| Summary: | The physical and chemical environment around corals, as well as their physiology, can be affected by interactions with neighboring corals. This study employed small colonies (4 cm diameter) of Pocillopora verrucosa and Acropora hyacinthus configured in spatial arrays at 7 cm s flow speed to test the hypothesis that ocean acidification (OA) alters interactions among them. Interaction effects were quantified for P. verrucosa using three measures of growth: calcification (i.e., weight), horizontal growth, and vertical growth. The study was carried out in May–June 2014 using corals from 10 m depth on the outer reef of Moorea, French Polynesia. Colonies of P. verrucosa were placed next to conspecifics or heterospecifics (A. hyacinthus) in arrangements of two or four colonies (pairs and aggregates) that were incubated at ambient and high pCO (~1000 µatm) for 28 days. There was an effect of pCO, and arrangement type on multivariate growth (utilizing the three measures of growth), but no interaction between the main effects. Conversely, arrangement and pCO had an interactive effect on calcification, with an overall 23 % depression at high pCO versus ambient pCO (i.e., pooled among arrangements). Within arrangements, there was a 34–45 % decrease in calcification for solitary and paired conspecifics, but no effect in conspecific aggregates, heterospecific pairs, or heterospecific aggregates. Horizontal growth was negatively affected by pCO and arrangement type, while vertical growth was positively affected by arrangement type. Together, our results show that conspecific aggregations can mitigate the negative effects of OA on calcification of colonies within an aggregation.
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