Sydney rock oysters (Saccostrea glomerata) at Australia’s Wingan Inlet. Photo Courtesy of Stevage

As Earth’s atmosphere soaks up an ever-increasing amount of carbon dioxide, our oceans are becoming more acidic, which is causing problems for animals that create hard shells, from crabs to shellfish. Now, new research has revealed a more subtle and sinister consequence of acidification. By the end of this century, ocean water could wreak havoc on many mollusks’ reproductive systems, reducing their reproductive output, delaying maturation, and most strangely of all, increasing the proportion of females.

Sydney rock oysters (Saccostrea glomerata) are found only in Australia and New Zealand. Like most oysters, its sex determination system is somewhat alien to our own. Some members of Saccostrea are males their whole lives, determined by genetics. Others are “protandrous hermaphrodites”, which start life as males, but become females later.

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The speed of the sex shift is thought to be determined by complex environmental inputs. Previous work hinted that the pH, or acidity, of the surrounding water might mess with how this process works, so a team of Australian and German scientists set out to directly test it. Their results, published Tuesday in Proceedings B of the Royal Society, suggest that given rising ocean acidity, for oysters, the future is female.

The researchers collected and raised young rock oysters in a hatchery setting, using different combinations of environmental conditions to simulate the oceans of the year 2100, assuming an uninhibited rise in atmospheric carbon dioxide. (As carbon dioxide dissolves in seawater, it forms carbonic acid, causing the pH to drop.)

Some oysters were raised under today’s carbon dioxide and ocean acidity levels, and optimal growth temperatures. Others were raised with the temperature four degrees Celsius hotter, and with carbon dioxide levels at twice the concentration of today. The rest were either raised with just the elevated temperature, or just the increased carbon dioxide. Over the next eight weeks, the researchers sampled the oysters to gauge their current sex, and development of their gonads and sex cells. The oysters were also induced to spawn so their reproductive output could be measured.

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Sydney rock oysters on a mangrove root. Photo Courtesy of Marc Lehmann.

They found that acidification markedly skewed the sex ratio. Within a single reproductive cycle, oysters raised in the 2100 acidification conditions had 16 percent more females than the control group, resulting in a 2.3:1 sex ratio.

Some oysters are thought to influence the switch to femaleness via chemical cues from neighbor oysters. It’s possible that the acidity of the water messes with this chemical signal. It’s also possible that female oysters tolerate the acidic water better than their male counterparts, simply skewing the ratio through higher survival rates.

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Acidification produced other strange effects, including reduced reproductive output, shorter shell length, and increased mortality. It’s worth noting that elevated temperature by itself actually helped in these regards, but once acidification effects were thrown in, any benefits were completely eclipsed.

The study adds an important dimension to the impacts of ongoing acidification. Screwing with sex ratios and reproductive health is a surefire way to make species less resilient to other threats. More research needs to be done with other aquatic invertebrates to verify how widespread the phenomenon observed in this study is, or if the effects seen in the oysters persist through multiple generations.

The discovery also complements sex determination disarray observed in other species due to the effects of rising CO2. For example, many reptiles use temperature to determine sex, so climate change is resulting in skewed ratios. Already, in one Australian sea turtle rookery, 99 percent of the hatchlings are female. That’s happening off of Florida, too. Bearded dragon lizards are also feeling the sex skewing.

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Reproductive problems arising from increased acidity may be far broader than just the ocean, since acidification is a growing issue in freshwater habitats too. If these types of impacts are widespread, the future of many aquatic ecosystems may be even rockier than expected.

Jake Buehler is a Seattle area science writer with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.