Water Flow and Hydromechanical Adaptations of Branched Reef Corals

Hydromechanical properties of colony form are important to the survival of branched reef corals. Our flume experiments and field observations of real and idealized model colonies show that water flows through a coral colony either horizontally or diagonally outward from the point of entry. In the latter case a stagnant region forms inside the colony. Flow patterns depend on colony size, external current velocity, and on several morphologic parameters of the colonies themselves, including: porosity, relative branch size, and branching pattern. Our data suggest that water velocity inside colonies has an upper limit (saturation velocity) which is dependent on colony form. Because it represents an optimal living velocity, saturation may be at least partly responsible for reef zonation by limiting species and growth forms to environments with the appropriate optimal velocities. Hydrodynamic considerations indicate that under the appropriate conditions of size and external current velocity, morphologically dissimilar colonies can have similar flow characteristics, and morphologically similar colonies can have dissimilar flow characteristics. This implies that atypically sized colonies may be dynamically suited to atypical hydrologic environments, and that some colonies may need to grow allometrically. Colonies of different branching types have different nutritive problems with respect to water flow. Loosely branched colonies waste their resources, but experience no adverse gradient of food availability inside the colony. Tightly branched colonies must contend with a marked decrease in food availability within the colony.