Behavioral Response of the Lemon Damselfish to Slowly Increasing Ocean Acidification

Atmospheric CO₂ currently exceeds the natural range seen in the last 650,000 years, with a correspondingly unprecedented rate of oceanic CO₂ uptake. CO₂ levels projected to occur in the next 100 years have been shown to dramatically affect fish behavior and physiology, with the potential for widespread impacts on marine ecosystem structure and function. However, experiments to date have exposed fish directly to high acidity levels, without determining whether a slow increase over a longer period of time might allow a plastic response to increasing acidity. Here we report the results of an experiment in which lemon damselfish (Pomacentrus moluccensis) were exposed to five phases of increasing CO₂, starting at 385ppm CO₂ and increasing in steps of 150ppm over the course of two months. One additional group was subjected to suddenly increased CO₂ at the highest level, and another experienced no change (the control). Three behavioral tasks were conducted with different groups of six control and six elevating CO₂ fish at each acidity level, with associated physiological measures. Aggression, interaction with a novel object, and shelter use was compared across each group, providing insight into the capacity for fish to show adaptive response to a slow increase in acidity.