Research on populations of green sea turtles suggests that specific pollutants that accumulate in female turtles could be passed onto their young and may cause them to become feminized. This situation may compound issues for a species that already has fewer male hatchlings.
“Green sea turtles are listed as endangered on the IUCN Red List of Threatened Species, threatened with risk of extinction due to poaching, collisions with boats, habitat destruction, and accidental capture in fishing gear,” study author Dr Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University, said in a statement.
“But they also face another more insidious threat linked to climate change”, he added.
For years now, scientists have been aware that the number of male green sea turtles (Chelonia mydas) has been shrinking. This is because embryos developing in the egg have temperature-dependent sex determination, which means that a growing number of turtles are developing into females as temperatures continue to rise.
In fact, in the northern part of the Great Barrier Reef in Australia, hundreds of female turtles are born for every male born.
“Our research shows that the risk of extinction due to a lack of male green sea turtles may be compounded by contaminants that may also influence the sex ratio of developing green sea turtles, increasing the bias towards females,” Dr Barraza said.
Barraza and his team studied the effects pollution had on the development of these turtles at a long-term monitoring site on Heron Island, which is a small coral sand cay in the southern Great Barrier Reef. Each year, between 200 and 1,800 female green sea turtles come to visit the area to breed.
At the moment, the sex ratio of turtles at the Heron Island site is more balanced than it is further towards the equator. Here, there are two to three female hatchlings for every male.
In order to study this phenomenon, the team collected 17 clutches of eggs within two hours of them being laid. They then reburied them next to probes that measured the temperature inside the nest and at the beach surface at hourly intervals.
Once the hatchlings emerged, their sex was identified and levels of the 18 metals were recorded. In addition, the team recorded the levels of organic contaminants like polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs).
“These contaminants are all known or suspected to function as ‘xenoestrogens’ or molecules that bind to the receptors for female sex hormones,” senior author Dr Jason van de Merwe, a marine ecologist and ecotoxicologist at the Australian Rivers Institute, explained.
“Accumulation of these contaminants by female turtles happens at foraging sites. As eggs develop within her, they absorb the contaminants that she accumulated and sequester them in the liver of the embryos, where they can stay for years after hatching.”
Although the ratio of male to female hatchlings emerging from the nests ultimately varied, predominantly more of the latter were produced in those with greater levels of estrogenic trace elements, such as antimony and cadmium.
“From these results we concluded that these contaminants mimic the function of the hormone estrogen, and tend to redirect developmental pathways towards females,” Dr Barraza said.
“Determining which specific compounds can change the hatchling sex ratios is important for developing strategies to prevent pollutants from further feminizing sea turtle populations,” Dr van de Merwe concluded.
“Since most heavy metals come from human activity such as mining, runoff, and pollution from general urban waste, the best way forward is to used science-based long-term strategies to reduce the amount of pollutants going into our oceans.”
The study was conducted as part of the World Wildlife Fund for Nature – Australia’s Turtle Cooling Project.
The paper is published in the journal Frontiers in Marine Science