Raised in acidified seawater, reef fishes act like risky teenagers

When you’re a tiny reef fish and the reef is your entire world, there are a few important strategies to avoid becoming the meal of a slightly larger reef fish. One of those strategies is: if you detect a nearby predator, swim the other direction. Seems simple enough, but some fish have the wrong idea.

Recent research has shown that when exposed to seawater with large concentrations of carbon dioxide – that sort of concentrations projected for the increasingly acidifying ocean this century – the behavior and decision-making abilities of reef fish are compromised. They swim towards, rather than away from, the scents that they’d normally avoid, such as those associated with predators. They can no longer use associative learning to connect predator-related threats with the possibility of an early demise. Juvenile reef fish also become more active when they develop in high-CO2 environments, and as a result engage in riskier behavior, swimming further from the sheltering environment of their coral homes. That behavior increases the chances of being eaten, and in an environment of constant risk-taking, the population structure of reef ecosystems could fundamentally change. In essence, climate change is causing reef fish to act like human teenagers.

But most experiments have been done with fish raised and tested in relatively controlled laboratory conditions. Marine biologist Philip L. Munday from Australia’s James Cook University, and colleagues, wanted to see whether the laboratory findings would hold up in wild fish, so they set out for a reef system in the waters around Papua New Guinea. Some reefs were located adjacent to natural volcanic seeps, which raise the level of CO2 concentration in the surrounding water. The seawater acidification levels near those seeps are similar to projections for global ocean CO2 concentration for the coming century. They compared those reefs to other nearby control ones, unaffected by the effects of volcanic seeps, and reported their findings earlier this month in Nature Climate Change.

To study the behavior and decision-making of the wild reef fish, juveniles from two damselfish species and two cardinalfish species were caught, brought onboard ship-based labs for testing, and then returned to their home reefs. Because they were wild-captured, the researchers could be sure that the fish had lived the entirety of their lives (from the end of the larval phase) in relatively acidified water, or in relatively un-acidified water for the control reefs.

As they expected, fish from the control reefs avoiding water containing the odor of a predator (the rockcod), spending 100 percent of their time in the odor-free water. The fish from the CO2-rich reefs did almost the exact opposite. They spent more than 90 percent of their time in the water containing the smell of a predator. Fish from the control reefs spent more than 80 percent of their time in their shelters, in relative safety. The damselfishes from the CO2 reefs spent only 12 percent of their time in shelter, while the cardinalfishes didn’t use their shelters at all! The CO2 reef fish didn’t only have impaired sensory abilities and decision-making skills, they also showed personality differences: they were much bolder, emerging from within their shelters six times faster than the after they were disturbed.

Together, Munday’s findings reveal two important conclusions. First, the results were strikingly similar to results from similar experiments with laboratory-raised fish, validating those prior experiments, and suggesting that future laboratory-based experiments should generalize well to natural ecosystems. Second, it shows that continuous exposure to seawater with elevated CO2 does not allow these reef fish to acclimate to the more stressful conditions, unlike recent studies with the corals themselves. That is, the fish don’t acquire any sort of CO2 tolerance as an effect of developing within that seawater.

The researchers paint a stark picture for the future of reef ecosystems. “If reef fish behavior does not adapt to rising CO2 levels over coming generations,” they say, “there could be serious consequences for the structure and function of future reef communities.” – Jason G. Goldman | 30 April 2014

Source: Munday P.L., Cheal A.J., Dixson D.L., Rummer J.L. & Fabricius K.E. (2014). Behavioural impairment in reef fishes caused by ocean acidification at CO2 seeps, Nature Climate Change, DOI:

Header image: Lemon Damselfish (Pomacentrus moluccensis), one of the species studied by Munday et al. via shutterstock.com