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Study Shows How Climate Change Impacts Mussel Population

The oceans are quickly acidifying with increasing climate change. Now, biologists from the University of Chicago have performed a novel study that reveals that mussels grown in an acidic experimental setting grew smaller shells when compared to those raised in normal environmental levels.

A new study from the University of Chicago shows that mussels raised in an acidic experimental environment grew smaller shells, but survived. Image Credit: Neil Schofield/Flickr.

However, the total survival rate of mussels raised under both sets of conditions remained the same.

Within the acidic environment, the surviving population of mussels was genetically different from the others. This indicated that genetic variants already existing in a subset of the natural mussel population enabled the species to adapt to the more aggressive setting.

For both conservationists and seafood lovers, this may be good news as the mussels look for ways to adapt to the fluctuating seas.

The hope is there are already a few individuals in the species that already have some genetic makeup that allows them to withstand the change in the environment. This effectively allows evolution to work a lot faster if you're not waiting around for some new mutation to arise.

Mark Bitter, Study Lead Author and Graduate Student, University of Chicago

The study has been published in the Nature Communications journal on December 20th, 2019.

While the discovery provides some hope, it does not promise that species will be able to completely endure the issues caused by climate change, the study authors cautioned. For example, pH is only one of the variables that are anticipated to change in the days to come.

We have already seen substantial declines in shell size of one mussel species in nature, but it does look like there is this capacity for this species to adapt, which is some good news amidst a lot of bleakness.

Mark Bitter, Study Lead Author and Graduate Student, University of Chicago

With the continuous burning of fossil fuels, the oceans absorb about one-third of the additional carbon discharged into the air. This causes a reduction in the pH levels of the seawater, rendering it more acidic. A few species of algae, as well as oysters and mussels, find it difficult to produce their tough, calcium carbonate shells in such a setting.

Professor Cathy Pfister, the senior author of the latest study and a top scholar of the dynamics of species in marine systems performed a study in 2016. This study demonstrated that today’s mussel shells that gathered in the Pacific Northwest were 32% thinner, on average, when compared to those from the 1970s.

During the period of three research journeys to France in 2016 and 2017, scientists gathered specimens of the Mediterranean mussel called Mytilus galloprovincialis. This is one of the numerous species offering a valuable food source across the world.

The researchers meticulously raised dozens of combinations from 16 males and 12 females to ensure larvae population that is genetically diverse—in total, 192 different combinations.

Using that initial population, the scientists divided the larvae into two types of groups—one group to grow in water that has the usual, ambient pH level of 8.1, and the other group to grow in seawater that has a pH of 7.4. (The lower 7.4 pH utilized in the experiment drops below the predicted global declines in the pH levels of seawater over the next century, but is likely to be faced by marine species living in coastal habitats before the end of this century.)

The mussel larvae, which totaled more than a million individuals, were maintained in an array of buckets that were pumped in with carbon dioxide-manipulated seawater. Then, over a period of approximately six weeks, Bitter and his coworkers took samples every few days to quantify the size of the shells and study the genetic makeup of the surviving larvae.

Generally, when compared to the mussels in lower pH water, the ones in the ambient pH conditions grew their shells at a quicker rate, although the population in the lower pH mostly caught up after a period of two weeks.

Bitter believes that this could be because the individuals that were most susceptible to lower pH conditions died before this point, while the survivors continued to grow at a normal level.

While analyzing the genetic difference in the two test groups, the scientists observed powerful signs of selection in the lower pH conditions. This implies that a special genetic background evolved among the mussels that were able to tolerate that setting.

After the sixth day, the researchers isolated the fastest shell growers from the slowest ones in each pH setting. Fitness is indicated by the size of the shell—the mussels that have the largest shells were probably the strongest competitors.

But if a single mussel develops its shell the fastest in present ocean conditions, does that imply that it is also going to do well in a low pH and more adverse situation?

The answer is no. 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.

Mark Bitter, Study Lead Author and Graduate Student, University of Chicago

Toward the end of the experiment, no variation was observed in the total survival of mussels grown in both the settings. This would appear like some unusual good news in terms of quickly progressing climate change—a species that already has the potential to adjust to more aggressive conditions. However, it is not the full picture, warned the authors.

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?” added Bitter. “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.”

According to him, the research demonstrated why it is crucial to focus on aquaculture and conservation efforts to sustain a wide genetic diversity among populations of mussels because the potential to adapt to near-future situations seems to already exist in the gene pool.

The study was sponsored by the France and Chicago Collaborating in the Sciences program, developed to promote collaboration among UChicago research teams and scientists at higher learning institutions in France.

The study is titled “Standing genetic variation fuels rapid adaptation to ocean acidification.” It was funded by the National Science Foundation, European Commission, the U.S. Department of Education, and the UChicago FACCTS program.


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