| Summary: | We tested the hypothesis that ocean acidification (OA) affects spatial competition among scleractinian corals. Competitive ability was evaluated indirectly by linear extension of Porites lutea and Montipora aequituberculata placed in intraspecific, interspecific, and control pairings (paired with dead coral skeleton) and exposed to ambient (400 μatm) and elevated (1000 μatm) pCO in experiments conducted in Moorea, French Polynesia, and Okinawa, Japan. High pCO had no effect on linear extension of M. aequituberculata in Moorea, but in Okinawa, it reduced linear extension 37%; high pCO had no significant effect on linear extension of P. lutea in Okinawa. Much of the negative effect of high pCO on linear extension for M. aequituberculata in Okinawa was due to reduced extension in control pairings, with corals engaged in intra- and interspecific competition unaffected by OA. Linear extension of M. aequituberculata and P. lutea in interspecific pairings decreased relative to control pairings at ambient pCO by 39 and 71%, respectively, indicating a strong effect of competition on extension rates. These differences however disappeared at elevated pCO when the linear extension of controls was depressed. Together, our results show that OA can negatively affect the linear extension of corals not engaged in competition, as shown in the control pairings, and suggest that OA does not directly affect the ability of corals to compete with one another for space.
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