Alteration of cleaner wrasse cognition and brain morphology under marine heatwaves

Abstract Heatwaves, exacerbated by global warming, are progressively affecting various ecosystems, with coral reefs among the most susceptible. Within these ecosystems, cleaner wrasses ( Labroides dimidiatus ) engage in cooperative interactions with client fish by removing ectoparasites and play an essential role in sustaining client abundance and diversity. In 2016, the northern section of the Great Barrier Reef experienced widespread and intense bleaching due to unparalleled ocean temperatures associated with a marine heatwave. While prior studies have connected changes in fish densities following this heatwave to modifications in cleaner fish cognitive performance, the immediate impact of heatwave exposure on cleaner fish cognition and brain structure has yet to be investigated. Here, we exposed cleaner wrasses to a laboratory‐simulated Category 1 marine heatwave for 55 days, mirroring the 2016 Great Barrier Reef event. Cleaners' cognitive performance was evaluated through a visual discrimination task during the heatwave and after a 14‐day recovery phase. This was followed by an analysis of brain development 30 days after the cessation of the marine heatwave. Our results demonstrate that although heatwave exposure temporarily hindered cognitive performance, these deficits were recoverable. Interestingly, cleaner fish brain morphology, measured after recovery, underwent significant changes. Specifically, despite cleaners exposed to heatwaves having notably larger brains, their telencephalon was substantially smaller, while their brainstem was enlarged. These findings indicate that while some cognitive effects may be reversible, marine heatwave exposure leads to lasting alterations in brain morphology, particularly in regions associated with higher cognitive functions and social behaviour. This raises questions about the potential impact on more complex tasks that rely on these brain regions. We argue that the significant disruptions in cleaners' cognitive performance observed months after the 2016 due to neurological impairments linked to brain morphological changes. If so, a mere recovery of fish densities may not necessarily lead to a restoration of cognitive performance, as experiencing marine heatwaves might induce life‐long morphological alterations in fish. Our results underscore marine heatwaves' intricate and enduring impact on cleaner fish, emphasizing the need for comprehensive strategies to safeguard these vital components of coral reef ecosystems. Read the free Plain Language Summary for this article on the Journal blog.