Swimming on acid: How ocean acidification affects animal behavior.

Once a very active little fish, Nemo now feels like staying at home more often. He also eats less and does not interact with many other fish. What happened? Ocean Acidification happened. This overlooked aspect of climate change affects many other animals just like Nemo. It is hard to say what will happen to them in the future. Mostly because the oceans are the home of many different species. Will an acidified ocean negatively affect the lives of every ocean dweller? And what consequences might this bring to the future of our planet?

Our story begins around 200 hundred years ago. After the industrial revolution, CO2 in the atmosphere increased faster than ever before. This not only had an effect on the air quality but also on the water.  Ocean acidification is a process where CO2 from the air dissolves in the sea, making it more acidic (Read more about it in our blog). The ocean covers around 70% of our planet’s surface. It also plays key roles in life on Earth, either directly or indirectly.

The ocean is home to many species of fish, mammals, birds, algae, shellfish, countless microorganisms, and many more. Every one of them is the product of millions of years of evolution. Throughout generations, they gradually adapted according to what suited them the best. Now imagine that suddenly, their surroundings start changing faster than ever before. Thus, the way the different groups of organisms affect each other, and their environment is also bound to change.

What does ocean acidification mean for marine animals and how can it affect them?

An acidic ocean, depending on the situation, can have direct and profound effects on a marine animal’s life. It can affect the way an animal perceives the world around itself. It can also affect their access to food and shelter. It is even more complicated when considering that other species are also affected. Because the oceans are the home of a large number of species across the planet, it is difficult to picture what could happen on a large, global scale. Some species that are not easily affected by acidification may gain great benefits from ocean acidification and thrive. Some others will probably disappear forever. Either way, whatever happens on a large scale depends on behavior, as this is the fastest way an animal can respond to changes in their environment.

One of the major issues of OA is the effects it can have on the ability of an animal to sense its surroundings properly. Scientists have found that an acidic ocean can mess directly with the brain of sea animals. They found that some damselfish and cardinalfishes living in water with high amounts of CO2 in the water change their personalities. Some became bolder, meaning that they care less about putting their lives at risk and more about getting the most out of their surroundings. Some others like the nibbler become extremely shy, losing opportunities for getting food and other things, in favor of staying under protection.

Some fish can get careless and pay less attention to their predators. This is due to the inability to recognize certain chemical signals that are in the water. For example, scientists have observed that in an can not only lose some of their ability to identify predators by smell but even some might be attracted to it. Picture a fish misreading a “warning, shark ahead” sign and confusing it for one that says “free cookies”. For predators is a similar story. Some sharks that have lived in acidic waters seem to lose some of their ability to find prey using their smell. Some others have been seen changing their diets when compared to non-acidified water. Thankfully for them, their prey is also acting strangely. This shift in the way predators and prey behave and interact is bound to have a big impact on the dynamics of their populations.

Now picture a beach. Thanks to the tides rising and falling, there are some small pools that form in parts of the beach. A small fish from the coasts of Chile called nibbler lives in those pools. According to recent research, the nibbler becomes shy when exposed to higher concentrations of CO2 in the water. This shyness means that these fish are less likely to: go exploring, find food, and even a partner. Because these pools are usually wide and very shallow, they are very sensitive to an increase in CO2 levels in the air. This means that the pH of the ponds goes down much faster than in the oceans. According to the article by Benitez: “The negative effects of increased CO2 (in the air) on the behavior of this intertidal fish can have significant ecological consequences, mainly in the tidal pools habitats where this group is dominant in terms of abundance and biomass. These changes could affect not only at a population level, if not also on the entire community.”

The consequences of changing behavior

One of the most likely scenarios is an immense loss of biodiversity. A rapidly changing environment is a very hostile place. Species that can adapt their behavior easily to changes in their surroundings will most likely benefit. On the other hand, more sensitive species will suffer the most. This could eventually lead to a complete shift in the species that inhabit the ocean. This would also decrease the number of species (and the dynamics between those species!). This may have consequences in predators like sharks, filtering animals like sponges, herbivores and prey, and even habitat builders like corals (Read more about it in on our blog).

There is still a lot of research that needs to be done about this topic, but one thing is certain: ocean acidification has a wide range of effects on animal behavior. In turn, this could affect the whole planet. The ocean of the future will probably look very different as it does today. If we do not do anything about it, we should expect changes that will last forever.

References

Nagelkerken, I., & Munday, P. L. (2016). Animal behaviour shapes the ecological effects of ocean acidification and warming: moving from individual to community‐ level responses. Global Change Biology, 22(3), 974-989.

Watson, S. A., Fields, J. B., & Munday, P. L. (2017). Ocean acidification alters predator behaviour and reduces predation rate. Biology letters, 13(2), 20160797.

Ashur, M. M., Johnston, N. K., & Dixson, D. L. (2017). Impacts of ocean acidification on sensory function in marine organisms. Integrative and comparative biology, 57(1), 63-80.

Munday, P., Cheal, A., Dixson, D. et al. Behavioural impairment in reef fishes caused by ocean acidification at CO2 seeps. Nature Clim Change 4, 487–492 (2014). https://doi.org/10.1038/nclimate2195

Benítez, Samanta & Duarte, Cristian & Opitz, Tania & Lagos, Nelson & Pulgar, José & Vargas, Cristian & Lardies, Marco. (2017). Intertidal pool fish Girella laevifrons (Kyphosidae) shown strong physiological homeostasis but shy personality: The cost of living in hypercapnic habitats. Marine Pollution Bulletin. 118. 10.1016/j.marpolbul.2017.02.011.