(CN) — A new study published on Monday revealed how pollutants are compounding the effects of rising temperatures on green sea turtles, causing the endangered reptile to produce mostly female offspring.
The research — published in Frontiers of Marine Science and led by Arthur Barraza from the Australian Rivers Institute at Griffith University — explored how pollution effects the development of green sea turtles on Heron Island in the southern Great Barrier Reef.
Scientifically known as Chelonia mydas, green sea turtles are an endangered species that received conservation protections from the National Marine Fisheries Service and U.S. Fish and Wildlife Service in 1977, and are currently recognized by the International Union for Conservation of Nature’s Red List of Threatened Species.
The large sea turtles’ global population has long faced existential threats like poaching, boat collisions, habitat destruction and accidental capture. The turtles’ survival is now made more complicated by climate change, particularly since the species have temperature-dependent sex determination.
On Heron Island, anywhere from 200 to 1,800 female green sea turtles arrive annually to nest, producing one male turtle for every two to three females. This sex ratio is less balanced for turtles in hotter climates, however, as males are only born in temperatures below 84 degrees Fahrenheit.
But while studying the green sea turtles with WWF Australia’s Turtle Cooling Project — which investigates how to counter anthropogenic influences on turtle sex ratios — Barraza and colleagues discovered yet another human-made threat to the island’s fragile population: contaminants that act as estrogen mimics.
"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,” Barraza said in a statement.
To reach this conclusion, the researchers observed 17 clutches that were reburied nearby automatic temperature probes within two hours of being laid. When the turtles hatched, the team euthanized them to dissect and examine their sexual organs and livers — the latter of which allowed researchers to measure contaminants using inductively coupled plasma mass spectrometry in addition to bioassays on cultured sea turtle cells.
While analyzing contaminates, the researchers focused on 18 metals, including chromium, antimony and barium. They also focused on organic contaminants like polybrominated diphenyl ethers, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. These types of chemicals — which can be bioaccumulated, carcinogenic or both — pollute the environment through manufactured products or, in the case of polycyclic aromatic hydrocarbons, through the burning of coal, gas, wood, garbage and tobacco.
In model organisms, the researchers these metals and chemicals are either known or suspected to act like xenoestrogens: substances that are molecularly similar to estrogen, allowing them to bind with receptors for female sex hormones.
“Accumulation of these contaminants by a female turtle happens at the site where she forages,” Barraza explained. “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.”
While most nests produced females, the researchers reported varying sex ratios between clutches, where some had been entirely male and others entirely female.
The researchers also found that livers with higher average levels of antimony and cadmium indicated a greater bias toward female hatchlings, leading them to conclude that such contaminants can mimic the hormone estrogen and direct developmental pathways.
Barraza indicated that, over time, this pattern could spell trouble for the imperiled species.
“As the sex ratio gets closer to 100% females, it will get harder and harder for adult female turtles to find a mate,” Barraza said. “This becomes especially important as climate change will continue to make nesting beaches warmer and more female-biased.”
Co-author Jason van de Merwe — who also hails from the Australian Rivers Institute at Griffith University — explained that determining which compounds may affect hatchling sex ratios is vital for preventing contaminants from further feminizing sea turtle populations.
The author said, “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 used science-based long-term strategies to reduce the input of pollutants into our oceans.”
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