Green turtles-Are they vegetarian?

by Matthew Norton

The short answer is yes and no. Adult green turtles (Chelonia mydas) are indeed herbivorous, feeding on seagrass, seaweed and pieces of mangrove trees, but in their younger years they are carnivores, eating a range of animals including jellyfish, worms, shellfish and fish eggs. As they grow they gradually include seaweed and plants into their diet until they become entirely reliant on them. Changes in diet during an animal’s lifetime, especially as they develop and grow towards adulthood, is common in nature, but this extreme shift between two very different types of food is less common.

Green turtle article image 1
Green turtles are found in tropical and subtropical oceans across the world, including the coastal waters of around 140 countries.

For young green turtles, an animal based diet actually makes sense because they are weaker swimmers compared to the adults. Studies that have monitored green turtle diving behaviour have found that the dives of the younger turtles were shorter and shallower, which suggest they haven’t quite mastered the skill of diving, or the ability to properly control their buoyancy. This makes it difficult to reach patches of seaweed and seagrass because they grow on the seafloor, but near the ocean surface there are small, floating animals that the younger turtles can feed on.

Green turtle article image 2
Green turtle hatchlings eat various animal life they find at, or near the ocean surface.

Given that younger green turtles can survive on floating animal food, maybe we should be focussing on why they don’t continue eating animals into adulthood. This is a good question for two reasons.

Firstly, all other sea turtle species, including the green turtle’s close relatives the flatback turtle (Natator depressus) and hawksbill turtle (Eretmochelys imbricata), are either carnivores, or omnivores (they eat both plants and animals).

Secondly, plants are generally harder for animals to eat because they contain, among other things, a lot of fibre and cellulose, a complicated sugar that plants use to build walls around their cells and is hard for animals to break down into the smaller sugars that they actually need. The fruits and vegetables that we eat are only edible because plants ‘design’ them to be eaten by animals for the purposes of seed dispersal.

Herbivorous animals play an important role in the food chain by supporting larger (in most cases) carnivorous animals. However, evolution does not really work towards setting up these food chains and so a more immediate explanation is needed. One possibility is that by adapting to survive on a more difficult food, such as plants, adult green turtles avoid competing for food with other sea turtles who still rely on animal prey. Such adaptations include their serrated jaws which acts like a saw scraping up plants and seaweed from surfaces and tearing them into manageable pieces. Their stomachs also digest plants, especially cellulose, better than most other reptile herbivores thanks to a rich culture of bacteria that are particularly effective at breaking down plant cell walls.

The way that green turtles eat seagrass and seaweed also helps them to make the most of this resource. For example they mainly graze the tips of seagrass, encouraging the growth of new plant material which, if they are quick enough, they can pick off before the fibres, cellulose and other harder digest material can build up. They can also be flexible with their diet depending on what is available with some populations eating mainly seagrass, while others eat more seaweed. Some have even suggested that, in rare cases, they may still be omnivorous into adulthood.

The act of consuming other organisms is essential to the survival of any animal (with some exceptions) and regardless of what species they target there will be challenges for them to overcome. These include catching enough food, digesting, or getting rid of the hard parts and competing with other animals for a particular food source. In the case of green turtles they use different strategies depending on their age and where they live and so make good use of the most convenient and profitable food resource they can find.

From a human perspective

Sea turtles face numerous threats in today’s world as a consequence of our actions, they get tangled in our fish nets, they accidently eat our plastic rubbish and they lose their eggs, shells and lives to poachers. In other scenarios the impacts we have are more subtle, but no less devastating. In particular the rising temperatures on green turtle nesting beach, as a result of human induced climate change, can cause numerous problems for the developing embryos. These include burning through their energy reserves (contained in the egg yolk) too fast during development, as well as abnormalities and weaker swimming ability in the hatchlings. Even more worrying is how increasing temperatures can play havoc with the birth rates of males and females.

Green turtle article image 3
Getting tangled in fishing nets (left), accidently eating plastic (centre) and poaching (right) are all major threats to turtles.

Determining whether an animal is born as a male, or a female is a more complex procedure than most of us realise, with different groups doing this in different ways. In some animals gender is determined by how many copies of their entire gene set they have, in others it’s determined by the combinations of specific chromosomes (bundles of genetic material) they inherit from their parents. In turtles, and other reptiles, genes play no role at all with gender being determined by the temperature at which the egg is incubated. Eggs incubated at 26-28oC produce male hatchlings and eggs at 30oC produce females.

Normally this would produce a healthy ratio of males and females, but with increasing temperatures, both globally and on particular nesting beaches, there is a very real risk of these ratios being skewed towards producing too many females and too few males. In some areas this has already yielded extreme results with over 99% of the population being female. The repercussions of this are pretty clear, without enough males to reproduce with the females the green turtle as a species is at risk of extinction.

There may be some nesting sites that can still produce male offspring due to the widespread range of green turtles, which exposes them to a range of temperatures despite the general trend of increasing temperature. Even within a single nesting beach, nesting temperature can still vary due to various factors, such as how high on the beach the nest is and how close it is to shade. This might be wishful thinking, but if green turtles are to survive in the future, then male producing nesting beaches should be prioritised for protection from poaching and other human threats.

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Thanks for reading

Sources

Wikipedia. 2018a. Green sea turtle. https://en.wikipedia.org/wiki/Green_sea_turtle. Last accessed 29/10/2018

Arthur et al. 2008. Ontogenetic changes in diet and habitat use in green sea turtle (Chelonia mydas) life history

Arthur and Balazs. 2008. A Comparison of Immature Green Turtle (Chelonia mydas) Diets among Seven Sites in the Main Hawaiian Islands1

Fourqurean et al. 2010. Effects of excluding sea turtle herbivores from a seagrass bed: overgrazing may have led to loss of seagrass meadows in Bermuda

Bjorndal. 2017. Chapter 8 in “The Biology of Sea Turtles Vol. I” (eds Lutz and Musick). ISBN. 978-0849384226

Carman et al. 2013. A jellyfish diet for the herbivorous green turtle Chelonia mydas in the temperate SW Atlantic

Reich et al. 2007. The ‘lost years’ of green turtles: using stable isotopes to study cryptic lifestages

Bolten. 2003. Chapter 9 in “The Biology of Sea Turtles Vol. II” (eds Lutz et al.). ISBN. 978-0849311239

Salmon et al. 2004. Ontogeny of diving and feeding behavior in juvenile seaturtles: leatherback seaturtles (Dermochelys coriacea L) and green seaturtles (Chelonia mydas L) in the Florida Current

Bjorndal. 1980. Nutrition and grazing behavior of the green turtle Chelonia mydas

Planck-Gesellschaft. 2010. Cellulose: Hard to digest, but full of energy. https://www.eurekalert.org/pub_releases/2010-07/m-cht072010.php. Last accessed 29/10/2018

Wikipedia. 2018b. Flatback sea turtle. https://en.wikipedia.org/wiki/Flatback_sea_turtle. Last accessed 29/10/2018

Wikipedia. 2018c. Hawksbill sea turtle. https://en.wikipedia.org/wiki/Hawksbill_sea_turtle. Last accessed 29/10/2018

Krestovnikoff. 2018. Nature’s Home: The RSPB Magazine. Winter 2018

National Wildlife Federation. 2018. Green Sea Turtle. https://www.nwf.org/Educational-Resources/Wildlife-Guide/Reptiles/Sea-Turtles/Green-Sea-Turtle. Last accessed 29/10/2018

Bjorndal. 1979. Cellulose digestion and volatile fatty acid production in the green turtle, Chelonia mydas

See Turtles. 2018. What Do Sea Turtles Eat? https://www.seeturtles.org/sea-turtle-diet/. Last accessed 29/10/2018

Hatase et al. 2006. Individual variation in feeding habitat use by adult female green sea turtles (Chelonia mydas): are they obligately neritic herbivores?

Gabbatiss. 2017. Plastic pollution: Turtles are dying after becoming tangled in fishing gear and household rubbish, new study finds. https://www.independent.co.uk/environment/plastic-pollution-turtles-dying-oceans-worldwide-tangled-waste-study-a8107616.html. Last accessed 29/10/2018

McGrath. 2018. ‘A single piece of plastic’ can kill sea turtles, says study.  https://www.bbc.co.uk/news/science-environment-45509822. Last accessed 29/10/2018

Seeturtles. 2018. Illegal poaching. https://www.seeturtles.org/illegal-poaching/. Last accessed 29/10/2018

Gempe and Beye. 2010. Function and evolution of sex determination mechanisms, genes and pathways in insects

Cook. 1993. Sex determination in the Hymenoptera: a review of models and evidence

Spotila et al. 1987. Temperature dependent sex determination in the green turtle (Chelonia mydas): effects on the sex ratio on a natural nesting beach

Booth and Astill. 2001. Incubation temperature, energy expenditure and hatchling size in the green turtle (Chelonia mydas), a species with temperature-sensitive sex determination

Hays et al. 2003. Climate change and sea turtles: a 150‐year reconstruction of incubation temperatures at a major marine turtle rookery

Fuentes et al. 2010. Past, current and future thermal profiles of green turtle nesting grounds: Implications from climate change

Welch. 2018. RISING TEMPERATURES CAUSE SEA TURTLES TO TURN FEMALE. https://news.nationalgeographic.com/2018/01/australia-green-sea-turtles-turning-female-climate-change-raine-island-sex-temperature/?user.testname=none. Last accessed 29/10/2018

Booth and Evans. 2011. Warm water and cool nests are best. How global warming might influence hatchling green turtle swimming performance

Image sources

Lindgren. 2013. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D. https://commons.wikimedia.org/wiki/File:Green_Sea_Turtle_grazing_seagrass.jpg

Grendelkhan. 2018. [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D. https://commons.wikimedia.org/wiki/File:Confiscated_musical_instrument_made_from_green_turtle.jpg

All other images are public domain and do not require attribution

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