June 16th is World Sea Turtle Day, made in honour of Dr. Archie Carr (June 16th, 1909 – May 21st, 1987), who has been regarded as the father of sea turtle biology. He founded the Sea Turtle Conservancy and turned the tide of sea turtle extinction into a road to recovery with many of his projects, from Operation Green Turtle to chairing the Marine Turtle Specialist Group of the IUCN for 20 years. His descriptive biology and ecological papers on the seven (or eight, depending on who you talk to) species of sea turtles have been the foundations for all sea turtle biologists worldwide to this day and for many years to come.
In celebration of World Sea Turtle Day, we will cover a bit on sea turtle biology and the fascinating life cycle of sea turtles, followed by the sea turtles found in Singapore and the perils they face. It is not all doom and gloom, as we will then talk about what you can do as the general public to make life a lot easier for our ancient and charismatic reptilian friends.
Sea Turtle Diversity
There are seven recognized species of sea turtles throughout the world: the Green Sea Turtle (Chelonia mydas), the Hawksbill Sea Turtle (Eretmochelys imbricata), the Loggerhead Sea Turtle (Caretta caretta), the Olive Ridley Sea Turtle (Lepidochelys olivacea), the Kemp’s Ridley Sea Turtle (Lepidochelys kempii), the Flatback Sea Turtle (Natator depressus), and the Leatherback Sea Turtle (Dermochelys coriacea). The first six fall into the family of Cheloniidae turtles (essentially sea turtles with scutes on their shells) while the Leatherback Sea Turtle is the only living representative of the Dermochelyiidae family (sea turtles with no scutes)1. Though these are the only living representatives of sea turtles, six are listed as vulnerable, endangered, or critically endangered under the International Union of Conservation of Nature Red List of Threatened Species2–6, while the Flatback Sea Turtle is listed as data deficient7.They are entirely marine, meaning they can only be found in the salt waters of the ocean, and despite being primarily sea-faring animals, they evolved from land turtles in the mid-Cretaceous8 and need to surface to breathe.
Sea Turtle Biology
The life of sea turtles begins with a clutch of about 100 eggs in the sand at the beach. The sea turtles will hatch from the eggs between 45 to 60 days of incubation, but will not emerge from their nest just yet. When they hatch, they are born curved and with the yolk still attached to the bottom-shell (plastron), and take four to five days to straighten out and to absorb the yolk into their system. After this, they head towards the surface in a mad flurry, but will stay just below the surface of the sand and wait for nightfall when temperatures are cool. At that point, they come out of the sand and make a mad dash down the beach, into the water, and a mad “swim frenzy” out into open-ocean to catch ocean currents (Flatback Sea Turtle hatchlings, which are endemic to Australia, will not swim out to open-ocean but will stay within the nearshore habitats). From this point on, sea turtles will spend the rest of their lives in the ocean (with the exception of females, which will emerge onto the beach to lay their eggs).
Many sea turtle biologists agree that this is a very important part of the sea turtle life cycle, as this is where imprinting occurs (the act of sea turtle hatchlings remembering where they hatched from so they can return to the same nesting site to lay eggs when they are older)9,10. This is also the most dangerous part of their lives; predators abound on land and in sea, and many sea turtle hatchlings fall prey to ghost crabs, feral animals, birds, sharks, fish11,12.
Once in open-ocean, hatchlings are at the mercy of ocean currents. Some hatchlings hide out in rafts of Sargassum seaweed for several years, where they feed on anything they can find, such as small crustaceans and fish. Once they reach about a meter in length, some sea turtle species will recruit back to the near-shore reefs, such as Green Sea Turtles, Hawksbill Sea Turtles, and Loggerhead Sea Turtles. Leatherback Sea Turtles, Kemp’s and Olive Ridley Sea Turtles are turtles that live in open-ocean, though Leatherback Sea Turtles are known to dive to substantial depths to feed on jellyfish.
Within these habitats, sea turtles will forage for a good part of their lives, but every two or three years sea turtles will migrate to the sites from which they hatched in an event called natal-homing, which can be accurate to within a few kilometers1. Here, they will mate in the near-shore habitats, and the females will emerge onto the beach to lay their eggs (oviposition). Much of the oviposition occurs at night to avoid predation on themselves and on their eggs, but many other factors will determine where a sea turtle may nest13–15. The female will first dig a body pit with the front flippers, followed by them turning 180º and digging an egg chamber with their hind flippers. They will then lay an average of 100 eggs in each egg chamber (depending on the species), where the whole process will begin again.
The Ridley Sea Turtles undertake a phenomenal mass-nesting event called the arribada, where thousands of individuals will emerge onto the beaches to nest at once. Several hypotheses have been raised as to why they aggregate to mate and nest en masse, from predator saturation to being able to find mates easier in the water16.
SEA TURTLES OF SINGAPORE
Two species of sea turtles frequent the waters of our little red dot: the Green Sea Turtles and the Hawksbill Sea Turtles17. A single Leatherback Sea Turtle specimen was recovered in 1883, but no other Leatherback Sea Turtle sighting in Singapore territorial waters has been reported since18.
So far, there has been no evidence of Green Sea Turtles nesting on Singapore beaches, but there have been several reports of Hawksbill Sea Turtles nesting and hatching from our beaches19,20. This may be attributed to the nest site selections of the two species, where Green Sea Turtles prefer level and open-expanses of beaches with deep nesting chambers13,21, while Hawksbill Sea Turtles are happy to nest in dense vegetation along beaches22. That said, because many of the beaches in Singapore are reclaimed and artificial23, the nesting habitats are barely suitable for sea turtles in general, let alone for Green Sea Turtles.
Threats within Singapore
Nesting Hawksbill Sea Turtles in Singapore are worth celebrating, especially for a heavily urbanized island-state like ours, but they still face a multitude of threats. A single news report on poached sea turtle eggs in Singapore highlights the dangers of this activity24, and possibly that many other poaching incidents go unreported. Females will not go onto well-lit beaches, while hatchlings can be mis- or disorientated and head towards artificial light sources rather than the ocean25. Climate change can spell disaster for sea turtles as well, as rising sea levels can drown and kill entire clutches of sea turtle eggs26. Further, because sea turtle gender is temperature-dependent, where warmer temperatures produce more females and cooler temperatures produce more males, global warming can cause clutches to be entirely female, leaving no males to mate with them and resulting massive population crashes27. That is assuming temperatures do not go above the lethal threshold, after which entire clutches of sea turtle eggs can die28. The heavy boat traffic in Singapore can also increase the potential of sea turtles dying from boat strikes.
There are future plans for managing the incidents of sea turtles within Singapore, which will possibly mitigate the impact on the species. The National Parks Board has announced that a sea turtle hatchery will be opened on Sisters’ Island Marine Park as a means of moving sea turtle clutches found on mainland Singapore that are in danger of mortality. This hatchery is scheduled to be operational by end-2017.
WHAT CAN WE DO?
The picture painted for sea turtles in Singapore is not a pretty one, but there are things that everyday Singaporeans can do to reduce the threats to sea turtles in Singapore:
- Ensure that trash you bring to the beaches is disposed of properly within the designated bins in our coastal parks. Better yet, adopt a pack-in-pack-out mentality of taking whatever trash is brought into our coastal parks back out of the coastal parks and disposed responsibly elsewhere.
- Do not approach a sea turtle emerging onto the beach to nest; sea turtles are incredibly sensitive to movement and lights, and may abort the entire procedure if they feel the slightest inclination that they are being threatened.
- Do not touch sea turtle hatchlings that are emerging from the ground as this is a very sensitive part of their life cycle. Give them plenty of space to go into the water, and maybe even remove trash that would be in their way.
- If you do see a sea turtle on the beach, whether it is a nesting female or a hatchling, call the National Parks Board on their hotline at 1800 471 7300. Take note of your location (barbecue pit number, the zone you are in, etc.) so they are able to come down to the site.
Rushan is an environmental science honours student in Murdoch University undertaking a project using airborne laser scanning to see what aspects of beach topography sea turtles prefer for a nesting site. He has volunteered and interned with the Juara Turtle Project (Tioman Island, Malaysia), Seamarc Pvt. Ltd. (Maldives), and the National Parks Board (Singapore) working on various projects with hatchery management, a captive-rear-and-release program, sea turtle nursing and husbandry, identification of individual sea turtles, satellite tagging (ARGOS SPOT5) and satellite tracking. Upon completion of his course in July 2017, he hopes to continue working with sea turtles in Singapore and abroad.
- Bowen, B. W. & Karl, S. A. Population genetics and phylogeography of sea turtles. Mol. Ecol. 16, 4886–4907 (2007).
- Mortimer, J. A. & Donnelly, M. Eretmochelys imbricata. The IUCN Red List of Threatened Species e.T8005A12881238 (2008). doi:http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T8005A12881238.en
- Casale, P. & Tucker, A. D. Caretta caretta. The IUCN Red List of Threatened Species e.T3897A83157651 (2015). doi:http://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T3897A83157651.en
- Abreu-Grobois, A. & Plotkin, P. Lepidochelys olivacea. The IUCN Red List of Threatened Species e.T11534A3292503 (2008). doi:http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T11534A3292503.en
- Marine Turtle Specialist Group. Lepidochelys kempii. The IUCN Red List of Threatened Species e.T11533A3292342 (1996). doi:http://dx.doi.org/10.2305/IUCN.UK.1996.RLTS.T11533A3292342.en
- Wallace, B. P., Tiwari, M. & Girondot, M. Dermochelys coriacea. The IUCN Red List of Threatened Species (2013). doi:http://dx.doi.org/10.2305/IUCN.UK.2013-2.RLTS.T6494A43526147.en
- Red List Standards & Petitions Subcommittee. Natator depressus. The IUCN Red List of Threatened Species e.T14363A4435952 (1996). doi:http://dx.doi.org/10.2305/IUCN.UK.1996.RLTS.T14363A4435952.en
- Wang, Z. et al. The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan. Nat. Genet. 45, 701–706 (2013).
- Brothers, J. R. & Lohmann, K. J. Evidence for geomagnetic imprinting and magnetic navigation in the natal homing of sea turtles. Curr. Biol. 25, 392–396 (2015).
- Lohmann, K. J., Lohmann, C. M. F., Brothers, J. R. & Putman, N. F. in Biology of Sea Turtles: Volume III (eds. Wyneken, J., Lohmann, K. J. & Musick, J. A.) 59–78 (CRC Press Taylor & Francis Group, 2013).
- Peterson, C. H., Fegley, S. R., Voss, C. M., Marschhauser, S. R. & VanDusen, B. M. Conservation implications of density-dependent predation by ghost crabs on hatchling sea turtles running the gauntlet to the sea. Mar. Biol. 160, 629–640 (2013).
- Burger, J. & Gochfeld, M. Avian predation on olive ridley (Lepidochelys olivacea) sea turtle eggs and hatchlings: Avian opportunities, turtle avoidance, and human protection. Copeia 1, 109–122 (2014).
- Zavaleta-Lizarraga, L. & Morales-Mavil, J. E. Nest site selection by the green turtle (Chelonia mydas) in a beach of the north of Veracruz, Mexico. Rev. Mex. Biodivers. 84, 927–937 (2013).
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- Bernardo, J. & Plotkin, P. T. in Biology and Conservation of Ridley Sea Turtles (ed. Plotkin, P. T.) 59 – 87 (John Hopkins University Press, 2007).
- Tan, R. Sea turtles. Wild Singapore (2010).
- Diong, C. H. in Assessment of the conservation status of the leatherback turtle in the Indian Ocean and South-East Asia. IOSEA Species Assessment: Volume I 120–121 (Secretariat of the Indian Ocean – South-East Asian Marine Turtle Memorandum of Understanding, 2006).
- Chua, G. Rare sea turtle spotted on beach at East Coast Park. The Straits Times (2013).
- Basu, R. Rollerbladers rescue baby turtles at East Coast. The Straits Times (2007). at <http://eresources.nlb.gov.sg/newspapers/Digitised/Article/straitstimes20060526-184.108.40.206.aspx>
- Chen, T. H. & Cheng, I. Breeding biology of the green turtle, Chelonia mydas, (Reptilia: Cheloniidae) on Wan-An Island, Peng-Hu Archipelago, Taiwan. I. Nesting Ecology. Mar. Biol. 124, 9–15 (1995).
- Zare, R., Vaghefi, M. & Kamel, S. Nest location and clutch success of the hawksbill sea turtle (Eretmochelys imbricata) at Shidvar Island, Iran. Chelonian Conserv. Biol. 11, 229–234 (2012).
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- Shaw, K. R. Effects of inundation on hatch success of loggerhead sea turtle (Caretta caretta) nests. (2013).
- Tomillo, P. S. et al. High beach temperatures increased female-biased primary sex ratios but reduced output of female hatchlings in the leatherback turtle. Biol. Conserv. 176, 71–79 (2014)
- Wood, A., Booth, D. T. & Limpus, C. J. Sun exposure, nest temperature and loggerhead turtle hatchlings: Implications for beach shading management strategies at sea turtle rookeries. J. Exp. Mar. Bio. Ecol. 451, 105–114 (2014).