Metabolic consequences of phenotypic plasticity in the coral Madracis mirabilis (Duchassaing and Michelotti): the effect of morphology and water flow on aggregate respiration

Phenotypic plasticity has been documented in a number of reef coral species for a variety of morphological traits, but its ecological importance is not well understood. In the branching coral Madracis mirabilis (Duchassaing and Michelotti) spacing among branches varies across environmental gradients and in general is inversely related to the rate of water movement. This polymorphism is due in part to variation in branch diameter which is phenotypically plastic in this species. Branch spacing can affect biomechanical processes such as the surface roughness of aggregated branches and flow dynamics within an aggregate, both of which could affect colony metabolism. We examined the metabolic consequences of variation in branch spacing and flow speed on small M . mirabilis aggregates to determine if plasticity for this trait could be beneficial. The dark respiration of aggregates with different amounts of branch spacing was measured under three flow speeds (3.1, 4.7 and 8.4 cm s −1) in respiration chambers. Aggregates with the greatest branch spacing had the highest respiration rates in all three flow conditions and the respiration of each morphology increased with flow speed. Increased branch spacing may decrease the thickness of the diffusive boundary layer, thereby maintaining mass transport and hence high respiration rates in low flow. This and previous studies suggest that phenotypically plastic branch spacing may represent an important adaptation in M . mirabilis, enabling aggregates to tolerate a variety of flow conditions.