Non-radial symmetry of the transport system of Acropora corals

Abstract

The branches of Acropora corals are formed by radially symmetric skeleton lined with soft tissue. However, the transport system does not conform to the same radial structure, but is bilaterally symmetric instead. The hydroplasm flows at constant velocity towards the apex (distally) along the upper side of the branch (facing the water surface) and towards the base of the branch (proximally) along its underside (facing the seabed). The axial and the largest canal do not play a major role in the transport of hydroplasm, however, if they do conduct any movement of fluid, it is directed proximally, i.e. towards the base of the branch. The results were obtained using the simple method of vital staining of broken off fragments of Acropora branches. Changing the position of the branch for a period of several weeks did not affect the pattern of hydroplasmic flow.