Coral-associated Crabs and Macroalgae Alter Disease Spread in Branching Corals on the Great Barrier Reef

Disease is an important driver of coral loss regionally and is projected to become more severe as temperatures increase around the world. Although there has been substantial research into the abiotic factors (e.g. temperature, nutrients) controlling coral diseases, we know significantly less about the biotic factors (i.e. species interactions) influencing disease dynamics. We examined how the species living on and within corals affect coral tissue loss from a white syndrome-like condition on Heron Island in the southern section of the Great Barrier Reef. We exposed Acropora aspera fragments in flow-through tanks to a fully crossed factorial experiment with three factors: the presence of a common symbiotic crab (Cyclodius ungulatus), contact with a common macroalgal complex, and simulated wounding mimicking fish predation. We found that crab presence increased coral survival from a white syndrome-type disease by over 25%, likely by removing macroalgae if present and by cleaning infected tissue. Conversely, contact with macroalgae dramatically increased coral mortality, with the chance of survival dropping to nearly 0 by the end of 25 days for corals that were in contact with algae. Wounding had no direct effect on coral health, but wounded corals with crabs did significantly better than corals with no wounding and crabs, which may be the result of coral-crab signaling. We suggest that A. aspera may produce nutrient-rich mucus when wounded, which attracts crab symbionts that help slow disease progression. These results suggest that incorporating biotic interactions into restoration designs may dramatically improve restoration outcomes and that adding beneficial symbionts may improve disease resilience at a local level.