Green Turtle Biology, Natural History and Conservation
Green Sea Turtles also known by their scientific name, Chelonia mydas, are one of seven different species of sea turtles. Chelonia mydas are distributed worldwide and live along the coast of over 140 countries (NOAA). Once fully grown, C. mydas are the largest of the hard-shelled turtle species and can reach up to 3-4 feet long and weigh up to 300 pounds. Their lifespan is unknown but estimated to be up to 60 years. Green sea turtles are unique in that they are herbivores and mainly eat seagrass and algae. They get their name from their diet which gives their cartilage and fat a greenish color. Currently, green sea turtles are listed as an endangered or threatened species under the Endangered Species Act.
As mentioned, prior, Green sea turtles are named for their green cartilage which comes from the chlorophyll the ingest from the foods they eat, not because of the coloring of their carapace. The carapace is the top of a turtle’s shell. Green sea turtles actually have dark brown or black shells. The underside, or plastron of C. mydas, is a pale-yellow color (NOAA). C. mydas are the largest hard-shell sea turtle species as mentioned before and are only smaller than the soft-shelled Leatherback sea turtle. Fully grown Green sea turtles have 5 scutes, or bony plates down the middle of their shell and 4 scutes on each side.
Throughout their life cycles C. mydas will occupy several different habitats. They are born on sandy beaches and hatch around 2 months after being laid. From there, they make the trek to the ocean. This journey from beach to ocean is very dangerous as baby sea turtles make the perfect prey for a wide variety of organisms. It is estimated that out of a nest of 100 sea turtles, only 1 will reach adulthood. However, once they do make it to the ocean, they stay there for several years, hiding in the sargassum and floating with the ocean current. Not much is known about this stage of a turtle’s life as it is difficult to gather good data. (Araujo et al., 2016) are working to use photogrammertry, which is a minimally invasive technique to better determine the life-history parameters of Green sea turtles. Combining photo-ID and photogrammetry can allow scientists to determine where Green sea turtles gather during various stages of their life. The photo-ID helps to keep visual records while the photogrammetry can measure straight carapace lengths and widths. Pairing photo-ID and photogrammetry has already been proven to be very effective in the Philippines by (Araujo et al., 2016). It is assumed the baby turtles use their time in the open ocean to grow by eating and hiding from predators (Stubbs et al., 2019). Due to the fact that this large period of the sea turtle’s life is largely unstudied, it is hard to determine the ages of puberty, life span and reproductive potential. However, it is estimated that Green turtles become sexually mature at around 25-35 years of age. It is unknown how long their reproductive lifespan is. Most scientists rely on counting the number of nesting females on beaches; however, this data is hard to use to determine abundance trends. (Stubbs et al., 2019) hope to use the Dynamic Energy Budget (DEB) theory to better determine growth and reproduction of sea turtles.
Once C. mydas, reach maturity the females will return to the beach, often the same beach they themselves were born on, to lay their eggs. In the United States, breeding season start in late spring and early summer. Males will mate every year once they reach adulthood, while females migrate between foraging area and nests every 2 to 5 years (NOAA). Adult female Chelonia mydas migrate between foraging and nesting sites. These two different sites are often thousands of kilometers apart (Dalleau et al., 2019). Studying these hotspots are important to understand the distribution and dynamics of C. mydas. (Dalleau et al., 2019) found that spatial distribution impacted the foraging hot spots and was constrained by regional landscapes, oceanic currents and distribution of feeding patches.
Sea turtles are very interesting in that they are temperature-dependent sex determination (TSD) organisms. This means that warmer temperatures create female offspring while cooler temperatures result in male offspring (Blechschmidt et al., 2020). Due to global warming, sea turtle nests are being incubated at warmer temperatures, resulting in higher numbers of female juveniles. A study in the Great Barrier Reef showed a largely female-skewed sex ratio where almost all juvenile turtles were female (Blechschmidt et al., 2020). This skew in the sex ratio has not caused problems yet, but in 20 or so years when the turtles are sexually mature, the females will have almost no males to mate with and this could very easily lead to extinction. (Blechschmidt et al., 2020) created a stochastic individual-based model inspired by C. mydas. They hoped the turtles would begin to nest on cooler beaches as their usual nesting beaches have risen in temperature, but the turtles returned back to the original nesting beaches most likely due to their return-specific migration patterns. This example shows how climate changes is impacting long lived species and threatening their survival.
A large threat all marine animals face is ingestion of plastics and microplastics in the ocean. (Schuyler et al., 2015) determined a risk analysis of sea turtles ingesting debris to find out just how much of a problem marine debris is to sea turtles. Their findings showed that ocean life-stage turtles face the highest risk of debris ingestion, with the olive ridley sea turtle being the most at-risk species. They also determined there was no difference in ingestion rates of stranded sea turtles versus turtles caught as bycatch (Schuyler et al., 2015). This mean stranded sea turtles do not represent a bias of debris ingestion rates. Overall, the study found that current data indicates that up to 52% of sea turtles have ingested debris.
Currently all Green sea turtles or Chelonia mydas, populations are listed as either endangered or threatened. The primary threats Green sea turtles face are bycatch, direct killing of turtles and eggs, vessel strikes and lost/alteration of their habitat (NOAA). There are many different nesting populations of C. mydas across the globe. The Central South Pacific and Central West Pacific populations are classified as endangered because of their continued vulnerability. However, there is some good news! The C. mydas population in Hawaii, also known as the Central North Pacific population, has been increasing by a rate of 5% over the past 20 years! Additionally, the nesting populations of Green turtles in Florida and the Pacific coast of Mexico are also on the increase thanks to successful conservation efforts (NOAA). Both of the populations were listed as “threatened” in 2016 and show to be on the mend! (Mazaris et al., 2017) completed a study on sea turtle population sizes and found very similar results as well. Levels of abundance were summed within regional management units (RMUs) and found that there were upward trends in 12 RMUs and only 5 RMUs with downward trends (Mazaris et al., 2017). This data indicates that conservation efforts globally are making a difference! To conclude, Green sea turtle populations are starting to make a turn for the better, but conservation efforts are still needed to bring their species, as well as other turtle species, back from the brink!
Araujo, G., Montgomery, J., Pahang, K., Labaja, J., Murray, R., & Ponzo, A. (2016). Using minimally invasive techniques to determine green sea turtle Chelonia mydas life-history parameters. Journal of Experimental Marine Biology and Ecology, 483, 25-30.
Blechschmidt, J., Wittmann, M. J., & Blüml, C. (2020). Climate Change and Green Sea Turtle Sex Ratio—Preventing Possible Extinction. Genes, 11(5), 588.
Dalleau, M., Kramer‐Schadt, S., Gangat, Y., Bourjea, J., Lajoie, G., & Grimm, V. (2019). Modeling the emergence of migratory corridors and foraging hot spots of the green sea turtle. Ecology and evolution, 9(18), 10317-10342.
Fisheries, N. (n.d.). Green Turtle. Retrieved July 25, 2020, from https://www.fisheries.noaa.gov/species/green-turtle
Mazaris, A. D., Schofield, G., Gkazinou, C., Almpanidou, V., & Hays, G. C. (2017). Global sea turtle conservation successes. Science advances, 3(9), e1600730.
Schuyler, Q. A., Wilcox, C., Townsend, K. A., Wedemeyer‐Strombel, K. R., Balazs, G., van Sebille, E., & Hardesty, B. D. (2016). Risk analysis reveals global hotspots for marine debris ingestion by sea turtles. Global Change Biology, 22(2), 567-576.
Stubbs, J. L., Mitchell, N. J., Marn, N., Vanderklift, M. A., Pillans, R. D., & Augustine, S. (2019). A full life cycle Dynamic Energy Budget (DEB) model for the green sea turtle (Chelonia mydas) fitted to data on embryonic development. Journal of sea research, 143, 78-88.
Research review paper written by summer intern, Meg A. Meg is a student at Roger Williams University studying Marine Biology.