Contact-free modulation of stony coral productivity by sessile reef organisms

Coral reefs are biodiversity and productivity hotspots1 where space limitation makes interactions between organisms inevitable. Biodiversity loss and shifts in community composition alter the interactions between coral reef organisms, however downstream effects on the productivity of individual species remain unexplored. Here, we quantified changes in stony coral productivity as immediate (photosynthesis, respiration) and long-term (photosynthesis, respiration, calcification, and symbiont density) response to contact-free interactions with various benthic organisms (stony corals, soft corals, macroalgae, sponges). By systematically excluding confounding biological and physical factors, we provide experimental proof that corals sense the presence of other organisms and modulate their productivity as a response. Each stony coral species had a characteristic response to contact-free stimuli, while the identity of the interaction partner was of subordinate importance. Our data highlight so-far unaccounted downstream effects that biodiversity loss and shifting coral reef communities may have through indirect modulation of productivity, resulting in disproportionate benefits for some species. The productivity response is probably mediated by secondary metabolites released into the water, which are distinguished as self- and non-self attributes as part of a historecognition system in corals. The communication pathways between sessile reef organisms that mediate these interactions remain to be investigated.