An abundant mutualist can protect corals from multiple stressors.

Mutualisms can increase the ability of foundation species to resist individual stressors, but it remains unclear whether mutualisms can also ameliorate co-occurring stressors for habitat-forming species. To examine whether a suspected mutualist could improve foundation species' resistance to multiple stressors, we tested how a common coral-dwelling crab affected corals exposed to macroalgal contact and physical wounding during a widespread heat stress event using flow-through tanks supplied with seawater from a nearby reef flat. High temperatures on the reef flat, which raised the temperature in our tanks, appeared to trigger rapid tissue loss in experimental corals, but the amount of tissue lost by corals was strongly determined by treatment. Macroalgal contact increased, while the presence of a crab decreased, the amount of tissue lost. Although the effect of wounding was not strong in isolation, when wounding occurred in the presence of a crab, coral tissue loss unexpectedly decreased below that of all other treatments. We propose that wounding increased coral resistance to stress by attracting crabs-a result that appeared supported in a field experiment. These results highlight that mutualisms can interact with stressors in unexpected ways, buffering the effects of both local and global stressors on foundation species.