Green sea turtles are a threatened species listed as endangered on the IUCN Red List of Threatened Species. They face multiple anthropogenic threats such as poaching, accidental capture in fishing gear, collisions with boats, and habitat destruction. These threats are compounded by climate change, which is causing more embryos to develop into females as temperatures rise. The resulting risk of extinction due to a lack of male green sea turtles may be further compounded by pollution.
A new study published in Frontiers in Marine Science reveals that contaminants from human activities may influence the sex ratio of developing green sea turtles, increasing the already existing bias towards females. The study was conducted by researchers at the Australian Rivers Institute at Griffith University, as part of WWF-Australia’s Turtle Cooling Project. The project aims to counter anthropogenic influences on turtle sex ratios.
The researchers studied the effects of pollution on the development of green sea turtles on Heron Island, a small coral sand cay in the southern Great Barrier Reef, where between 200 and 1,800 females come to nest every year. The sex ratio on the island is currently more balanced than nearer the equator, with approximately two to three females hatching for every male.
The researchers collected 17 entire clutches within two hours after the eggs were laid, and reburied them nearby adjacent to automatic temperature probes. When the hatchlings emerged, they were euthanized, and their sex was determined by dissecting out and examining the sexual organs. The researchers focused on 18 metals such as chromium, antimony, and barium, as well as organic contaminants like polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs).
The study found that the greater the average amount of the heavy metals antimony and cadmium in the hatchlings’ liver, the greater the bias towards females within the nest. The final sex ratio varied from 100% males to 100% females between clutches, although the majority of nests produced mainly female hatchlings. The authors concluded that these contaminants mimic the function of the hormone estrogen, and tend to redirect developmental pathways towards females.
The authors emphasized that accumulation of these contaminants by a female turtle happens at the site where she forages. As eggs develop within her, they absorb the contaminants that she accumulated. These then are sequestered in the liver of the embryos, where they can stay for years after hatching. The authors also noted that determining which specific compounds could change the hatchling sex ratios is important for developing strategies to prevent pollutants from further feminizing sea turtle populations.
Dr Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University and first author of the study, said: “As the sex ratio gets closer to 100% females, it will get harder and harder for adult female turtles to find a mate. This becomes especially important as climate change will continue to make nesting beaches warmer and more female-biased.”
Senior author Dr Jason van de Merwe, from the same institute, added: “Since most heavy metals come from human activity such as mining, runoff, and pollution from general urban center waste, the best way forward is to use science-based long-term strategies to reduce the input of pollutants into our oceans.