A study of two damselfish species, published in Scientific Reports this week, reveals that when levels of dissolved carbon dioxide (CO2) in the oceans rise, the current dominant species is most affected, while the ‘underdog’ adapts more successfully.

Dissolved CO2 levels in the ocean are rising in line with atmospheric CO2 and are projected to increase from 390 to 900 microatmospheres by 2100. Ocean acidification and elevated water temperature affect live coral, which is an important source of food and shelter for many coral-reef fishes, some of which are more susceptible than others to the effects of increased CO2.

Mark McCormick and colleagues examined how increased CO2 levels affected interactions between two damselfish species known to compete for space. They found that when CO2 levels rise, the species that is dominant under current conditions -- Pomacentrus amboinensis -- may not fare as well as its competitor, Pomacentrus moluccensis. This reversal of the competitive hierarchy was particularly marked in degraded habitats. Improved understanding of the complex interactions between species will be important for predicting the likely composition of marine ecosystems under future climate change and ocean acidification, the authors conclude.