ABSTRACT: Competitive and facilitative interactions are common throughout the rocky subtidal and may be important when determining organism distribution and abundance. In this study, we used the corallimorpharian Corynactis californica to examine these relationships as they develop spatially and temporally between 2 kelp forest habitats. Sampling along Santa Catalina Island, California, USA, demonstrated that C. californica densities are up to 4-fold greater under canopies of the stipitate kelp Eisenia arborea than in areas just outside the canopy. Stipitate kelp canopies can act as foundation species by modifying critical resources (e.g. light and flow) that mediate understory assemblages. It was determined that the understory algal community is 17% less dense and is distinguished by branching red algae, whereas the non-shaded community is characterized by dense stands of foliose and decumbent brown algae. This study quantified the relationship among benthic macroalgae and C. californica to determine whether macroalgae were negatively or positively affecting C. californica distributions. Permanent 25 × 25 cm quadrats were established in both the canopy and open habitat, and understory algal biomass was manipulated with 3 treatments: (1) 100% removal; (2) thinning by 50%; and (3) no removal (control). All treatments were censused and maintained monthly for 1 yr. C. californica abundance increased in the control treatments in both habitats, and to the greatest extent under E. arborea canopies, suggesting a positive interaction between understory macroalgae and C. californica. The results of this study indicate that E. arborea creates a unique habitat that differs in water flow, light attenuation, and in composition and density of macroalgae. The combination of these factors under the E. arborea canopy may support large aggregations of C. californica (up to 3200 m^–2) in an otherwise sub-optimal habitat compared to subtidal vertical walls.