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

Ocean acidification, the assimilation of atmospheric CO2 by the oceans that decreases the pH and CaCO3 saturation state (Omega) 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 pCO(2) treatments (410, 770, and 1220 mu atm) 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 pCO(2). Juvenile mortality was highest under high pCO(2) (1220 mu atm) and low food (21-day intervals). Feeding rate had a greater impact on calcification of B. elegans than pCO(2). While net calcification was positive even at 1220 mu atm (similar to 3 times current atmospheric pCO(2)), overall calcification declined by similar to 2545 %, and skeletal density declined by similar to 35-45% as pCO(2) increased from 410 to 1220 mu atm. Aragonite crystal morphology changed at high pCO(2), becoming significantly shorter but not wider at 1220 mu atm. 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.