Corals rely almost exclusively on the ambient flow of water to support their respiration, photosynthesis, heat exchange, and reproduction. Coral tentacles extend to the flow, interact with it, and oscillate under the influence of waves. Such oscillating motions of flexible appendages are considered adaptive for reducing the drag force on flexible animals in wave-swept environments, but their significance under slower flows is unclear. Using in-situ and laboratory measurements of the motion of coral tentacles under wave-induced flow, we investigated the dynamics of the tentacle motion and its impact on mass transfer. We found that tentacle velocity preceded the water velocity by ~1/4 of a period. This out-of-phase behavior enhanced mass transfer at the tentacle tip by up to 25% as compared with an in-phase motion where drag is minimal. The enhancement was most pronounced under flows slower than 3.2 cm s^-1, which are prevalent in many coral-reef environments. We found that the out-of-phase motion results from the tentacles’ elasticity, which can presumably be modified by the animal. Our results suggest that the mechanical properties of coral tentacles, and, may represent an adaptive advantage that improves mass transfer under the limiting conditions of slow ambient flows. Because the mechanism we describe operates by enhancing convective processes, it is expected to enhance other fitness-determining transport phenomena such as heat-exchange and particle capture.

Table S2. Flow conditions and the measured phase difference in the field experiments on individuals of A. diaphana (A) and colonies of D. favus (B). u^c is the horizontal current velocity, A is the average amplitude of the horizontal wave-induced velocity u^w, SD is its standard deviation, f is wave frequency calculated as the inverse of the period, and Δφ is the phase difference between the water and tentacle velocities.

 

Table S3. Conditions of the D. favus laboratory experiments. D_T and L_T are the tentacle's diameter and length, u^c the horizontal current velocity, A the amplitude of the wave-induced horizontal velocity u^w, f the wave frequency calculated as the inverse of the period f=1/T, and Δφ the phase difference calculated as the difference between the phase in the sine-fitted horizontal water and the tentacle velocities. The Reynolds number, Re=U_mD_T/ν, and the Keulegan–Carpenter number, KC=U_mT/D_T where calculated using the maximal velocity of the horizontal phase-averaged period, U_m=max⁡(u), and the kinematic viscosity of sea-water ν.

 

Table  S4. Flow conditions during the G. fascicularis laboratory experiments. u^c is the current velocity, A is the wave amplitude of the wave-induced horizontal velocity u^w, f is the wave frequency calculated as the inverse of the period f=1/T, and Δφ is the phase difference between the water and tentacle velocities.