|Irradiance is an abiotic factor that strongly affects the metabolism of endosymbiotic Symbiodinium within photosynthetic cnidarians such as rose tip sea anemones Condylactis gigantea. Fluctuations in the intensity of irradiance or photosynthetically active radiation (PAR) within and among environments can cause both behavioral and physiological changes to the holobiont, which consists of host sea anemones and their endosymbiotic Symbiodinium. Through laboratory experiments, we investigated: 1) anemone behavioral selection of PAR levels when placed in light tunnels (i.e., phototaxis), and 2) growth and physiological changes to the holobiont (both host anemone and Symbiodinium) when exposed to 3 PAR treatments (high, medium, and low) over 6 weeks. In terms of behavioral responses, we observed that anemones in light tunnels locomote to select locations exposed to a narrow range of PAR (~40-80 µmol photons m-2 s-1, equivalent to irradiance at ~ 30 m depth on the open surface of some coral reefs). They exhibit this narrow range of irradiance selection when exposed to spatial variation in PAR levels over both broad (~10-300+ µmol photons m-2 s-1) and fine scales (~20-140 µmol photons m-2 s-1). Physiologically, anemones exhibit a more complex reaction to irradiance. Anemone growth did not differ among irradiance treatments, however anemones increased their tentacle crown surface area to wet mass ratio in response to low PAR. Alteration of this ratio may enhance light capture by endosymbiotic Symbiodinium in their tentacles when exposed to low PAR, allowing the algae to translocate more photosynthate to the host. We observed that the Symbiodinium rapidly acclimatize during the first two weeks of exposure to altered irradiance. Speifically, significant changes in microalgal abundance and in chl α concentration per microalgal cell occurred in response to irradiance treatments. As large fleshy cnidarians, these sea anemones are able to respond both behaviorally and physiologically to alteration of PAR intensity, indicating an ability to acclimate to different irradiance environments, especially to low irradiance. These processes allow C. gigantea to occupy low-light microhabitats such as reef crevices, and also to occur over a wide depth range on Caribbean coral reefs. Based on these results, we recommend that conservation management of this species focus on protecting deep mesophotic reefs and highly rugose reefs where individuals may thrive in low-light environments. These types of habitats may serve as refuges for populations to reseed shallow reefs impacted by bleaching. We also recommend aquaculture conditions to enhance the growth and survival of this species in culture, thereby reducing the collection pressure on natural populations.