Acropora reticulata : structure, mechanics and ecology of a reef coral

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Abstract
Skeletal structure of the reef coral Acropora reticulata is of clear mechanical and, thereby, ecological significance. The skeletal material is linearly elastic and brittle. The strength, by contrast with stiffness, of excised material varies with anatomic direction in a manner dictated by organization of the skeletal elements. In the stem, the organization imposed by axial growth and the existence of coenosteum between secondarily fused branches creates functional cleavage planes that determine the directional strength and fracture characteristics of the material. In the stem, a porous region constructed of concentric skeletal annuli surrounds the denser, centrally located set of branches, or primordia of the stem. While pores among the annuli reduce the bending strength of the stem, they also harbour skeletogenic tissue that may prevent fatigue failure from incessant flexing in surge. Shape and size, which vary among colonies, determine the hydrodynamic forces and moments that stress and break colony skeletons. Shape and skeletal strength determine, in turn, the resistance by a coral to catastrophic fracture of the stem. Shape and branching pattern in the fan beyond the stem determine the extent and pattern of breakage produced by debris swept about in storms. Below the upper limit to bathymetric distribution colonies should vary systematically in shape and size according to depth, in a way that suits each to the mechanical environment. In addition, topographic specialization for growth, feeding or reproduction among polyps in a colony may have resulted from the predictable, but limited damage inflicted by debris. In consequence, the mechanically functional organization of the skeleton can be said to affect the distribution, abundance and, perhaps, the life-historical characteristics of Acropora reticulata .