What impact do heat waves have on fish brains?

MARE researchers Beatriz Pereira, Lígia Cascalheira, Rui Rosa and José Ricardo Paula are the authors of the article “Alteration of cleaner wrasse cognition and brain morphology under marine heatwaves”, in which they assess the impact of heatwaves on the cognitive performance of cleaner wrasse. 

Heat waves, intensified by rising global temperatures, have profoundly affected marine ecosystems. Among the most affected are coral reefs, which are essential to marine biodiversity and their interactions. An example of these complex interactions is the relationship established between cleaner wrasses (Labroides dimidiatus) and their customers.

These small fish play an essential role in removing ectoparasites from client fish, thus maintaining the health and diversity of marine populations. However, warming waters have shown significant negative impacts on their behavior and physiology.

To try to understand and quantify these impacts, a team of researchers from MARE simulated a marine heatwave in the laboratory. The aim was to assess the cognitive performance of cleaner wrasse in extreme environmental conditions. The fish were then exposed to a heatwave for 55 days, and their cognitive performance was assessed using a visual discrimination task during this period and after 14 days of recovery. The brain morphology of the wrasse was also analyzed 30 days after the recovery period.

This study showed that although the cleaner wrasse showed a recovery in their cognitive performance, they temporarily maintained some deficits. Most alarming, however, were the morphological changes in the fish's brains. Although the total brain size of the exposed fish was significantly larger, the telencephalon (the region associated with higher cognitive functions and social behaviors) was noticeably reduced, while the brain stem showed a considerable increase. This imbalance in brain structure suggests that, although cognitive effects may be reversible in the short term, changes in brain regions associated with higher cognitive functions (such as social interactions) can have lasting impacts.

This means that the structural changes caused by heat waves can interfere with ecological interactions and the vital role of cleaner fish in coral reefs. 

This study highlights the importance of conservation strategies that go beyond the physical protection of reefs and fish populations. Preserving the ecological function of cleaner fish, which are fundamental to the health of coral reefs, also depends on actions that minimize the impacts of global warming and marine heat waves, ensuring the recovery not only of populations, but also of the cognitive and social capacities of these species.