The ocean picks up our slack when it comes to climate change by absorbing a substantial amount of the carbon dioxide we produce. While this helps to mitigate global warming, it also increases the acidity of seawater in a process called ocean acidification. Unsurprisingly, there is global concern over the impact of ocean acidification on marine life, but understanding the extent of its effects is an active and somewhat contentious area of research. One of the main controversies centers on whether ocean acidification affects fish behavior. Trevor Hamilton from MacEwan University and Alberta University, Canada, and Martin Tresguerres from the University of California, San Diego, USA, worked with an international group of colleagues to examine the impact of current and predicted changes in ocean acidity on the behavior of bicolor damselfish (Stegastes partitus) in Panama.
The team started by holding damselfish in seawater, either at current ocean acid levels or at those predicted with climate change, for 5 days. Then, they videoed the fish's response when placing them in four testing arenas that assessed their movement, boldness, aggression and anxiety. While most behaviors were not affected by ocean acidification, anxiety was higher in the acidic seawater than in the current-day seawater.
After establishing that the damselfish's behavior was impacted by ocean acidification, the team wanted to figure out how it happened. They hypothesized that increases in ocean acidity affect the dopamine response in the brain. Changes in this neurotransmitter can influence animal movement, boldness and more, which would explain why the fish behaved differently in the more acidic seawater. To test this, they exposed the fish to a drug that mimics dopamine and ran the same behavioral tests on them. The team discovered that the dopamine mimic affected the fish in current-day seawater – the animals moved less and they were bolder and more anxious – but the behavior of the fish in acidic water did not change. This confirmed the team's suspicion that ocean acidification disrupts dopamine signaling to impact fish behavior.
But, as the researchers ran their experiment, they noticed something odd. While their fish tanks were constantly supplied with fresh seawater, the tank water grew more acidic throughout the study, likely because of the fish's own carbon dioxide production. As damselfish spend much of their time in small coral reef crevices where water mixing is minimal, the team wondered how acidic the seawater was in the fish's natural habitat and how it compared to the global-scale future predictions that had informed their study design. They collected water samples from the crevices where the damselfish live and outside in the open reef. Surprisingly, seawater from the damselfish crevices was acidic, like the climate change scenario used in the study, while the surrounding reef seawater was similar to normal current conditions. Importantly, these acidic readings were not caused by climate change; they are just a natural phenomenon in the reefs.
These findings demonstrate that there is no universal ‘normal’ or ‘climate change scenario’ that can be applied to an entire ecosystem. Instead, there are smaller microhabitats within an ecosystem that may differ in their environmental conditions. With this new information in hand, the researchers speculated that the increased anxiety they observed in damselfish exposed to acidic water may help the fish seek shelter in crevices to avoid being eaten by bigger fish. One has to wonder whether these tiny fish have figured out a way to use acidic water to their advantage and what other treasures of information we can unlock if we follow up our experimental observations and always consider the ecology of the animals we study.
