Coral tentacle elasticity promotes an out-of-phase motion that improves mass transfer

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-pha...