Fish unable to rapidly adapt to ocean acidification

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In high-CO₂ water, offspring don't do much better than their parents.

Apart from strengthening the greenhouse effect, our emissions of carbon dioxide also affect the chemistry of the oceans. When CO₂ dissolves in water, it lowers the pH, which makes it more difficult for organisms to make calcium carbonate shells. The low pH also has some direct physiological effects on other marine organisms like fish. The big question mark for the future is whether these organisms can adapt or evolve to better deal with a higher-CO₂ world. A new study in Nature Climate Change digs into the adaptation part of that question.

The study, led by Megan Welch at James Cook University, follows up on a previous experiment we covered in 2012. In that work, researchers put spiny damselfish hatchlings in tanks with varying levels of CO₂ and tested several behaviors.
First, researchers put the fish in a split tank with one side containing the odor of a predator, and then they measured how much time the fish spent in each side. High CO₂ made the animals much less likely to avoid the predator cue.
Second, the team tested the fishes' preference for right or left turns—behavior analogous to humans being right- or left-handed and connected to the development of important escape reactions. High CO₂ had an impact here, too, impairing the development of that preference.
The researchers were able to work out that these changes resulted from carbon dioxide interfering with a specific chemical transmitter in the brain.
But what if those fish had offspring? Would the next generation be better acclimated?

Combinations

To find out, the researchers raised spiny damselfish in low-, medium-, and high-CO₂ water covering a range of CO₂ we’re likely to see between now and the end of the century. When those fish had offspring, they were split up and moved into low-, medium-, and high-CO₂ water themselves. That resulted in every possible pairwise combination—medium-CO₂ fish from low-CO₂ parents, low-CO₂ fish from high-CO₂ parents, and so on.
The young were then run through similar tests as before—one to see if they would recognize and avoid a chemical cue signalling a predator attack, and one to see if they would develop a left/right preference. Let’s take the offspring of fish that had lived in low-CO₂ water first. Those that were also put into low-CO₂ water spent less than 10 percent of their time in the presence of the predator alarm cue. As in previous experiments, those that were put into high-CO₂ water actually spent about 80 percent of their time in the section with the predator cue. No surprises yet, as this is basically a replication of the earlier experiment.
How about the offspring of high-CO₂ parents? Those that were also put into high-CO₂ water did no better, hanging out with the predator cue nearly 80 percent of the time. They didn’t seem to get any benefit from their parents having acclimated to these conditions. Those that were put into low-CO₂ water, however, didn’t behave normally. They spent almost 30 percent of their time with the predator cue, as if suffering a generational hangover from their parents’ environment.
The story was largely similar with right/left preference. Among those with low-CO₂ parents, those in low-CO₂ water themselves displayed a normal preference, while those in higher-CO₂ water displayed much less preference. Again, low-CO₂ fish with high-CO₂ parents fell short of normal. But here, at least, second generation high-CO₂ fish did slightly better than first generation high-CO₂ fish, indicating some benefit from their parents. Even so, it wasn’t much, and they were nowhere near normal.
Researchers don’t know yet how these shifts were passed down to the offspring, but it’s a good bet that it’s an epigenetic change—one that alters gene activity without altering the DNA itself. Changes could have come with the parental chromosomes or could have been locked in place during the development of the embryo. Either way, repeating the experiment with subsequent generations of offspring should yield the same results—epigenetic changes aren't likely to accumulate.
Some other experiments like this, focused on other marine organisms and other impacts of CO₂, have found that a second generation raised in high-CO₂ conditions is able to reverse some of the problems that hampered their parents. That doesn’t seem to be the case for these behavioral impacts, which could be because there’s less genetic flexibility to increase tolerance.
If epigenetics can't help you adapt, you're left with evolutionary adaptation. It wasn’t clear from the experiment how much genetic variation there might be among the fish—variation that would allow natural selection to make the population more resilient to high-CO₂ conditions. It will take more work to breed enough generations to find out. In the mean time, the rest of the fish in the sea will be part of humanity’s great carbon emission experiment.