Supplementary material from "The past, present and future of cleaner fish cognitive performance as a function of CO₂ levels"

Ocean acidification is one of the many consequences of climate change. Various studies suggest that marine organisms' behaviour will be impaired under high CO₂. Here, we show that the cognitive performance of the cleaner wrasse, Labroides dimidiatus, has not suffered from the increase of CO₂ from pre-industrial levels to today, and that the standing variation in CO₂ tolerance offers potential for adaptation to at least 750 µatm. We acclimated cleaners over 30 days to five levels of pCO_2, from pre-industrial to high future CO₂ scenarios, before testing them in an ecologically relevant task—the ability to learn to prioritize an ephemeral food source over a permanent one. Fish learning abilities remained stable from pre-industrial to present-day pCO₂. While performance was reduced under mid (750 µatm) and high CO₂ (980 µatm) scenarios, under the former 36% of cleaners still solved the task. The presence of tolerant individuals reveals potential for adaptation, as long as selection pressure on cognitive performance is strong. However, the apparent absence of high CO₂ tolerant fish, and potentially synergistic effects between various climate change stressors, renders the probability of further adaptation unlikely.