Photochemical Efficiencies in Reef-Dwelling Anthozoans: Insights from “Survivor” Species

In recent decades, populations of many coral species have declined dramatically on reefs worldwide. A major factor in coral mortality has been photo-oxidative stress associated with both solar irradiance and elevated temperatures. While many studies have focused on species that have declined, fewer efforts have focused on the “survivor” species, those that have maintained relatively stable populations or even increased in abundance. The objective of this study was to assess temporal variability in photochemical efficiencies (Fv/Fm) as an indicator of potential photo-oxidative stress in the dinoflagellate symbionts in three species, the scleractinians Siderastrea siderea and Montastraea cavernosa, and the zoanthid Palythoa caribaeorum, whose populations have remained relatively stable along the Florida reef tract. Coral colonies with no visual indication of disease or bleaching were assessed quarterly in 2012 and 2013, at sites at 6 or 18 m depths. Colonies were dark-acclimated prior to measurements using pulse-amplitude modulated fluorometry. The mean Fv/Fm values for P. caribaeorum colonies assessed at 6 m depth were consistently the lowest (0.59, 0.02 SE). Siderastrea siderea assessed at both 6 and 18 m revealed significantly lower Fv/Fm values (p=0.0006) for those living at 6 m (0.64, 0.02 SE) than for those living at 18 m (0.68, 0.01 SE) depths. The Fv/Fm values for colonies of M. cavernosa assessed at 18 m also averaged 0.68 (0.01 SE). Thus, photochemical efficiencies were lower in colonies living at 6 m than in colonies living at 18 m, while no significant seasonal differences were found in the mean Fv/Fm values in the examined species. This study provides the first known report of photochemical efficiency in P. caribaeorum.