Sea anemones in the genus Anthopleura are conspicuous community members on California rocky shores. They live symbiotically with algae and exchange nitrogen for photosynthetically derived sugars and lipids. The mutualism between host and endosymbiont has been well-studied, but the impact of this mutualism in a larger context of community and ecosystem ecology has not. Here I investigate how sea anemone diets affect the mutualism between host and symbiont, which invertebrate prey are chosen and digested by sea anemones, and how favorable microhabitat created by sea anemones is a direct result of the mutualism between sea anemones and algal endosymbionts. In chapter 1, I found evidence for a trade-off between autotrophy of endosymbionts and heterotrophic diet. When food was increased, sea anemones down-regulated endosymbionts, and when it was decreased, they up-regulated endosymbionts. However, the direction of the response was sea anemone species specific. In chapter 2, I provide evidence for sea anemone prey choice and predation avoidance by common prey species. All snail species that we tested escaped sea anemones alive if they were undamaged, but crustaceans, while sometimes escaping ingestion, were always digested. We suggest that the snail species Tegula funebralis may release a chemical deterrent to predation after ingestion by the sea anemone. In chapter 3, I describe how the mutualism between sea anemone and algal endosymbionts creates favorable microhabitat that ameliorates abiotic stressors during low tides. Mobile invertebrates take advantage of this microhabitat and stay next to sea anemones during low tides. Temperature and relative desiccation were lower and mobile invertebrate richness and biomass were higher in sea anemone habitat as compared to adjacent rock habitat, suggesting a facilitative role for this sea anemone-algal mutualism. Taken together, these results reveal the complex trophic and non-trophic interactions caused by this temperate cnidarian-algal mutualism.