Green sea turtles
11-13-2023

Sex ratio of green sea turtles is approaching 100 percent females due to pollution

Green sea turtles (Chelonia mydas), are listed as endangered on the IUCN Red List of Threatened Species. They face extinction risks from poaching, boat collisions, habitat destruction, and accidental capture in fishing gear. Yet, another insidious threat linked to climate change is emerging: temperature-dependent sex determination. 

Hundreds of females for every male

As temperatures rise, an increasing number of embryos develop into females. In the northern Great Barrier Reef off Australia, this has led to a significant skew, with hundreds of females born for every male.

Adding to this concern, researchers have now shown that pollution may compound the risk of extinction due to a lack of male green sea turtles. 

“Here we show that contaminants from human activities may also influence the sex ratio of developing green sea turtles, increasing the already extant bias towards females,” said lead author Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University.

Focus of the study

The study focused on the effects of pollution on green sea turtles on Heron Island, a small coral sand cay in the southern Great Barrier Reef. This site, where 200 to 1,800 females nest annually, has a more balanced sex ratio than locations closer to the equator, with approximately two to three females hatching for every male.

The researchers collected 17 entire clutches shortly after laying and monitored temperatures with automatic probes. Upon emergence, the hatchlings were euthanized, their sex determined, and liver contaminants measured using ICP-MS and bioassays on cultured sea turtle cells.

Various pollutants 

The study focused on metals like chromium, antimony, barium, and organic pollutants such as PAHs, PCBs, and PBDEs, known or suspected as “xenoestrogens.”

“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,” Barazza explained.

Heavy metals 

The experts found a direct correlation between the amount of heavy metals antimony and cadmium in the hatchlings’ liver and a bias towards females within the nest. These contaminants mimic estrogen, influencing developmental pathways towards females. 

Sex ratio 

“As the sex ratio gets closer to 100 percent 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,” Barazza said.

“Determining which specific compounds could change the hatchling sex ratios is important for developing strategies to prevent pollutants from further feminizing sea turtle populations,” added senior author Jason van de Merwe, a marine biologist and ecotoxicologist at Griffith. 

Study implications

“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.”

This study – published in the journal Frontiers in Marine Biology – highlights the multifaceted threats to green sea turtles, emphasizing the urgent need for comprehensive strategies to mitigate the impacts of both climate change and pollution on these endangered species.

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