Fish pretend it’s night to cope with warming oceans

As CO₂ levels rise, more of the gas is dissolving in the oceans, where it turns into carbonic acid. Since industrial times, the ocean has become 30% more acidic.

How will marine organisms fare in this new life aquatic? Scientists are just beginning to find out, but the early evidence does not bode well for some species. The higher acidity can dissolve the shells of oysters and clams, for example, and scramble the chemical signals on which fish rely to avoid predators.

But the spiny damselfish (Acanthochromis polyacanthus) may be one of the lucky ones.

In recent years, researchers found that some individuals within populations along Australia’s Great Barrier Reef are resilient to acidification. A study published in Nature Climate Change in August by a team led by Timothy Ravasi, a biologist with King Abdullah University of Science and Technology in Saudi Arabia, reveals their secret.

{%recommended 1480%}

Surprisingly, it lies in their ability to change their circadian rhythms.

In coral reefs, CO₂ levels naturally rise as the sun goes down and seaweed and other marine plants shut down photosynthesis (which absorbs CO₂) for the night.

Previous studies showed that damselfish have evolved the ability to tolerate these nightly spikes of CO₂ and the slight rise in acidity that follows. Subsequent research on the effects of higher CO₂ levels found that some individual fish are more resilient than others.

What is it about these fish, Ravasi and his team wondered, that makes them so tough? And what are their prospects for surviving future acidity levels as climate change worsens?

To find out, they caught wild spiny damselfish from the Great Barrier Reef and divided them into two groups – one tolerant and one sensitive. Then they placed the fish in tanks of seawater with CO₂ levels raised to about 750 parts per million – the level expected by the end of the century if CO₂ emissions are not curtailed – and waited for them to reproduce.

To the team’s surprise, some of the offspring hung on. “We thought they’d all die,” Ravasi says. “But we found the baby fish seemed to be adapted.”

Thinking some kind of genetic advantage might explain their adaptability, they mined a vast amount of data on the specific types of RNA and protein molecules (produced under the direction of specific genes) that were generated in the brain of the fish.

The team, which included researchers from James Cook University in Australia, found that certain genes governing circadian rhythm went into overdrive. “Basically the offspring of the tolerant fish set their circadian clock [as if it were] always night,” Ravasi says. The study found other genetic differences between the tolerant and sensitive offspring too, but the biggest was the change in the circadian rhythm genes in the tolerant juveniles.

‘THIS IS A MILESTONE, IN THAT IT PUTS TOGETHER EVOLUTION, ECOLOGY AND THE MOLECULAR MECHANISMS THAT DRIVE THE ENTIRE PROCESS’

While their resiliency is a hopeful sign that at least some marine fish are equipped with strategies that could help them adapt to a more acidic future, “we don’t want to be too optimistic,” Ravasi adds. “We don’t know how other fish species will react. They might not be as strong as our little fish.”

Giacomo Bernardi, a marine ecologist at the University of California at Santa Cruz, who was not involved in the study, says it adds some key insights to the tiny but growing body of work on how marine organisms will respond as climate change worsens. “This is a milestone, in that it puts together evolution, ecology and the molecular mechanisms that drive the entire process,” he says.

The bigger question is how individual genetic changes will affect the ways species interact with each other and their environment, Bernardi adds. “What will happen to the genes of the entire ecosystem – prey, predators, corals and plants?”

Ravasi and his team are now performing their cross-generation experiment on clownfish, another inhabitant of the Great Barrier Reef, and kingfish, a commercially fished species that lives in the open sea. Other research is taking the same approach with coral.

With conditions changing so rapidly, the work can’t happen quickly enough, Ravasi says. “There’s going to be a fast change in ocean chemistry,” he says. “We really need to understand how marine organisms will react.”