Effects of future climate on coral-coral competition

than interspecific competition [41], or may not directly affect interspecific competition [40]. Unlike these results, our data suggest that OA and warming increased the susceptibility of the stronger competitor, P. astreoides, to the weaker competitors, M. cavernosa and O. faveolata, in areas of direct contact at day 7 without impacting the negative effects that P. astreoides had on the weaker competitors. Outside of direct contact areas, none of the corals exhibited evi- dence of reductions in photosynthetic efficiency from competitive interactions or OA and temperature stress at day 7 (Figs 1C, 4A and 4B). The mechanism altering the susceptibility of P. astreoides to competition with O. faveolata and M. cavernosa in areas of direct contact under OA and warming is unclear. It is possible that the change in susceptibility could have been exacerbated by differences in salinity between present and future treatments. While cor- als are known to have a limited tolerance for changes in salinity [57], the small differences in salinity between treatments (<0.6 ppt) were unlikely to have affected coral performance. Previ- ous research evaluating effects of salinity on Orbicella faveolata larvae (which we assume are less robust than adults) found that differences of 4 ppt (36 vs. 32 ppt) affected larval survival but that a difference of 2 psu (36 vs 34 ppt) did not [58]. The change in the susceptibility of P. astreoides to the other two corals is more likely to be due to oxidative stress, which is often a

precursor to coral bleaching [59–61]. changes in the microbiome [62], altered chemical defenses [63], faster mesenterial filament production [40], or other mechanisms. Regardless of the factor, these results suggest that the OA and warming may alter coral-coral interactions among some of the most common species remaining on degraded Caribbean reefs. While both experiments demonstrated an effect of OA and warming on coral-coral interac- tions at day 7, the second experiment ran for an extra 7 days to evaluate if results changed over time. Other experiments have suggested that ecological interactions can outweigh the negative effects of OA and warming over longer time scales [40]. As with these studies, OA and warm- ing no longer significantly affected photosynthetic efficiency in areas of direct contact at day 14 (Fig 3). Increased variation of photosynthetic efficiencies in the no contact locations for both present and future treatments (Fig 3A and Fig 3C) corresponded with an increase in the size of the tissue necrosis area from day 7 to day 14 for P. astreoides suggestive of continued competition (Fig 5C and 5D). These results mirror other findings suggesting that OA can speed the production of mesenterial filaments while not affecting the final outcome of compe- tition [40]. Across both experiments, areas of discoloration and/or tissue mortality formed in response to competition, but the sizes of these areas were unaffected by OA and warming. In the first experiment, damage did not expand beyond the areas of direct contact, and there was no sig-

nificant difference in the size of the lesion area between the two coral species (Fig 2). In the sec- ond experiment, P. astreoides developed areas of discoloration twice the size of those formed by O. faveolata, and these extended beyond the areas of direct contact by day 7 (Fig 5). This may be evidence of the formation of sweeper tentacles known to form on some corals [31, 36], although the evidence of such formation was not visually obvious in this study. The area of dis- coloration on P. astreoides shrank 41% between days 7 and 14, leaving tissue mortality and bare skeleton in the areas of direct contact with recovered tissue outside areas of direct contact. Some studies have suggested that environmental conditions can play a role in coral tissue recovery [64, 65], however they focused more on strong variations between environmental field conditions without evaluating the specific conditions responsible. In this study, there was no evidence that returned tissue coloration (suggestive of tissue recovery) was dependent on present or future warming conditions supporting one other study that found tissue recovery of Porites spp. to be unaffected by OA [66]. Across the two experiments, OA and warming only affected competition in areas of direct contact. Areas of the coral not in direct contact and the no-contact control corals did not PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 11 / 16