Our results indicate that coral growth in the wild may be much more variable than previously thought, as zooplankton feeding and oxygen have profound effects on calcification. We do not yet know, however, how high coral feeding rates have to get to actually disrupt calcification. The prey concentrations we used are only found under aquaculture conditions, although on reefs, corals can feed on bacteria and phytoplankton as well, which seem to be more abundant in the wild compared to filtered aquarium water (Feldman et al. 2011). Future research will have to determine the dose-response relationship between feeding and dark calcification rates. As nocturnal feeding may negatively affect coral growth, it seems puzzling that in the wild, many coral species expand their tentacles to feed at night. A possible explanation for this phenomenon is that at night, zooplankton concentrations are significantly higher than during the day (Holzman et al. 2005; Yahel et al. 2005a,b). The concentration of copepods, for example, can increase fivefold at night (Yahel et al. 2005a). As plankton is a source of essential nutrients, the inhibition of calcification during the night may be outweighed by the nutrition corals gain from feeding.