Interaction of hydrodynamics and corallivory control growth of branching Acropora

Coral community shifts are a common, recent phenomenon across reefs in the Caribbean and Indo-Pacific, and have been related in time and space to catastrophic natural disturbances, especially storms, crown-of-thorns starfish (COTS) outbreaks, bleaching events, and disease epidemics. Many shifts have been characterized by the loss of branching acroporid corals. Recent work also indicates that chronic low-level (‘press’) disturbances also lead to community shifts by influencing the rates of recovery of different coral taxa following catastrophes. Corallivory is a press disturbance that often impacts acroporid corals more than other taxa, thus influencing coral community recovery. We investigated how hydrodynamic conditions (mean current velocity and variance) influence the growth of Acropora elseyi with and without partial predation, mainly by butterflyfishes, in Moorea, French Polynesia. Coral growth was greatest under conditions of relatively high current velocities (40-60 cm sec^-1) accompanied by moderate levels of velocity variance (a combination of waves and turbulence). But corals performed well in these conditions only when protected from predation: unprotected corals were attacked at the highest rates and grew at the lowest rates. Corallivores had the least impact where velocity variance was greatest, conditions that otherwise generated low rates of coral growth for colonies protected from corallivores. Results from our experiment indicate that abiotic forcing influences coral performance directly through various processes, including material flux, but that coral response, and thus the potential for coral recovery, is highly context dependent, especially where hydrodynamic-habitat interactions influence the abundance of predators and predator attack rates.