA Technical Memorandum NMFS-SEFSC-470, 53-112.
  • Valente ALS, Cuenca R, Zamora M, Parga ML, Lavin S, Alegre F and Marco I 2007. Computed tomography of the vertebral column and coelomic structures in the normal loggerhead sea turtle (Caretta caretta). The Veterinary Journal 174: 362-370.

Sea turtles can survive in the wild with only three flippers as many sporadic sightings of turtles with such injuries show – they learn to adapt to a missing limb just like humans. If they are missing a front flipper, they learn to compensate by using their opposite back flipper when swimming, for example. We currently do not know if a missing limb is going to significantly influence the general life span of these turtles, as systematic studies are not easy to conduct.

One or two missing hindlimbs is going to have a great impact on female turtles, as they use these flippers to dig their nests. Unsuccessful digging can lead to abandoned nesting attempts.

A hawksbill turtle missing a front flipper, Maldives. Underwater. Image.
A hawksbill turtle missing a front flipper, Maldives. © Chiara Fumagalli.
A female green turtle missing a back flipper on a reef, Maldives. Image.
A female green turtle missing a back flipper, Maldives.

References:

  • Spotila JR & Tomillo PS (eds.) 2015. The Leatherback Turtle: Biology and Conservation. Johns Hopkins Press, Maryland, US.

Most barnacles do not hurt sea turtles, as they are only attached to the shell or skin on the outside. Others though burrow into the skin of the host and might cause discomfort and provide an open target area for following infections.

Excessive barnacle cover can be a sign of general bad health of a turtle. Usually sea turtles are debilitated first, and then become covered in an extensive amount of other organisms, such as barnacles and algae.

Luckily turtles are very resilient and can sometimes recover from such infestations.

Turtle with high barnacle cover on face Maldives
Hawksbill turtle, Orimas, with high barnacle cover ©Karin Nistler, Prodivers.
Hawksbill turtle having recovered from high barnacle cover
Hawksbill turtle, Orimas, recovered from high barnacle cover ©Stephanie Köhnk.

References:

  • Monroe R & Limpus CJ 1979. Barnacles on turtles in Queensland waters with descriptions of three new species. Memoires of the Queensland Museum 19: 197-223.
  • Ross A & Frick MG 2007. From Hendrickson (1958) to Monroe & Limpus (1979) and Beyond: An Evaluation of the Turtle Barnacle Tubicinella cheloniae. Marine Turtle Newsletter 118: 2-5.

No. Once a nest has been laid, the female never returns to it. The eggs and hatchlings are left to fend for themselves and locate the water upon emerging.

   

Reference:

  • Spotila JR 2004. Sea Turtles: A Complete Guide to Their Biology, Behaviour and Conservation. The John Hopkins University Press, Baltimore, USA.

Yes, sea turtles have tails. In fact, once sea turtles reach sexual maturity, the size of the tail can be used to reliably distinguish between male and female sea turtles. Males develop much longer tails – which may extend past their rear flippers – whereas females tails remain much shorter.

The tail of both male and female sea turtles contain a cloaca – a posterior opening for digestive, urinary and reproductive tracts – and, as such, the tail plays a crucial role in sea turtle reproduction.

An adult male green turtle from behind showing a long tail, Maldives. Image.
An adult male green turtle has a long tail.
Adult female green turtle with small tail swimming in sea grass meadow, Maldives, image.
An adult female green turtle has a short tail.

References:

  • Godley, B.J., Broderick, A.C., Frauenstein, R., Glen, F. and Hays, G.C. 2002. Reproductive seasonality and sexual dimorphism in green turtles. Marine Ecology Progress Series, 226, 125-133.
  • Hendrickson, J.R. 1958. The green turtle Chelonia mydas in Malaya and Sarawak. Proc Zool Soc Lond, 130, 455-535.

There are several theories as to how sea turtles are able to the same beach to nest, but none have yet been proven. The most common theories are:

  • They can detect both the angle and intensity of the earth’s magnetic field. Using these two characteristics, a sea turtle may be able to determine its latitude and longitude, enabling it to navigate virtually anywhere. Early experiments seem to show that sea turtles have the ability to detect magnetic fields. Whether they actually use this ability to navigate is the next idea being investigated.
  • It is believed that hatchlings imprint the unique qualities of their natal beach while still in the nest and/or during their trip from the nest to the sea. Beach characteristics used may include smell, low-frequency sound, magnetic fields, the characteristics of seasonal offshore currents and celestial cues.
  • Younger female turtles may follow older, experienced nesting turtles from their feeding grounds to the breeding site.
Two green sea turtles leaving tracks in the sand as they return to sea after nesting on a beach in Oman. Image.
Two green sea turtles returning to sea after nesting on a beach in Oman. ©Zoe Cox
A nesting green turtle makes her way around beach furniture, Maldives. Image
A nesting green turtle makes her way around some beach furniture at a resort in the Maldives. © Chiara Fumagalli

References:

  • Lohmann KJ and Lohmann CMF 1992. Orientation to oceanic waves by green turtle hatchlings. Journal of Experimental Biology 171: 1-13.
  • Lohmann KJ, Lohmann CMF, Ehrhart LM, Bagley DA and Swimg T 2004. Geomagnetic map used in sea-turtle navigation. Nature 428: 909-910.

During the course of incubation, the embryo grows inside the egg from a few cells at the beginning to a self-sufficient animal hatching (coming out of the egg) some 50 to 80 days later. Hatching typically begins with the baby turtle using a small, point keratinous bump on the tip of their snout (called caruncle) to break the egg.

As there are many eggs in each nest, and they are buried several centimetres in the sand, baby turtles will hatch almost synchronously with their siblings to allow simultaneous digging to make their way out of the nest faster! This behaviour is called ‘social facilitation’ as the synchronous effort of many individuals might be needed to dig successfully through the column of sand above the nest chamber. This effort may take 3-5 days in total as intense digging activity also needs some time of resting.

Once one of the hatchlings decides it is time to continue, its digging will trigger its siblings to do the same. Hatchlings are also clever in avoiding high temperatures. Emerging during daylight and under the hot sun can cause heat exhaustion, and the hatchlings will be more exposed to predators. Therefore, most hatchlings will emerge after the sand cools in the late afternoon or at night, or during cool cloudy days. Hatching simultaneously is also a clever idea to decrease the overall predation rate of the baby turtles as they leave the beach and swim out to sea. Once they emerge at the sand surface, they will quickly crawl towards the ocean.

Green turtle hatchling emerging from the egg. Image.
A green turtle hatchling emerging from the egg.
Leatherback hatchlings, Pacuare Reserve, Costa Rica
Leatherback hatchlings, Pacuare Reserve, Costa Rica © Joana Hancock

References:

  • Carr, A., & Hirth, H. (1961). Social facilitation in green turtle siblings. Animal Behaviour, 9(1-2), 68-70.
  • Rusli, M. U., Booth, D. T., & Joseph, J. (2016). Synchronous activity lowers the energetic cost of nest escape for sea turtle hatchlings. Journal of Experimental Biology, 219(10), 1505-1513.
  • Saito, T., Wada, M., Fujimoto, R., Kobayashi, S., & Kumazawa, Y. (2019). Effects of sand type on hatch, emergence, and locomotor performance in loggerhead turtle hatchlings. Journal of experimental marine biology and ecology, 511, 54-59.

The olive and kemp’s ridley sea turtles are the smallest species, growing only to about 70 cm (just over 2 feet) in shell length and weighing up to 45 kg (100 lbs). Leatherbacks are the largest sea turtles. On average leatherbacks measure 1.5 – 2m (4-6 ft) long and weigh 300 – 500 kg (660 to 1,100 lbs). The largest leatherback ever recorded was 2,56 m (8.4 ft) long and weighed 916 kg (2,019 lbs) !

Kemp’s Ridley

Nesting female kemp's ridley turtle, nicknamed
Nesting female kemp’s ridley turtle, nicknamed “Mij”, laying 116 eggs on Galveston’s East Beach. ©Ron Wooten, Wildscreen Exchange.

55.6-66.0 cm carapace length, weight range of 25-54 kg for nesting females.

References:

  • Marquez-M R 1994. Synopsis of Biological Data on the Kemp’s Ridley Turtle, Lepidochelys kempi (Garman, 1880). NOAA Technical Memorandum NMFS-SEFSC-343.

Olive Ridley

Female Oliver ridley turtle nesting on the beach during arribada
Nesting female olive ridley turtle ©Susie Gibson.

Curved carapace length 52.5-80.0 cm, weight less than 50 kg (average 35.7 kg) for nesting females.

References:

  • Qureshi M 2006. Sea turtles in Pakistan. In: Shanker K and Choudhury BC (Eds.). Marine Turtles of the Indian Sub- continent. Heydarabad: India Universities Press, pp. 217–224.Reichart HA 1993.
  • Reichart HA 1993. Synopsis of biological data on the olive ridley sea turtle Lepidochelys olivacea (Eschscholtz 1829) in the western Atlantic. NOAA Technical Memorandum NMFS-SEFSC-336.

Hawksbills

Adult male hawksbill turtle swimming in the blue, Maldives. Image
Adult male hawksbill, Maldives.

Nesting females reported between 53.3 and 95.5 cm carapace length, with weight between 27.2 and 86.2 kg.

References:

  • Witzell WN 1983. Synopsis of biological data on the hawksbill turtle, Eretmochelys imbricata (Linnaeus, 1766). No. 137. Food & Agriculture Org.

Green turtles

Adult green turtles mating, Malsdives. Image.
Adult green turtles mating, Maldives.

Nesting green females reported curved carapace length 75-134 cm, weight (after egg deposition) 45-250 kg (!).

References:

  • Hirth HF 1997. Synopsis of the Biological Data on the Green Turtle Chelonia mydas (Linnaeus, 1758). Vol. 2. Fish and Wildlife Service, US Department of the Interior.

Flatbacks

Nesting flatback sea turtle ©Lyndie Malan / CC BY-SA.

Ones study (Ref. 1) found nesting females have a mean curved carapace length 86.3 cm, and mean weight of 67.4 kg. Another study (Ref. 2) found flatbacks to be between 87.5-96.5 cm.

References:

  • Schäuble C, Kennett R and Winderlich S 2006. Flatback Turtle (Natator depressus) nesting at Field Island, Kakadu National Park, Northern Territory, Australia, 1990-2001. Chelonian Conservation and Biology 5: 188-194.
  • Limpus CJ 1971. The Flatback Turtle, Chelonia depressa Garman in Southeast Queensland, Australia. Herpetologica 27: 431-446.

Loggerheads

Adult female loggerhead, Oman.

The largest loggerhead was reported stranded in 1938 on the welsh coast (Tenby, Pembrokshire) with a carapace length of 146.7 cm. The turtle was highly emaciated and missing a front flipper. It was reported to weigh only 27.8 kg, which is severely underweight for a turtle of that size. On average nesting and therefore adult female loggerheads have a curved carapace length of 65.1-114.9 cm and weigh between 40.0 and 180.7 kg. Males fall into the same size range (79.0-104.0 cm curved carapace length).

References:

  • Brongersma LD 1972. European Atlantic turtles. Zoologische Verhandlingen 121, Leiden.
  • Dodd C 1988. Synopsis of the biological data on the loggerhead sea turtle. Ecology 88: 1-119.

Leatherbacks

Leatherback Turtle, Claudia Lombard, USFWS
Adult leatherback turtle ©Claudia Lombard, USFWS.

143.8-169.5 cm curved carapace length, weight 259-506 kg recorded for nesting females all around the world. Largest ever recorded specimen was found dead on a beach on the coast of Wales. The adult male turtle weighed 916 kg and its shell was 256.5 cm long. An autopsy revealed that it had drowned.

References:

  • Eckert KL and Luginbuhl C 1988. Death of a Giant. Marine Turtle Newsletter 43: 2-3.
  • Eckert KL, Wallace BP, Frazier JG, Eckert SA and Pritchard PCH 2012. Synopsis of the Biological Data on the Leatherback Sea Turtle (Dermochelys coriacea). Biological Technical Publication BTP-R4015-2012, US Fish & Wildlife Service.

There is no way to determine the exact age of a sea turtle from its physical appearance other than to establish if it is a hatchling, juvenile or adult, depending on its size.

In the Indian Ocean, hawksbills and greens are smaller than their counterparts elsewhere. Good information on size at maturity does not exist for the northern Indian Ocean. It’s tricky to tell visually if an animal is mature or not, especially females: they can be big enough but not actually be sexually mature.

After it’s death, the age of a turtle can be determined by a technique called “skeletochronology”, whereby the humerus (arm bone) is examined. These bones reveal growth rings that allow the turtle’s age to be calculated, much like we can calculate the age of a tree.

Green turtle hatchling Claire, 5 cm long, at ORP turtle rescue centre, Maldives. Image.
Green turtle hatchling Claire, only 5 cm long.
Juvenile hawksbill turtle, 45 cm long, swimming on a reef, Maldives. Image.
Euan, a juvenile hawksbill turtle, 45 cm long.
Hope, an olive ridley turtle hatchling 13 cm long at the ORP rescue centre Maldives. Image.
Baby Hope, an olive ridley hatchling, 13 cm long.
An adult male green turtle, Bjorn, approximately 100cm long, swimming in the blue, Maldives. Image.
An adult male green turtle, Bjorn, approximately 100cm long. ©Lisa Bauer.

References:

  • Snover ML 2002. Growth and ontogeny of sea turtles using skeletochronology: methods, validation and application to conservation. PhD Thesis, Duke University, USA.

Leatherbacks can dive to a depth of more than 1,000 meters (3,000 feet) in search of their prey, jellyfish. The hard-shelled species dive at shallower depths, typically up to 175 meters (500 feet) though olive ridleys have been recorded at over 200 meters (660 feet).

The leatherback is adapted to deep dives because it lacks a rigid breastbone. Its leathery shell also absorbs nitrogen, reducing problems arising from decompression during deep dives and resurfacing (i.e., “the bends”).

Green turtle diving in the blue, Maldives. Image.
Green turtle diving, Maldives.
Aguadormi, an identified Hawksbill sea turtle swimming in the blue, Maldives. Image.
Hawksbill Turtle, Maldives.

References:

  • García-Párraga D, Crespo-Picazo JL, Bernaldo de Quirós Y, Cervera V, Martí-Bonmati L, Díaz-Delgado J, Arbelo M, Moore MJ, Jepson PD and Fernández A 2014. Decompression sickness (‘the bends’) in sea turtles. Diseases of aquatic organisms 111: 191-205.
  • Houghton JDR, Doyle TK, Davenport J, Wilson RP and Hays GC 2008. The role of infrequent and extraordinary deep dives in leatherback turtles (Dermochelys coriacea). The Journal of Experimental Biology 211: 2566-2575.
  • McNahon CR, Bradshaw CJA and Hays GC 2007. Satellite tracking reveals unusual diving characteristics for a marine reptila, the olive ridley turtle Lepidochelys olivacea. Marine Ecology Progress Series 329: 239-252.

Most sea turtles are predated during their hatchling stage. Their small size and limited swimming speed makes them easy targets for crabs, sea birds, and wild and domestic mammals on their way from their nest to the beach. Synchronized, mass hatching is their main strategy to avoid predators at this stage.

Once in the water, hatchlings are still highly predated by carnivorous fish, sea birds, and pretty much any animal with a big appetite and a big mouth!

As they grow older, their hard-shell provides them a shield from predator attacks, making them harder to get eaten. Sharks and killer whales are the main predator of adult sea turtles. Shark avoidance by sea turtles is hard to study in the wild, and most of what we know is directly observing how sea turtles behave around these predators. Encounters between sharks and turtles are difficult to observe, as shark populations are now severely depleted throughout the world.

However, some studies looking at  turtle diving behavior suggest that U-shaped dives of green turtles might function as both resting dives and predator avoidance, while a slow approach to the surface to breathe, using a passive ascent under positive buoyancy, allows the turtles to scan the habitat for predators before surfacing. If an attack is imminent, sea turtles have been seen turning their shell to the shark’s mouth as it approaches, thus preventing the shark from biting their flippers or soft tissues, and swimming fast in the opposite direction.

A feral dog patrolling the beach as an olive ridley turtle returns to sea after nesting. Image.
A feral dog patrolling the beach as an olive ridley turtle returns to the sea, Costa Rica. ©Ben Cherry, Wildlife Exchange.
Hawksbill turtle with a possible shark bite, on a reef in Maldives. Image.
Hawksbill turtle with a possible shark bite on a reef in Maldives. ©Stephanie Köhnk.

References:

  • Heithaus, M. R., Wirsing, A. J., Thomson, J. A., & Burkholder, D. A. (2008). A review of lethal and non-lethal effects of predators on adult marine turtles. Journal of Experimental Marine Biology and Ecology, 356(1-2), 43-51.

Just like other reptiles, sea turtles have lungs. They have a slightly different structure than mammalian lungs, but work just as well when it comes to exchanging gases (oxygen and carbondioxide). The lungs are located right under the carapace and the vertebral column.

Ventilation of the lungs (breathing) is achieved by movements of the muscles attached to the pelvic and shoulder girdles and to the plastron. You can sometimes see turtles ‘rocking‘ their shoulders when they are not underwater; this movement of the muscle masses around the shoulder also helps them breathe by changing the pressure inside the lungs.

Sea turtle coming up to breath
Sea turtle coming up for air, Maldives.
A green turtle comes up for a breath, Maldives. Image.
A green turtle comes up for a breath, Maldives.

References:

  • Wyneken J 2001. The Anatomy of Sea Turtles: Part II. U.S. Department of Commerce NOAA Technical Memorandum NMFS-SEFSC-470, 53-112.

Sea turtles can perform a variety of thermoregulatory behaviours to stay warm in colder waters, such as basking at the surface of the water during the warmer times of the day, and in some cases, on beaches! Also, they tend to restrict their movements to warmer, tropical and temperate waters, or microclimates where water heats up faster (eg. shallow reefs and seagrass lagoons).

Leatherback turtles, however, are an exception. They may be found feeding in very colder waters (e.g. north of the arctic circle), where jellyfish are abundant. This is only possible as their black carapace acts as a good insulator, as well as their large layer of body fat (blubber) under their skin. This species also possess physiological control mechanisms that promote optimal heat exchange between their circulatory system and muscles. 

Green sea turtles seek out shallow sea grass lagoons where the water is warm, such as this one in Maldives. Image.
Green sea turtles seek out shallow sea grass lagoons, such as this one in Maldives.
Nesting leatherback sea turtle, Gandoca-Manzanillo National Wildlife Refuge, Costa Rica, image
Leatherback turtles can be found in cooler waters than other turtle species. ©Joana Hancock.

References:

  • Bostrom, B. L., Jones, T. T., Hastings, M., & Jones, D. R. (2010). Behaviour and physiology: the thermal strategy of leatherback turtles. PLoS One, 5(11).
  • Paladino, F. V., O’Connor, M. P., & Spotila, J. R. (1990). Metabolism of leatherback turtles, gigantothermy, and thermoregulation of dinosaurs. Nature, 344(6269), 858-860.

The male sea turtle climbs onto the female turtle’s back and holds on to her carapace with the long, sharp claws of his front flippers. The way he hooks on to the edge of the female’s shell often results in a scratched shell and bleeding wounds in the soft parts of her body. Copulation can take place on the surface or under water.

Both male and females turtle reproductive organs are located at the base of their tails in their cloaca – a combined intestinal, urinary, and reproductive organ. Male sea turtles have a very long tail while female sea turtles have a short tail. The male’s penis is located in his cloaca. He reaches his tail underneath the posterior end of the female’s shell to inseminate her cloaca.

Read more about how sea turtles mate here.

A pair of green sea turtles mating in Maldives. Video © Sasha Haslim.

Sea turtles use different cues to navigate the oceans. During the very early stage, when a turtle first enters the sea, it uses the direction of the waves for orientation. Usually, swimming directly perpendicular towards the waves will take hatchlings directly seaward and away from the shore. The juvenile turtles do not need to see the waves for that, but use the orbital movement of the waves, most likely detected with the inner ear (sense of balance).

Part of their navigation system in the open ocean is the earth’s magnetic field! Depending on the specific location on earth, the magnetic field has a specific inclination and intensity. Sea turtles seem to be able to sense this signature. When turtles are placed experimentally in the magnetic field of a different location with the use of an electric coil, they will adapt swimming directions as if they have changed to the location of the magnetic field!

Green turtle swimming in the big blue towards the sunlight. Image.
Green turtle swimming in the big blue, Maldives.
Hawksbill turtle swimming in the blue surrounded by fish. Image.
Hawksbill turtle swimming in the blue, Maldives.

References:

  • Lohmann KJ and Lohmann CMF 1992. Orientation to oceanic waves by green turtle hatchlings. Journal of Experimental Biology 171: 1-13.
  • Lohmann KJ, Lohmann CMF, Ehrhart LM, Bagley DA and Swimg T 2004. Geomagnetic map used in sea-turtle navigation. Nature 428: 909-910.

When sea turtles are juveniles, it is very difficult to tell their sex by eye as they do not differ externally. However, after reaching sexual maturity male sea turtles develop a long tail, which houses the reproductive organ. The tail may extend past the hind flippers.

Female turtles have a short tail, which generally doesn’t extend more than 10 cm (4 inches) past the edge of the carapace. Male sea turtles (except leatherbacks) have elongated, curved claws on their front flippers to help them grasp the female when mating.

The sex of a sea turtle embryo is determined by the temperature of the sand: warm temperatures result in more females while cooler temperatures result in more males.

Male green turtle tail
Male green turtle tail
Female green turtle tail
Female green turtle tail

Climate change can impact sea turtles in various ways. Firstly, increasingly warm seas pose a threat to vital sea turtle habitats and food sources, such as coral reefs. Cooler ocean temperatures are associated with higher productivity, thus providing more food for many organisms – including sea turtles. Warmer oceans and less food available leads to decreased nesting activity and fewer sea turtles being born. These effects have been monitored in some populations, e.g. in the Western Pacific Ocean and Western Atlantic.

Secondly, sea level rise can be very detrimental for sea turtle nesting beaches and it can be increasingly difficult for turtles to find appropriate spaces to deposit their eggs. This will also lead to decreased nesting activity and fewer sea turtles being born.

In addition to that, the sex of a sea turtle is not determined purely genetically as it is in humans: the temperature during the development of the embryo will determine whether a male or a female turtle hatches. Higher temperatures result in females, lower in males. Increasing sand temperatures on nesting beaches can therefore shift the sex ratio of hatchlings to almost entirely female. As a result, turtles can have a problem reproducing in the future.

A juvenile hawksbill turtle looking for food at a bleached coral reef in Maldives. ©Kylie Merritt.
Increased sand temperature on nesting beaches can shift the sex ratio of sea turtle hatchlings to almost entirely female. Green turtle hatchlings, Maldives, image
Increased sand temperature on nesting beaches can shift the sex ratio of sea turtle hatchlings to almost entirely female. ©Chiara Fumagalli.

References:

  • Abella Perez E, Marco A, Martins S and Hawkes L 2016. Is this what a climate change-resilient population of marine turtles looks like? Biological Conservation 193: 124-132.
  • Chaloupka M, Kamezaki N and Limpus C 2008. Is climate change affecting the population of the endangered Pacific loggerhead sea turtle? Journal of Experimental Marine Biology and Ecology 356: 136-143.
  • Fuentes MMPB, Limpus CJ and Hamann M 2011. Vulnerability of sea turtle nesting grounds to climate change. Global Change Biology 17: 140-153.
  • Pike DA, Antworth RL and Stiner JC 2006. Earlier Nesting Contributes to Shorter Nesting Seasons for the Loggerhead Sea Turtle, Caretta caretta. Journal of Herpetology 40: 91-94.

In the Maldives, the turtles we observe on the reefs show extremely high site fidelity, meaning they do not, in general, move from reef to reef to find food but have a “home reef”.

Upon reaching maturity most species travel long distances every few years for a breeding migration (from their feeding grounds to their breeding sites and back). These migrations can be hundreds or thousands of kilometers and take several months.

The leatherback turtle can travel 16,000 km (10,000 miles) or more each year, crossing the entire Pacific Ocean in search of jellyfish, while loggerheads have been tracked traveling from Japan to Baja, a distance of 13,000 km (8,000 miles). The longest recorded green turtle migration was 3,979 km (2,472 miles) from Chagos to Somalia.

Sea turtles are generally slow swimmers traveling at a speed of 2.8 to 10 km/h (1.7 to 6.2 mp/h) with slight variation between the species.

The leatherback sea turtle has been recorded swimming as fast as 35 km/h (22 mph), according to the San Diego Zoo. This speed is usually just achieved during brief bursts, for example due to flight reactions.

References:

  • Eckert SA 2002. Swim speed and movement patterns of gravid leatherback sea turtles (Dermochelys coriacea) at St Croix, US Virgin Islands. Journal of Experimental Biology 205: 3689-3697.
  • Papi F, Luschi P, Croisio E and Hughes GR 1997. Satellite tracking experiments on the navigational ability and migratory behaviour of the loggerhead turtle Caretta caretta. Marine Biology 129: 215-220.
Green turtle swimming in the blue, Maldives. Image.
Green turtle swimming in the deep blue, Maldives.
Green turtle swimming neat a reef, Oman. Image
Green turtle swimming neat a reef, Oman.
Male hawksbill popping head above surface for air, Maldives. Image.
Marvin, male hawksbill, coming up for a breathe, Maldives.

Sea turtles can hold their breath for several hours, depending on their level of activity.

If they are sleeping, they can remain underwater for several hours. In cold water during winter, when they are effectively hibernating, they can hold their breath for up to 7 hours. This involves very little movement.

Although turtles can hold their breath for 45 minutes to one hour during routine activity, they normally dive for 4-5 minutes and surfaces to breathe for a few seconds in between dives.

However, a stressed turtle, entangled in a ghost net for instance, quickly uses up oxygen stored within its body and may drown within minutes if it cannot reach the surface.

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References:

  • Hays GC, Akesson S, Broderick AC, Glen F, Godley BJ, Luschi P, Martin C, Metcalfe JD & Papi F 2001. The diving behaviour of green turtles undertaking oceanic migration to and from Ascension Island: dive durations, dive profiles and depth distribution. Journal of Experimental Biology 204: 4093-4098.
  • Hays GC, Hochscheid S, Broderick AC, Godley BJ & Metcalfe JD 2000. Diving behaviour of green turtles: dive depth, dive duration and activity levels. Marine Ecology Progress Series 208: 297-298.
  • Hochscheid S, Bentivegna F & Hays GC 2005. First records of dive durations for a hibernating sea turtle. Biology Letters 1: 82-86.
  • Lutz PL and Musik JA (eds.) 1996. The Biology of Sea Turtles Volume I. CRC Press.

The actual documentation of a sea turtle’s age in the wild is difficult or nearly impossible. Individual turtles can be tracked for a shorter time of six month to three years with the help of satellite transmitters. Longterm studies rely on capture-recapture principle, just like our turtle photo id project. Each photo of a turtle represents a recapture event documenting that the individual is still alive.

A study of nesting green turtles in Hawaii observed female turtles returning to nest for up to 38 years after they were first identified. Assuming the average age at first nesting activity of 24 years, this would show that green turtles can live to up to at least 62 years.

Similar estimates have been made for loggerhead turtles.

An adult green turtle resting on a reef in Maldives, image
An adult green turtle
Hawksbill turtle resting on the reef, Maldives
An adult hawksbill turtle

References:

  • Dodd C 1988. Synopsis of the biological data on the loggerhead sea turtle. Ecology 88.
  • Humburg IH and Balazs GH 2014. Forty Years of Research: Recovery Records of Green Turtles Observed or Originally Tagged at French Frigate Shoals in Northwestern Hawaiian Islands, 1973-2013. NOAA Technical Memorandum NMFS-PIFSC-40.

It is difficult to predict the exact incubation time for turtle eggs. The hatching date depends on variables such as the temperature during incubation and the depth of the nest, for example.

In the Maldives, sea turtle nests incubate for approximately 49 to 62 days, whereas in colder regions around the world incubation can take up to 80 days.

In Kenya, turtle nests take between 40 and 73 days to incubate before hatching, varying between different nesting seasons and beaches along the coast. The shortest incubation period was recorded on the northern part of the coast.

Incubation time in Iran, in the Gulf of Oman, is at mean 61.7 days.

Loggerhead turtle nest, Cap Verde, image
Loggerhead turtle nest, Cap Verde.
Green turtle hatchlings in the Maldives emerge from their nest after between 49 and 62 days, image
Green turtle hatchlings in the Maldives emerge from their nest after between 49 and 62 days.

References:

  • Hudgins JA, Hudgins EJ, Ali K and Mancini A 2017. Citizen science surveys elucidate key foraging and nesting habitat for two endangered marine turtle species within the Republic of Maldives. Herpetology Notes 10: 463-471.
  • Matsuzawa Y, Sato K, Sakamoto W and Bjorndal KA 2002. Seasonal fluctuations in sand temperature: effects on the incubation period and mortality of loggerhead sea turtle (Caretta caretta) pre-emergent hatchlings in Minabe, Japan. Marine Biology 140: 639-646.
  • Okemwa GM, Nzuki S and Mueni EM 2004. The Status and Conservation of Sea Turtles in Kenya. Marine Turtle Newsletter 105: 1-6.
  • Olendo MI, Okemwa GM, Munga CN, Mulupi LK, Mwasi LD, Mxolisi Sibanda HBM and Ong’anda HO 2019. The value of long-term, community-based monitoring of marine turtle nesting: a study in the Lamu archipelago, Kenya. Oryx, doi:10.1017/S0030605317000771.
  • Sinaei M, Bolouki M, Ghorbganzadeh-Zaferani G, Matin MT, Alimoradi M and Dalir S 2018. On a Poorly Known Rookery of Green Turtles (Chelonia mydas) Nesting at the Chabahar Beach, Northeastern Gulf of Oman. Russian Journal of Marine Biology 44: 254-261.
Nesting Green Sea Turtle

While there is no particular season for green sea turtle nesting in the Maldives, on Six Senses Laamu and the surrounding islands it usually happens between March and August. This green sea turtle crawled on land early in the morning of the 26th of June and dug a nest to lay her eggs.When sea turtles come on land they are more vulnerable because they can only move very slowly and are not well adapted for life out of the water. It is critical to not disturb a sea turtle attempting to nest as she may otherwise abort and crawl back into the ocean. Once a turtle starts laying eggs however, she enters a trance-like state from which she can’t be disturbed. This gives us the opportunity to carefully approach her and collect data such as carapace size, number of eggs and ID-shots of the nesting turtle. Once finished, she thoroughly covers the egg chamber containing 70-125 eggs on average and crawls back down the beach into the ocean.What an amazing event to witness!#greenseaturtle #turtlenest #maldives #seaturtleconservationPacsafe Sea Turtle Conservancy SEE Turtles National Geographic OceanCare

Posted by Olive Ridley Project on Thursday, June 28, 2018
Video © Beth Faulkner, Manta Trust.

The number of eggs in a nest, called a clutch, varies by species. On average, sea turtles lay 110 eggs in a nest, averaging between 2 to 8 nests a season. The smallest clutches are laid by Flatback turtles, approximately 50 eggs per clutch. The largest clutches are laid by hawksbills, which may lay over 200 eggs in a nest.

In the Maldives predominantly green and hawksbill turtles are nesting. On average green turtles lay a mean of 110 eggs per nest with the largest clutches ever recorded of up to 238 eggs! Our own studies show clutch size to be 82-174 eggs (Lhaviyani Atoll).

Mean clutch size for hawksbills is significantly larger than that with around 150 eggs per nest in the Caribbean, variation is roughly the same with 86-206 eggs per nest. In the Indian Ocean data from the Seychelles shows even higher mean clutch size with 182 eggs per nest (160-242 range).

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References:

  • Hirth HF 1997. Synopsis of the Biological Data on the Green Turtle Chelonia mydas (Linnaeus, 1758). Vol. 2. Fish and Wildlife Service, US Department of the Interior.
  • Bjorndal KA, Carr, A, Meylan AB and Mortimer JA 1985. Reproductive Biology of the Hawksbill Eretmochelys imbricata at Tortuguero, Costa Rica, with Notes on the Ecology of the Species in the Caribbean. Biological Conservation 34: 353-368.
  • Diamond AW 1976. Breeding Biology and Conservation of Hawksbill Turtles, Eretmochelys imbricata L., on Cousin Island, Seychelles. Biological Conservation 9:199-215. (Hughes 1974b)

It is very hard to say how many sea turtles are left. Sea turtles are not easy to count, so we use different methods to estimate population sizes.

How many sea turtles are left? One method of estimating turtle populations is to measure the annual number of nesting events in a population. Olive ridley arribada nesting event in Michoacan Mexico; image
An olive ridley arribada nesting event in progress in Michoacan, Mexico. © Susie Gibson.

One such measure used is the annual number of nesting events in each population. Since turtles can lay more than one clutch per year, the number of nests does not directly translate to adult females in a population. Additionally, sea turtles do not reproduce every year. An average of 2-6 years (depending on the species) can pass between active reproduction for each female.

Scientists take several factors into account when they convert observed nesting activity into the estimated population size. These include remigration interval, proportion identified & resighted females, sex ratio etc. A recent publication evaluating this process recommends caution that our current overall estimates of population sizes might still be too optimistic.

Recent estimates show us that there are nearly 6.5 million sea turtles left in the wild with very different numbers for each species, e.g. population estimates for the critically endangered hawksbill turtle range from 83,000 to possibly only 57,000 individuals left worldwide. Kemp’s Ridley and Flatback turtles each with a very narrow distribution could have less than 10,000 individuals left for each species (medium estimates: 25,000 and 69,000 respectively).

In general it is best to evaluate conservation efforts by assessing trends. We can see that conservation measures are fruitful in certain areas, because the general numbers of observed turtle nests increased over the years, even though we do not have specific numbers for individual turtles in the area.

References:

  • Casale P & Ceriani SA 2020. Sea turtle populations are overestimated worldwide from remigration intervals: correction for bias. Endangered Species Research 30: 141-151.
  • Mazaris AD, Schofield G, Gkazinou C, Almpanidou V & Hays GC 2017. Global sea turtle conservation successes. Science Advances 3.
  • Spotila JR 2004. Sea Turtles: A complete guide to their biology, behaviour, and conservation. John Hopkins University Press, Maryland, US.
  • Wallace BP 2020. How many sea turtles are there? SWOT Report XV: 41.
Green turtle with excessive barnacles, Oman, image
Green turtle with barnacles, Oman ©Zoe Cox.
Sea turtle with a single barnacle, Maldives, image
Green turtle with a single barnacle, Maldives ©Stephanie Köhnk.

Barnacles are a highly specialized group of crustaceans. They have developed a sessile lifestyle as adults, attaching themselves to various substrates such as rocks, ships, whales or to sea turtles. Most commonly found barnacles on sea turtles belong to the genus Chelonibia, named after their host (Chelonia = turtle).

Initially, barnacles produce larvae. These early life stages are still mobile and facilitate further distribution. After the first six different so-called nauplius larvae, a seventh non-feeding larva develops: the cyprid. This is the stage which settles on a new substrate. The cyprid larvae has special attachment devices which allow it to hold onto the substrate, e.g. cup-shaped attachment organs on the antennae. Once settled, the barnacle develops into an adult and attaches in various ways: gripping the skin, cementing to the shell or boring into it.

Adult barnacles are filter feeders, thus benefit from a constant flow of water around them. As sessile creatures they can achieve that by a) settling in an area with pronounced water movement (e.g. close to shore) or b) settling on a moving substrate such as a sea turtle.

Even though barnacles are quite safely attached, barnacles actually are capable of moving as adults! They most likely achieve this through an extension of their cemented base as well as through muscle activity.

Most barnacles do not hurt sea turtles as they are only attached to the shell or skin on the outside. Others though burrow into the skin of the host and might cause discomfort and provide an open target area for following infections.

Excessive barnacle cover can be a sign of general bad health of a turtle. Usually sea turtles are debilitated first, and then become covered in an extensive amount of other organisms, such as barnacles and algae.

Luckily turtles are very resilient and can sometimes recover from such infestations.

Read more about Sea Turtle Hitchhikers – The Symbiotic Relationships of Sea Turtles here.

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References:

  • Monroe R & Limpus CJ 1979. Barnacles on turtles in Queensland waters with descriptions of three new species. Memoires of the Queensland Museum 19: 197-223.
  • Moriarty JE, Sachs JA & Jones K 2008. Directional Locomotion in a Turtle Barnacle, Chelonibia testudinaria, on Green Turtles, Chelonia mydas. Marine Turtle Newsletter 119: 1-4.
  • Zardus, JD & Hadfield MG 2004. Larval development and complemental males in Chelonibia testudinaria, a barnacle commensal with sea turtles. Journal of Crustacean Biology 24: 409-421.
  • Monroe R & Limpus CJ 1979. Barnacles on turtles in Queensland waters with descriptions of three new species. Memoires of the Queensland Museum 19: 197-223.Ross A & Frick MG 2007.
  • From Hendrickson (1958) to Monroe & Limpus (1979) and Beyond: An Evaluation of the Turtle Barnacle Tubicinella cheloniae. Marine Turtle Newsletter 118: 2-5.
Hawksbill turtle eating jelly fish
A hawksbill turtle eating a jellyfish. ©Lauren Arthur.
A green turtle eating sea grass.

Each sea turtle species feeds on a specific diet and all lack teeth:

  • Loggerheads feed mainly on hard-shelled organisms such as lobsters, crustaceans, and fish.
  • Green turtles are vegetarian and prefer sea grasses, sea weeds and algae as adults, however, green turtle hatchlings are omnivorous, eating jellyfish, snails, crabs, and shrimp.
  • Leatherbacks feed mostly on jellyfish.
  • Hawksbills have a bird-like beak that is used to cut through soft coral, anemones and sea sponges.
  • Olive ridleys are omnivorous, mostly eating jellyfish, snails, crabs, and shrimp but they will occasionally eat algae and seaweed as well.

Hungry Hungry Hawksbill

Hungry hungry hawksbill

We can celebrate turtles every day of the week! Happy #TurtleThursday everyone. Did you know that hawksbill sea surtles get their name from the narrow, elongated head that tapers to a beak-like jaw?It allows them to forage in crevices in coral reefs where they feed mainly on sponges but also anemones, soft coral or jellyfish.Sometimes it seems like they get in too deep and even stuck searching for their favourite food!This juvenile hawksbill was filmed by Beth Faulkner during a research dive for the Manta Trust in Laamu Atoll. Neither divers nor a camera can distract this turtle when it's lunch time!#turtle #hawksbill #coralreef #underwater #maldives Pacsafe SEE Turtles Sea Turtle Conservancy National Geographic Six Senses Laamu Fourth Element OceanPositive

Posted by Olive Ridley Project on Thursday, July 5, 2018
Watch the video of a very hungry hawksbill turtle in the Maldives.

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Apart from the most obvious – swimming – sea turtles use their flippers for a variety of other things as well. During foraging, their flippers allow them to hold onto prey, swipe it aside to tear off bits or leverage against the substrate again to remove substantial parts of their food. Loggerhead and green turtles have also been observed to use the forelimbs to remove sediment – so essentially to dig up their food.

Additionally, sea turtles use their flippers during mating & nesting. Male turtles hold onto the carapace of the female by hooking on with a large claw on each forelimb. Female turtles move up the beach, pulling with the forelimbs and pushing with the hindflippers. They use the hindlimbs to dig a nest, which is later closed & covered/hidden with the use of all four extremities.

A green turtle seen holding on to his female mating partner with his front flippers, image
A green turtle seen holding on to his female mating partner with his front flippers.©Nina Roth.
A female olive ridley turtle digging a nest with her hind flippers, image
A female olive ridley turtle digging a nest with her hind flippers. ©Susie Gibson.

References:

  • Fujii JA, McLeish D, Brooks AJ, Gaskell J & Van Houtan KS 2018. Limb-use by foraging marine turtles, an evolutionary perspective. PeerJ 6.
  • Spotila JR & Tomillo PS (eds.) 2015. The Leatherback Turtle: Biology and Conservation. Johns Hopkins Press, Maryland, US.
  • Wyneken, J. (1996). 7 Sea Turtle Locomotion: Mechanisms, Behavior. The biology of sea turtles, 1, 165.


Sea turtles are ectotherms (cold-blooded), meaning that they can not internally maintain their body temperature and must absorb heat from the surrounding environment to stay warm. For this reason, they are particularly sensitive to ambient temperature and seek to occupy warmer waters, typically in the tropical and temperate zones and ideally over 20ºC. At temperatures of less than 10ºC, see turtles may have trouble keeping warm, and may become cold-stunned, and sometimes, die of hypothermia.

Adult female green turtle on a colourful reef, Lhaviyni Atoll, Maldives. Image.
Holiday, female green turtle, relaxing on a reef in tropical waters.

References:

  • Hochscheid, S., Bentivegna, F., & Speakman, J. R. (2002). Regional blood flow in sea turtles: implications for heat exchange in an aquatic ectotherm. Physiological and Biochemical Zoology, 75(1), 66-76.
  • Schofield, G., Bishop, C. M., Katselidis, K. A., Dimopoulos, P., Pantis, J. D., & Hays, G. C. (2009). Microhabitat selection by sea turtles in a dynamic thermal marine environment. Journal of Animal Ecology, 78(1), 14-21.

Sea turtles have several natural predators; these vary depending upon the sea turtle’s life stage

Racoons, foxes, coyotes, feral dogs, ants, crabs, armadillos and mongooses can unearth and eat sea turtle eggs before they have the chance to hatch; crabs and birds can eat hatchlings as they run from the nest to the ocean, and fish (including sharks) and dolphins can eat hatchlings as they move from coastal waters towards the open ocean. 

Although sea turtles have fewer predators as they increase in size, sharks and killer whales can predate adult sea turtles in-water, and jaguars and crocodiles have been known to predate adult female sea turtles as they climb ashore to nest.

Vultures pray on olive ridley turtle eggs Orissa India. Image
Vultures pray on Olive Ridley turtle eggs as a female nests on a beach in Orissa, India. © Emma Doyle
Hawksbill turtle with a possible shar bite, on a reef in Maldives. Image.
A hawksbill turtle with a possible shark bite, Maldives.

References:

  • Heithaus, M.R. 2013. 10 Predators, Prey, and the Ecological Roles of Sea Turtles. The Biology of Sea Turtles, 3, 249.
  • Heithaus, M.R, Wirsing, A.J, Thomson, J.A. and Burkholder, D.A. 2008. A review of lethal and non-lethal effects of predators on adult marine turtles. Journal of Experimental Marine Biology and Ecology, 356(1-2), 43-51.

In the past 100 years, human demand for turtle meat, eggs, skin, and shells have reduced their populations. Destruction of feeding and nesting habitats and pollution of the world’s oceans are all taking a serious toll on the remaining sea turtle populations.

Many breeding populations have already disappeared, and some species are being threatened to extinction. The natural obstacles faced by young and adult sea turtles are staggering, but it is the increasing pressures from the presence of humans that are threatening their future survival.

One species of sea turtles, the hawksbill can affect reef diversity and succession by simply… eating! Hawksbills prefer eating sponges above anything else, which is very helpful to maintain a high coral cover on a reef. Scleractinian corals and sponges commonly compete for space on reefs, with sponges being more often the superior competitor. Sponges also compete for space, so predation by hawksbills is believed to have a major role in maintaining sponge species diversity.

Coral reefs are thought to be the most diverse ecosystem on the planet, providing habitats and shelter for thousands of marine organisms. Many fish spawn on the coral reefs and juvenile fish spend time there before heading out to deeper waters when they mature. Coral reefs also protect coastlines from wave action and storms and are an important revenue generator for many nations through tourism.

A hawksbill turtle having a bite to eat on a coral reef, Maldives. Samha, hawksbill turtle Maldives. Image.
A hawksbill turtle having a bite to eat on a coral reef, Maldives.
A hawksbill turtle having a bite to eat on a coral reef, Maldives. Marvin, hawksbill turtle Maldives. Image.
A hawksbill turtle having a bite to eat on a coral reef, Maldives.

References:

  • Bjorndal, K. A., & Jackson, J. B. (2002). 10 Roles of sea turtles in marine ecosystems: reconstructing the past. The biology of sea turtles, 2, 259.
  • Jackson, J. B. (1997). Reefs since columbus. Coral reefs, 16(1), S23-S32.

Sea turtles have many recognized roles in the evolution and maintenance of the structure and dynamics of marine ecosystems; they are an integral part of the interspecific interactions in marine ecosystems as prey, consumer, competitor, and host. They also serve as significant conduits of nutrient and energy transfer within and among ecosystems; and can also substantially modify the physical structure of marine ecosystems. 

Sea turtles are an important part of the planet’s food web and play a vital role in maintaining the health of the world’s oceans. They regulate a variety of other organisms simply through eating them. For example, green turtles mainly feed on seagrass. By grazing on seagrass meadows, they prevent the grass from growing too long and suffocating on itself. Nice and healthy seagrass beds again perform a multitude of so-called ecosystem functions: they are a nursery ground for many marine species and additionally are an important carbon sink and oxygen provider in the ocean. 

Another example are hawksbill turtles, who are mostly focused on eating sponges. Their sponge consumption is very important for a healthy coral reef by keeping the fast-growing sponges at bay and giving slower growing corals the chance to grow. Coral reefs are thought to be the most diverse ecosystem on the planet, providing habitats and shelter for thousands of marine organisms. Many fish spawn on the coral reefs and juvenile fish spend time there before heading out to deeper waters when they mature. Coral reefs also protect coastlines from wave action and storms and are an important revenue generator for many nations through tourism.

Leatherbacks eat jellyfish. Keeping the jellyfish population in check is important. Jellyfish prey on fish eggs and larvae and too many jellyfish means fewer fish.

Loggerheads feed on hard-shelled prey, such as crustaceans. By breaking up these shells, they increase the rate at which the shells disintegrate and, as a result, increase the rate of nutrient recycling in the ocean bottom ecosystems. 

Sea turtles also provide habitat for many marine organisms! Barnacles, algae and small creatures called epibionts attach themselves to the turtle and by carrying these around, the sea turtles provide a food source for fish and shrimp. In fact, some fish species obtain their diet strictly from epibionts found on sea turtles.

Apart from that, sea turtles provide an important food source for other organisms, especially in their early life stages. Ants, crabs, rats, raccoons, foxes, coyotes, feral cats, dogs, mongoose and vultures are known to dig up unhatched turtle eggs; the eggs are a nutrient-rich source of food. Juvenile turtles are a food source for various sea birds, fish and invertebrates. Adult sea turtles are preyed upon by sharks and killer whales.

Unhatched eggs and empty eggshells remaining inside nests on the beaches are a fertilizer for beach vegetation – they provide nutrition for plant growth with helps stabilize the shoreline as well as provide food for a variety of plant eating animals.

Because sea turtles can migrate huge distances, they also play an important role in generating and maintaining diversity throughout the world’s oceans by transporting the organisms that live on them to and from reefs, seagrass beds and the open ocean.

Adult male green turtle feeding on sea grass, Lhaviyani Atoll, Maldives. Image.
Sky, male green turtle, feeding on sea grass, Maldives
Juvenile hawksbill eating coral, Maldives. Image.
Brownie, juvenile hawksbill turtle, ready to feast on sponges, Maldives

References:

  • Bjorndal, K. A., & Jackson, J. B. (2002). 10 Roles of sea turtles in marine ecosystems: reconstructing the past. The biology of sea turtles, 2, 259.
  • León, Y. M., & Bjorndal, K. A. (2002). Selective feeding in the hawksbill turtle, an important predator in coral reef ecosystems. Marine Ecology Progress Series, 245, 249-258.

The exploitation of sea turtles has been a traditional activity among many coastal communities for centuries. Most indigenous tribes along the coast of many countries, specially in the tropics, capture sea turtles for subsistence, and as a source of animal protein. Sea turtles can be very delicious and nutritious. This artisanal and subsistence take, primarily for local consumption, was likely occurring at sustainable levels at first.

The consumption and trade of sea turtles became more common in the temperate zones during the colonization of the New World. Whalers and ocean explorers exploited sea turtles as a supply of fresh meat during the long journeys across the Atlantic ocean between the Caribbean sea to Europe, as turtles were found to withstand several weeks alive inside the boats without food, or water!

Sea turtle meat was introduced in Europe as a delicacy by the end of the 19th century, with green turtle soup becoming particularly popular in Great Britain. Then new markets started to develop in Europe and Asia throughout the first half of the 20th century, peaking between the 1950s and the early 1970s.

Ongoing sea turtle consumption (legal and illegal) occurs in many parts of the world, including Australia, the Caribbean, the Indian Ocean, Central and South America, among many other nations. Apart from the meat, coastal communities may use internal organs such as kidney and liver for soup; oil extracted from the fat as a cure for respiratory problems, especially in children, while the blood can be drunk raw as a remedy for anemia and asthma.

In the case of the hawksbill turtle, the richly patterned scutes that cover the carapace and plastron of the hawksbill (called tortoiseshell or bekko) have been considered a precious material — on par with ivory, rhinoceros horn, gold, and gems — for thousands of years; trade between some countries remains legal, and public sale of products, mainly for international tourists, occurs in many countries, legally and illegally. Read more about why hawksbill turtles are critically endangered.

Hawksbill turtle patient Crush at ORP turtle rescue centre, Maldives
Hawksbill turtle patient Crush has a beautiful shell
Hawksbills are often poached for their beautiful shell. Jewellery made of tortoise shell for sale. Image.
Jewellery made of tortoise shell for sale.

References:

  • Aguirre, A. A., Gardner, S. C., Marsh, J. C., Delgado, S. G., Limpus, C. J., & Nichols, W. J. (2006). Hazards associated with the consumption of sea turtle meat and eggs: a review for health care workers and the general public. EcoHealth, 3(3), 141-153.
  • Humber, F., Godley, B. J., & Broderick, A. C. (2014). So excellent a fishe: a global overview of legal marine turtle fisheries. Diversity and Distributions, 20(5), 579-590.
  • Mancini, A., & Koch, V. (2009). Sea turtle consumption and black market trade in Baja California Sur, Mexico. Endangered Species Research, 7(1), 1-10.
  • Meylan, A. B., & Donnelly, M. (1999). Status justification for listing the hawksbill turtle (Eretmochelys imbricata) as critically endangered on the 1996 IUCN Red List of Threatened Animals. Chelonian conservation and Biology, 3(2), 200-224.

The sex of sea turtles is not determined purely genetically, as it is in humans. The temperature during the development of the embryo will determine whether a male or a female turtle hatches. Higher temperatures result in females, lower in males.

Increasing sand temperatures on nesting beaches can therefore shift the sex ratio of hatchlings to almost entirely female. As a result, turtles may have a problem reproducing in the future.

Nesting female olive ridley turtles, Aribada. Image.
Nesting female olive ridley turtles.
Olive ridley turtle hatchling leaving the nest. Image
Olive ridley turtle hatchling leaving the nest.

References:

  • Abella Perez E, Marco A, Martins S and Hawkes L 2016. Is this what a climate change-resilient population of marine turtles looks like? Biological Conservation 193: 124-132.
  • Chaloupka M, Kamezaki N and Limpus C 2008. Is climate change affecting the population of the endangered Pacific loggerhead sea turtle? Journal of Experimental Marine Biology and Ecology 356: 136-143.
  • Fuentes MMPB, Limpus CJ and Hamann M 2011. Vulnerability of sea turtle nesting grounds to climate change. Global Change Biology 17: 140-153.
  • Pike DA, Antworth RL and Stiner JC 2006. Earlier Nesting Contributes to Shorter Nesting Seasons for the Loggerhead Sea Turtle, Caretta caretta. Journal of Herpetology 40: 91-94.

Sea turtles drink seawater to hydrate. Although sea turtles are physically adapted to a saline environment, they need to be able to excrete excess salt. As reptilian kidneys are unable to excrete large volumes of salt via urine, sea turtles evolved specialised secretory glands (lachrymal glands) located in the corner of each eye to remove excess salt. The liquid secreted gives the appearance of tears, hence why turtles are often reported to “cry” .

A rescued olive ridley turtle and ghost net victim seems to be crying but is excreting excess salt from the glands in the corner of her eye. Image.
A rescued olive ridley turtle and ghost net victim seems to be crying, but is excreting excess salt from the glands in the corner of her eye.
Why do sea turtles cry? A green turtle excreting excess salt seems to be crying. Image.
A green turtle excreting excess salt may seem to be crying. ©Ellie Butler.

References:

  • Reina, R.D. Jones, T.T. and Spotila, J.R. 2002. Salt and water regulation by the leatherback sea turtle Dermochelys coriacea. Journal of Experimental Biology, 205(13), 1853-1860.

Sea turtles eggs must incubate in moist sand. For this reason, every year, some beaches around the tropical and temperate world are visited, mostly at night, by adult females who come ashore to dig a nest chamber and there, deposit their eggs.

It is common belief that only females come ashore. However, in some places in the world where waters are a little bit colder, turtles of both sexes may be found basking in the sun in the beach! This basking behaviour in sea turtles is linked to thermoregulation (regulation of the body temperature), and is commonly seen for green turtles of the Galapagos, Hawaiian and Wellesley archipelagos. As sea turtles are cold-blooded animals, this behaviour helps the turtles in warming their internal temperatures by 3ºC! There are also some speculations that basking may aid immune function, predator avoidance, digestion, egg development and even prevent unwanted courtship.

A nesting green turtle, Laamu Atoll, Maldives. Image
A nesting green turtle, Laamu Atoll, Maldives.
Olive ridley turtles nest together in a mass nesting event called an arribada, here from Mexico. Image.
Olive ridley turtles nest together in a synchronised mass nesting event called an arribada, here from Mexico. ©Susie Gibson.

References:

  • Mrosovsky, N. (1980). Thermal biology of sea turtles. American Zoologist, 20(3), 531-547.
  • Spotila, J. R., & Standora, E. A. (1985). Environmental constraints on the thermal energetics of sea turtles. Copeia, 694-702.
  • Van Houtan, K. S., Halley, J. M., & Marks, W. (2015). Terrestrial basking sea turtles are responding to spatio-temporal sea surface temperature patterns. Biology letters, 11(1), 20140744.

Sea turtles are considered to be what is called a “keystone species”. The herbivorous green turtle and the sponge-eating hawksbill turtles are integral keystone species to any tropical marine ecosystem by performing critical ecological roles that are essential for the structure and function of these ecosystems.

For example, it has been suggested that the dramatic decline of these species in the Caribbean has radically reduced, and qualitatively changed, grazing and excavation of seagrasses, as well as depredation on marine sponges. This has in turn resulted in loss of production to adjacent ecosystems, such as coral reefs and disrupted entire food chains.

The leatherback turtle, which feed mainly on gelatinous prey such as jellyfish and salps, may consume the equivalent weight of an adult lion in jellyfish (440 lbs.) in a day. Recent studies show that jellyfish population’s blooms are more frequent and are continuously increasing in size and therefore have become problematic and are in fact a threat to the natural balance of marine ecosystems and towards human beings.

For this reason, the Leatherbacks play an essential role in controlling jellyfish population numbers and prevent the take over from a once fish populated ecosystem to a jellyfish abundant ecosystem in locations where fish stocks are already depleted.

A hawksbill turtle making a meal of a jelly fish, Maldives. Image.
Hawksbill turtle making a meal of a jelly fish, Maldives. ©Lauren Arthur.
A green turtle eating sea grass, Maldives. Image.
A green turtle eating sea grass, Maldives.

References:

  • Eckert, K. L., & Hemphill, A. H. (2005). Sea turtles as flagships for protection of the wider Caribbean region. Maritime Studies, 3(2), 4.
  • Jackson, J. B. (1997). Reefs since columbus. Coral reefs, 16(1), S23-S32.
  • Williams, J. (2015). Are jellyfish taking over the world. Journal of Aquactic and Marine Biology, 3, 00026.
  • Wilson EG, Miller KL, Allison D, Magliocca M (2010) Why healthy oceans need sea turtles: the importance of sea turtles to marine ecosystems. Oceana protecting the world’s oceans, p.9.

Sea turtles scratch their shells to clean them. This self-grooming behaviour helps them remove epibionts such as barnacles or algae. Excessive epibiont growth would otherwise impair the turtle’s movement and swimming ability.

Green turtle with excessive barnacles, Oman
Green turtle with barnacles, Oman ©Zoe Cox

References:

  • Frick MG and McFall G 2007. Self-Grooming by Loggerhead Turtles in Georgia, USA. Marine Turtle Newsletter 118: 15.
  • Schofield G, Katselidis KA, Dimopolous P, Pantis J and Hayes GC 2006. Behaviour analysis of the loggerhead turtles Caretta caretta from direct in-water observations. Endangered Species Research 2: 71-79.

Sea turtles are ectotherms (cold-blooded), meaning that they can not internally maintain their body temperature and must absorb heat from the surrounding environment to stay warm. If exposed to temperatures under 10ºC for a prolonged time, sea turtles will suffer from “cold-stunning”, or hypothermia. When this happens, the turtles will suffer from many problems, increasing a decreased metabolic rate, slow heart beat, feeling lazy, for instance. If turtles remain cold for too long, they can die as their body literally shuts down.

Green sea turtle swimming in the blue, Maldives. Image.
Green sea turtle swimming in warm tropical water in Maldives.

References:

  • Anderson, E. T., Harms, C. A., Stringer, E. M., & Cluse, W. M. (2011). Evaluation of hematology and serum biochemistry of cold-stunned green sea turtles (Chelonia mydas) in North Carolina, USA. Journal of Zoo and Wildlife Medicine, 42(2), 247-255

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