Impacts of food availability and pCO₂ on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral Balanophyllia elegans

Abstract. Ocean acidification, the assimilation of atmospheric CO₂ by the oceans that decreases the pH and CaCO₃ saturation state (Ω) of seawater, is projected to have severe adverse consequences for calcifying organisms. While strong evidence suggests calcification by tropical reef-building corals containing algal symbionts (zooxanthellae) will decline over the next century, likely responses of azooxanthellate corals to ocean acidification are less well understood. Because azooxanthellate corals do not obtain photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on energy available for calcification. The solitary azooxanthellate orange cup coral Balanophyllia elegans often lives in low-pH, upwelled waters along the California coast. In an 8-month factorial experiment, we measured the effects of three pCO₂ treatments (410, 770, and 1220 μatm) and two feeding frequencies (3-day and 21-day intervals) on "planulation" (larval release) by adult B. elegans, and on the survival, skeletal growth, and calcification of newly settled juveniles. Planulation rates were affected by food level but not pCO₂. Juvenile mortality was highest under high pCO₂ (1220 μatm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO₂. While net calcification was positive even at 1220 μatm (~3 times current atmospheric pCO₂), overall calcification declined by ~25–45%, and skeletal density declined by ~35–45% as pCO₂ increased from 410 to 1220 μatm. Aragonite crystal morphology changed at high pCO₂, becoming significantly shorter but not wider at 1220 μatm. We conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food.