According to the study published on Friday in Nature Communications, over the course of three research trips to France in 2016 and 2017, researchers collected specimens of Mytilus galloprovincialis, the Mediterranean mussel, one of several species that provide a valuable food source worldwide.
They carefully bred dozens of combinations from 12 females and 16 males to ensure a genetically diverse population of larvae -- 192 different combinations total.
With that starting population, they divided the larvae into two groups: one to develop in water with the normal, ambient pH level of 8.1, and the other in seawater with a pH of 7.4.
The mussel larvae, over a million individuals in total, were kept in a series of buckets, with carbon dioxide-manipulated seawater pumped in. Over the span about six weeks, the researchers took samples every few days to measure shell size and analyze the genetic makeup of the surviving larvae.
In general, the mussels in the ambient pH conditions grew their shells at a faster rate than the ones in lower pH water, although after two weeks the low pH population mostly caught up. This may be because the individuals most vulnerable to low pH conditions died before this point, and the survivors were able to continue growing normally.
When the researchers analyzed the genetic variation in the two test groups, they saw strong signatures of selection in the low pH conditions, meaning that a unique genetic background emerged among the mussels able to withstand that environment. After day six, they separated the fastest shell growers from the slowest in each pH environment.
Shell size is an indicator of fitness -- the mussels with the largest shells were likely the strongest competitors. But if one mussel grows its shell the fastest in current ocean conditions, does that mean it's also going to do well in a more extreme, low pH scenario?
"The answer is no," said Mark Bitter, a UChicago graduate student who led the study. "There seems to be a very unique kind of genetic makeup of the individuals that end up growing best in the low pH environment, relative to the ambient conditions."
At the end of the experiment, there was no difference in the total survival of mussels raised in either environment. This would seem like rare good news in the context of rapidly advancing climate change: a species that already has the ability to adapt to harsher conditions.
But the researchers warned it's not the full picture.
"Some of these individuals are really good at dealing with this huge reduction in pH. But what if you also reduce salinity or change the temperature substantially?" Bitter said. "Just because you can run a marathon doesn't mean you can turn around and swim right after that. It's a multi-stressor scenario."