Environmental versus intrinsic determination of colony symmetry in the coral Pocillopora verrucosa

The morphology of corals is strongly dependent on environmental conditions. Different morphologies can be induced by flow and light due to their effects on respiration, production, calcification and prey capture. Yet, colonies of many branching corals exhibit a radial symmetry, possibly indicating an intrinsic determination of colony morphology. The scleractinian coral Pocillopora verrucosa (Ellis and Solander, 1786) is a common species in the Red Sea, displaying striking flow-dependent plasticity in colony morphology. Branches of this coral are thicker and more compact in habitats exposed to stronger flow, but the colonies are usually radially symmetric. The objective of this study was to experimentally examine whether the colony symmetry in this species is determined by intrinsic or extrinsic factors. Six corals were exposed in situ for 4 mo to a unidirectional flow generated with submerged pumps, creating asymmetric flow, stronger at the side facing the pump. The up-current side of the corals developed higher concentrations of chlorophyll and proteins, greater density of zooxanthellae, and displayed a more compact morphology and longer linear extension. While asymmetry in photosynthesis and photosynthates may disappear due to within-colony translocation, our findings on asymmetry in skeletal growth and morphology indicate that environmental conditions generate lasting asymmetry in corals. Current measurements indicate that the ubiquitous symmetry observed in P. verrucosa is apparently due to a corresponding symmetry in the flow.