For an oyster, gender is more than a matter of genetics—it’s also about the environment. Water temperature, salinity, pollution, and other factors determine whether an oyster will be male or female. And a recent study added something new to the list: acidity.

The oceans are becoming more acidic as they absorb carbon dioxide from the air. Over the past couple of centuries, they’ve taken up about a third of all the CO2 added to the atmosphere through the burning of fossil fuels. Today, the concentration of CO2 in the oceans is at its highest in 800,000 years. By the end of the century, it could be at its highest in 20 million years.

The more-acidic waters make it harder for oysters and other creatures to make their shells. And researchers looked at the impact on the sex of oysters. They gathered oysters from the wild and from hatcheries—both in China—and put them in tanks with different levels of acidity. The oysters in the more-acidic water spawned about three times more females than males.

The scientists then placed the new generation in two locations in the wild, with different levels of acidity. Both groups spawned more females than males, but the ratio was higher in the more-acidic waters.

Researchers conducted lab studies to understand how this happens. They found that the higher acidity turned on female-producing genes, and turned off the male-producing genes.

So oysters face one more threat from the world’s changing oceans.

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