Predation and thermal stress affect color change in the symbiotic sea anemone Aiptasia

Bleaching is the disruption of the symbiotic relationship between anthozoans and zooxanthellae. The term bleaching refers to the host appearing lighter--sometimes becoming completely white--as a result of losing their symbiotic dinoflagellates, their photosynthetic pigments, or both. Research has demonstrated that many abiotic factors, such as temperature, ultraviolet radiation, and salinity, cause bleaching. However, we know little about the role that biotic factors, such as predation, may play in coral bleaching. Additionally, little is known about the combined effects of different stressors, and whether these effects are additive or not. If effects are synergistic and difficult to predict, then much more research is needed to understand how coral reefs will respond to climate change. Using Aiptasia spp. as a model for coral, I investigated whether predators affect the anemones\u27 response to thermal stress with a fully crossed 2-factor experiment. Using digital photography and color analysis to measure red, green, and blue color change, I determined that predation caused anemones to become darker over time when compared to anemones that were not preyed on. Specifically, there was a significant predator effect when analyzing the change in blue and green color values. The magnitude of mean color change for the predator treatment was up to three times the change for the anemones that were not preyed on. These results suggest that predation actually makes the organism more resilient, thereby lessening the bleaching response. This would imply that a certain degree of coral grazing may be beneficial, which indicates that overfishing may have effects more far-reaching than previously suspected. Additionally, the effects of temperature and predators were additive, which suggests that we can predict the combined effects of thermal and predation stress from experiments that manipulated only a single factor