Endangered turtle population under threat as pollution may lead to excess of females being born

Published in Frontiers in Marine Science, the researchers concluded that exposure to heavy metals cadmium and antimony and certain organic contaminants, accumulated by the mother and transferred to her eggs, may cause embryos to be feminized in green sea turtles (Chelonia mydas), a species already at risk of extinction from a current lack of 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," said author Dr Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University.

"But they also face another more insidious threat linked to climate change. Sea turtles' embryos developing in their eggs have temperature-dependent sex determination, which means that more and more develop into females as temperatures keep rising."

In the northern part of the Great Barrier Reef off Australia, hundreds of females are born for every male.

"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.

"We studied the effects of pollution on the development of green sea turtles at a long-term monitoring site 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."

At the Heron Island study site, the sex ratio is currently more balanced than nearer the equator, with two to three females hatching for every male.

Conducted as part of WWF-Australia's Turtle Cooling Project researching ways to counter the occurrence of female-bias nests at warm beaches due to climate change, the authors collected 17 clutches of eggs within two hours of being laid and reburied them next to probes recording the temperature every hour inside the nest and at the beach surface.

When the hatchlings emerged, their sex was determined and levels of the 18 metals, as well as 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," said senior author Dr Jason van de Merwe, a marine ecologist and ecotoxicologist at the Australian Rivers Institute.

"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 final sex ratio varied between clutches, most nests produced predominantly female hatchlings, with the greater the amount of estrogenic trace elements, particularly antimony and cadmium, in the hatchlings' liver, the greater the female bias within the nest.

"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.

"As the sex ratio gets closer to 100% females, it gets harder and harder for adult female turtles to find a mate, which is particularly important in the face of climate change already making nesting beaches warmer and more female-biased."

"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 added.

"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."

This study was supported by funding from the World Wildlife Fund for Nature -- Australia (WWF-AU)