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Marine Turtles: Unique and Amazing Travellers

Every year, marine turtles travel thousands of kilometres between feeding grounds, developmental habitats and nesting beaches. Some individuals cross the High Seas[1] and the waters of many different nations during a single migration. Throughout their lives, they depend on a network of habitats connected across entire ocean basins. Yet these same migrations also expose them to threats across multiple jurisdictions, making international cooperation essential for their conservation.

Built for life at sea

Sea turtles are highly adapted for life in the ocean through a body design that maximizes swimming efficiency. Their fore flippers are shaped like long, stiff paddles that generate powerful forward thrust through a wing-like motion, while the hind flippers act as rudders, providing steering and stability in the water. Their shells are also shaped to improve movement through water. Most sea turtles have flattened, streamlined shells that reduce drag, making swimming more efficient while still providing protection.

Together, these adaptations allow sea turtles to travel long distances with remarkable efficiency, although they reduce mobility on land and limit the ability to fully retract the head into the shell because of the enlarged muscles needed to power swimming [1].Different species have distinct shell designs in response to their habitats; for example, hawksbill turtles (Eretmochelys imbricata) possess unique armouring features associated with life on coral reefs [2].

Different again: Leatherback turtles (Dermochelys coriacea) are built differently from other sea turtles. Instead of a rigid shell covered in hard scutes, they have elastic skin over small bony plates linked by flexible collagen-rich joints, allowing the body to compress during deep dives [3]. Their teardrop-shaped shell and prominent longitudinal ridges help water flow smoothly over the body, reducing drag during sustained swimming. Combined with their powerful, elongated flippers, these features support an efficient, wide-ranging life in the open ocean [1]. © U.S. Fish and Wildlife Service Southeast Region

 

 

Credit: Michael Landress
Salt secretion around the eyes of a 
green turtle (Chelonia mydas). 
© Michael Landress

Life in the ocean requires sea turtles to regulate salt, oxygen and energy efficiently. Unlike terrestrial vertebrates, they rely on specialized salt glands located behind the eyes to remove excess salt absorbed from seawater. These glands produce highly concentrated secretions, often visible as fluid around the eyes, allowing turtles to maintain the balance of water and salts in their bodies while living in a marine environment [4].

Sea turtles can remain submerged for long periods because of their low metabolic rates and high tolerance to low oxygen levels. In some turtles for instance, heart rate is driven mainly by activity rather than submergence itself, suggesting that they adjust oxygen use according to whether they are resting or swimming [5]. Their cells also produce energy differently, helping them meet the demands of long-distance swimming, migration and diving [6].

 

 

Their specialized physiology allows marine turtles to make use of a wide range of marine environments, from shallow coastal habitats to the open ocean, where they can access prey at multiple depths and across a variety of habitats, providing important nutritional and survival advantages.

One life, many habitats

Marine turtles do not belong to a single place. Instead, their life cycle depends on a network of connected habitats spread across oceans. Eggs develop on nesting beaches, but hatchlings soon enter the sea and begin journeys that may span entire ocean basins. Light from the seaward horizon helps hatchlings find the sea, while incoming waves guide their initial movement offshore. As they move farther from the coast, magnetic cues can help them maintain direction during migration [12]. Once offshore, currents influence where hatchlings and small juveniles disperse and which developmental habitats they reach [13,14].

Simplified life cycle
Simplified illustration of how a marine turtle's life cycle can connect nesting, developmental and feeding habitats across multiple maritime jurisdictions and the High Seas. Arrows are schematic and do not represent specific locations, the movements of an individually tracked turtle or a documented life cycle. Infographic: IOSEA Marine Turtle MOU Secretariat.

Young turtles may spend several years in oceanic habitats. As they grow, food becomes limiting and at that point most species move into coastal feeding grounds, where they continue to develop before reaching adulthood. Leatherback turtles and some loggerhead populations are an exception, as they remain largely oceanic throughout their lives [13]. 

As adults, many sea turtles migrate between feeding grounds and nesting areas, and some show strong fidelity to the same migratory routes and destinations across successive breeding cycles [16]. This return to the region where they were born, known as natal homing, is thought to depend largely on Earth’s magnetic field: turtles can detect regional magnetic signatures,  and developing embryos or hatchlings likely imprint on the magnetic characteristics of their natal area when young, later using them to navigate back as adults [17,18]. Adult hard-shelled turtles have recorded maximum migrations of around 1,050 to 2,850 km between breeding and feeding areas, while leatherbacks and juveniles can travel much farther across ocean basins [19]. For example, leatherbacks travel some 13,000 km between feeding areas in California and nesting areas in Indonesia.

Individual feeding grounds can bring together turtles originating from several nesting populations [13], and turtles from one nesting area can disperse to multiple different feeding areas. Viewed at a larger scale, these movements form complex networks in which some marine regions act as important hubs, while particular migratory routes provide key connections between otherwise distant regions [20].

These networks also extend across political boundaries and human cultures. A turtle may move through several national jurisdictions and the High Seas during its lifetime, meaning that protection in one location addresses only part of its range [21]. Global tracking analyses have shown that the High Seas can play an important role in connecting coastal regions for several marine turtle species [20].

As marine turtles are transboundary animals, their conservation must also operate across borders. Protecting nesting and feeding habitats is only part of the challenge; ensuring that turtles are protected along the migratory corridors and through the oceanic areas that connect these habitats requires coordination across the turtles' range [22]. People from far distant regions may have cultural connections to turtles of the same population, highlighting the importance of culturally sensitive policy to ensure effectiveness.

This need for cooperation lies at the heart of the IOSEA Marine Turtle MOU, which brings together Range States and other partners to coordinate conservation for migratory marine turtles across the Indian Ocean and South-East Asia region.

References

  1. Hendrickson, J. R. (1980). The Ecological Strategies of Sea Turtles 1 (Vol. 20). https://academic.oup.com/icb/article/20/3/597/213477
  2. Salmon, M., Coppenrath, C., & Higgins, B. (2018). The early ontogeny of carapace armoring in hawksbill sea turtles (Eretmochelys imbricata), with comparisons to its close relatives (Loggerhead, Caretta caretta; Kemp’s ridley, Lepidochelys kempii). Journal of Morphology, 279(9), 1224–1233. https://doi.org/10.1002/jmor.20844
  3. Chen, I. H., Yang, W., & Meyers, M. A. (2015). Leatherback sea turtle shell: A tough and flexible biological design. Acta Biomaterialia, 28, 2–12. https://doi.org/10.1016/j.actbio.2015.09.023
  4. Prange, H. D. (1985). Renal and Extra-Renal Mechanisms of Salt and Water Regulation of Sea Turtles: A Speculative Review (Vol. 1985, Issue 3). https://www.jstor.org/stable/1444769?seq=1&cid=pdf-
  5. Williams, C. L., Sato, K., & Ponganis, P. J. (2019). Activity, not submergence, explains diving heart rates of captive loggerhead sea turtles. Journal of Experimental Biology, 222(8). https://doi.org/10.1242/jeb.200824
  6. Ramos, E. K. D. S., Freitas, L., & Nery, M. F. (2020). The role of selection in the evolution of marine turtles mitogenomes. Scientific Reports, 10(1), 16953. https://doi.org/10.1038/s41598-020-73874-8
  7. Hays, G. C., Houghton, J. D., & Myers, A. E. (2004). Pan-Atlantic leatherback turtle movements. Nature, 429(6991), 522-522.
  8. López-Mendilaharsu, M., Rocha, C. F., Domingo, A., Wallace, B. P., & Miller, P. (2009). Prolonged deep dives by the leatherback turtle Dermochelys coriacea: pushing their aerobic dive limits. Marine Biodiversity Records, 2.
  9. Sakamoto, W., Uchida, I., Naito, Y., Kureha, K., Tujimura, M., & Sato, K. (1990). Deep diving behavior of the loggerhead turtle near the frontal zone. Nippon Suisan Gakkaishi, 56(9), 1435-1443.
  10. Hill, J. E., Robinson, N. J., King, C. M., & Paladino, F. V. (2017). Diving behavior and thermal habitats of gravid hawksbill turtles at St. Croix, USA. Marine biology, 164(1), 17.
  11. Hochscheid, S., Godley, B. J., Broderick, A. C., & Wilson, R. P. (1999). Reptilian diving: highly variable dive patterns in the green turtle Chelonia mydas. Marine Ecology Progress Series, 185, 101-112.
  12. Fuxjager, M. J., Davidoff, K. R., Mangiamele, L. A., & Lohmann, K. J. (2014). The geomagnetic environment in which sea turtle eggs incubate affects subsequent magnetic navigation behaviour of hatchlings. Proceedings of the Royal Society B: Biological Sciences, 281(1791). https://doi.org/10.1098/rspb.2014.1218
  13. Godley, B. J., Barbosa, C., Bruford, M., Broderick, A. C., Catry, P., Coyne, M. S., Formia, A., Hays, G. C., & Witt, M. J. (2010). Unravelling migratory connectivity in marine turtles using multiple methods. Journal of Applied Ecology, 47(4), 769–778. https://doi.org/10.1111/j.1365-2664.2010.01817.x
  14. Lambardi, P., Lutjeharms, J. R. E., Mencacci, R., Hays, G. C., & Luschi, P. (2008). Influence of ocean currents on long-distance movement of leatherback sea turtles in the Southwest Indian Ocean. Marine Ecology Progress Series, 353, 289–301. https://doi.org/10.3354/meps07118
  15. Flanders Marine Institute (2024). The intersect of the Exclusive Economic Zones and IHO sea areas, version 5. Available online at https://www.marineregions.org/. https://doi.org/10.14284/699
  16. Broderick, A. C., Coyne, M. S., Fuller, W. J., Glen, F., & Godley, B. J. (2007). Fidelity and over-wintering of sea turtles. Proceedings of the Royal Society B: Biological Sciences, 274(1617), 1533–1539. https://doi.org/10.1098/rspb.2007.0211
  17. Lohmann, K. J., & Lohmann, C. M. F. (2019). There and back again: Natal homing by magnetic navigation in sea turtles and salmon. In Journal of Experimental Biology (Vol. 222). Company of Biologists Ltd. https://doi.org/10.1242/jeb.184077
  18. Benhamou, S., Sudre, J., Bourjea, J., Ciccione, S., de Santis, A., & Luschi, P. (2011). The role of geomagnetic cues in green turtle open sea navigation. PLoS ONE, 6(10). https://doi.org/10.1371/journal.pone.0026672
  19. Hays, G. C., & Scott, R. (2013). Global patterns for upper ceilings on migration distance in sea turtles and comparisons with fish, birds and mammals. Functional Ecology, 27(3), 748–756. https://doi.org/10.1111/1365-2435.12073
  20. Kot, C. Y., Åkesson, S., Alfaro-Shigueto, J., Amorocho Llanos, D. F., Antonopoulou, M., Balazs, G. H., Baverstock, W. R., Blumenthal, J. M., Broderick, A. C., Bruno, I., Canbolat, A. F., Casale, P., Cejudo, D., Coyne, M. S., Curtice, C., DeLand, S., DiMatteo, A., Dodge, K., Dunn, D. C., … Halpin, P. N. (2022). Network analysis of sea turtle movements and connectivity: A tool for conservation prioritization. Diversity and Distributions, 28(4), 810–829. https://doi.org/10.1111/ddi.13485
  21. Shillinger, G. L., Palacios, D. M., Bailey, H., Bograd, S. J., Swithenbank, A. M., Gaspar, P., Wallace, B. P., Spotila, J. R., Paladino, F. v., Piedra, R., Eckert, S. A., & Block, B. A. (2008). Persistent leatherback turtle migrations present opportunities for conservation. PLoS Biology, 6(7), 1408–1416. https://doi.org/10.1371/journal.pbio.0060171
  22. Hays, G. C., Mortimer, J. A., Ierodiaconou, D., & Esteban, N. (2014). Use of Long-Distance Migration Patterns of an Endangered Species to Inform Conservation Planning for the World’s Largest Marine Protected Area. Conservation Biology, 28(6), 1636–1644. https://doi.org/10.1111/cobi.12325

[1] High Seas: ocean areas beyond national jurisdiction, beyond the maritime zones where individual countries exercise jurisdiction.

This article is one of a series celebrating the 25th anniversary of the IOSEA Marine Turtle MOU. Please refer to all here:

  1. From Perth to Manila: How a Regional Vision for Marine Turtles Became Reality
  2. The IOSEA Marine Turtle MOU – 25 Years of Collaboration for the Conservation of Marine Turtles
  3. Marine Turtles: Unique and Amazing Travellers
  4. Protecting Marine Turtles Takes a Village... and a Common Vision

As we celebrate 25 years of the IOSEA Marine Turtle MOU, we showcase some of the people, initiatives and communities that have contributed to marine turtle conservation across the region. Visit our Story Map to discover their stories.