Background/Question/Methods Mutualisms – including algal-cnidarian symbioses – can profoundly alter the structure of ecological communities. Without these positive interactions, some communities would fundamentally change or collapse. Here we present evidence for a new facilitation cascade on California, USA, rocky shores. Sea anemones in the genus Anthopleura, which host symbiotic unicellular algae, must maintain relatively low body temperatures and simultaneously remain exposed to sunlight to keep their photosynthesizing algae. If temperatures are too high, the partnership between host and symbiont breaks down. To maintain consistent, cool body temperatures during exposure to stressful environmental conditions at low tide, sea anemones slowly release water. We observed mobile invertebrates congregating next to anemones during low tides, possibly to avoid high temperatures and desiccation. We hypothesize that the mutualism between sea anemones and algal symbionts facilitates biodiversity, as the release of water not only cools the anemones, but it also provides a favorable microhabitat for a diverse assemblage of associated invertebrates. We experimentally manipulated sea anemone presence at three sites, surveyed mobile invertebrates, and measured temperature and relative desiccation in plots over several weeks before and following manipulations. Results/Conclusions We found that species richness and density of mobile invertebrates was higher in sea anemone microhabitat plots (5 cm border around anemone) than on adjacent bare rock plots regardless of tide type (spring or neap), sea anemone species (A. elegantissima or A. sola), or site. Temperatures were up to 9° C lower and relative desiccation was 30% lower next to anemones than on adjacent rock. To experimentally test the role of anemones, we manipulated habitats by adding or removing sea anemones in the same survey plots. Removing sea anemones decreased mobile invertebrate species richness relative to the anemone microhabitat control, and adding anemones enhanced richness as compared to our bare rock control. The presence of sea anemones in the upper intertidal zone – which otherwise consists largely of bare rock on these shorelines – shapes the community of mobile invertebrates by reducing abiotic stressors and creating a unique microhabitat. Our results highlight the importance of cnidarian endosymbiosis and its cascading effect on temperate rocky-shore biodiversity, providing evidence for a new, community-wide role for this mutualism.