Ocean acidification is a threat to shell-making organisms across the seas, but one species of oyster may provide a unique exception to this rule.

In a study published March 27 in Science of the Total Environment, researchers show how an edible species of oyster, Crassostrea hongkongensis, appeared to grow faster and larger and survive better in a wild habitat after being reared in a low-pH environment. The surprising observation may have positive implications for maintaining production of oysters as oceanic conditions change.

"We tend to think that ocean acidification always presents a threat, but sometimes I feel like the issue has been overgeneralized in the general public," said lead author James Lim, a marine science Ph.D. student at the University of Hong Kong who wrote the paper as part of his thesis. "There's always both sides in every situation, and we have to think of both and what their causes are."

To test the effect of low pH on the local oyster, Lim and his colleagues gathered 250 adult specimens and divided them among tanks simulating acidic conditions at a pH of 7.4, and control tanks closer to a typical ocean pH of 8.0. After four weeks, researchers bred these adult oysters at their respective pH levels, and allowed their offspring to grow out of their larval stage.

Once the oysters metamorphosed into juveniles, Lim and his colleagues removed them from the lab and planted them in a local estuary, where they remained for 10 months. When the researchers returned, they found that their hypothesis was proven wrong. 

"It was like, whoa; I hypothesized that this oyster species would be vulnerable to ocean acidification, but some of them were actually able to survive, and others were able to thrive in these acidifying conditions," Lim said in an interview with The Academic Times.

The oysters that had been bred and raised in a low-pH environment tended to grow faster and bigger and survive at higher rates than those subjected to the control pH, suggesting that this oyster species is successfully managing the stress of ocean acidification by making itself stronger.

According to Lim, these oysters may be demonstrating a form of epigenetic adaptation, where an organism's original DNA structure remains intact, but the molecular mechanism behind it changes. Where typical genetic adaptation can take thousands or millions of years, epigenetic adaptation can occur over one generation, or in as little time as a few hours.

Environmental stressors, such as shifts in temperature or pH, can trigger an epigenetic adaptation. When this happens, chemicals from a methyl group can attach to an organism's DNA, making it "methylated," and, in the case of the Hong Kong oyster, resulting in the improved fitness and altered shell formation of oysters grown in water treated with carbon dioxide.

Even though the oysters that "grew up" in the acidified environment appeared to outperform the others at the aquaculture field site, Lim and his colleagues still are not sure how positive the adaptive response is, because the microstructure of the acidified oyster shell may still be inferior to that of an oyster used to normal ocean pH. This possibility, according to James, is currently being studied.

Despite the uncertainty around the integrity of acidified oyster shells, Lim and his colleagues believe the aquaculture industry, specifically oyster farmers, will stand to benefit from their findings. 

China alone accounts for over 85% of total oyster production worldwide, with the Hong Kong oyster being "an important fisheries resource that is cultivated in the coastal waters of the South China Sea," according to previous research. In 2015, China produced 4.6 million tons of oysters, where the U.S. yield in 2017 was 18,000 tons, worth $186 million.

"We all know ocean acidification is going to impact the shellfish industry — that's very clear," said Vengatesen Thiyagarajan, Lim's research supervisor and a marine sciences professor at the University of Hong Kong. "To tackle this climate change and environmental problems, we have to adapt quite quickly."

Lim and Thiyagarajan believe their research could aid in oyster production through climate change and ocean acidification. If they can determine what genes govern the physiological changes of the oysters grown in acidified conditions, oyster farms could use that information to reproduce them on a wide scale.

"If the upscaled mechanism is epigenetically or genetically inherited, that would be fantastic news for the industry," Thiyagarajan said.

Even though Crassostrea hongkongensis appears to handle low pH environments well, prior research suggests that the same is not true for many other species, oyster or otherwise. Recent papers from Proceedings of the Royal Society B and the Journal of Experimental Biology show that ocean acidification damages both the hearing capabilities and olfactory neurons, which relate to sense of smell, of two different fish species. 

"There are definitely species out there, whether clams or snails, that are vulnerable to ocean acidification," said Lim. "But it is not entirely true for some species, where inside themselves they are able to make use of this threatening process, and adapt to be able to survive."

In future papers, Lim and his colleagues want to expand their experiments to include other species of oysters, as well as other effects of pollution and climate change, such as fluctuations in salinity, to see how oysters may respond to those, as well. 

The study, "Transgenerational responses to seawater pH in the edible oyster, with implications for the mariculture of the species under future ocean acidification," published March 27 in Science of the Total Environment, was authored by James Lim, Xin Dang and Vengatesen Thiyagarajan, the University of Hong Kong.