Flow-dependent growth in the zooxanthellate soft coral Sinularia flexibilis

Growth characteristics of colonies of the branching zooxanthellate octocoral Sinularia flexibilis, with potential pharmaceutical importance, were measured over a range of water velocities. The highest mean specific growth rate (μ d− 1) was found at a flow velocity of 11 cm s− 1. An optimal range of water turbulence was found at a Reynold's number of  ≈ 10,000, with a minimum thickness of boundary layer for rapid mass transfer. There was a similar dependency on water velocity for the contents of zooxanthellae, chlorophyll a, and protein, indicating that photosynthesis also runs at an optimum rate at 11 cm s− 1, thus maximizing coral growth. Moreover, the corals showed morphological responses to the changes in water velocity: increase in the number of protruding branches (buds) in proportion to increased flow and then decrease at higher flows, as well as reduced sizes of the colonies at high velocities.