The sex ratio of endangered sea turtles is badly skewed by pollution

Photo: Pexels/Stephen Leonardi

Exposure to heavy metals such as cadmium and antimony as well as certain organic contaminants accumula in female green sea turtles, which then pass these toxins to their eggs, causing embryos to be feminized. This is now adding to the travails of the reptiles already at risk of extinction.

Green sea turtles (Chelonia mydas), which are listed as endangered on the IUCN Red List of Threatened Species, face a variety of threats with poaching, habitat loss, collisions with boats and  accidental capture in fishing gear being primary among them, experts say.

“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,” explains Arthur Barraza, a researcher at the Australian Rivers Institute of Griffith University who was a lead author of a new study.

In their research Barraza and his colleagues have found that hundreds of females are born for every male in the northern part of the Great Barrier Reef off Australia. This trend is alarming as such a prominent sex imbalance in the species poses grave risks to their reproductive prospects.

Worse is that heavy metals in ocean water near coastlines appear to be contributing to this imbalance.

“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,” Barraza says.

“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 this island the sex ratio is currently more balanced than nearer the equator with two to three females hatching for every male.

The research is part of an effort to counter the occurrence of female-biased nests at warming beaches. To do this, the scientists 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),” the scientists report.

“These contaminants are all known or suspected to function as ‘xenoestrogens’ or molecules that bind to the receptors for female sex hormones,” notes 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,” the scientist elucidates.

Although the sex ratio varied between clutches most nests predominantly produced female hatchlings.These hatchlings had greater amounts of estrogenic trace elements, particularly antimony and cadmium, in their liver. In fact, the greater the amounts of contaminants were, the greater the female bias became within a nest, according to the researchers.

“From these results we concluded that these contaminants mimic the function of the hormone estrogen, and tend to redirect developmental pathways towards females,” Barraza explains.

“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,” he adds.

Most heavy metals in the sea result from human activities such as mining, runoff, and pollution from general urban waste, the scientists say.

“[T]he best way forward is to use science-based long-term strategies to reduce the amount of pollutants going into our oceans,” Van de Merwe stresses.