In contravention of our first hypothesis, pCO2 alone did not cause a decline in areal symbiont densities in P. damicornis following approximately eight weeks of exposure. This finding juxtaposes with a number of studies that did find reductions in areal symbiont densities under higher pCO2 (Tremblay et al., 2013, Krief et al., 2010, Kaniewska et al., 2012, Muehllehner, 2013, Horwitz and Fine, 2014, Hii et al., 2009), and is in harmony with several studies that did not find an effect of pCO2 alone on areal symbiont densities in corals (Wall et al., 2014, Ogawa et al., 2013, Godinot et al., 2011, Towle et al., 2015).