ON THE ORIENTATION OF SEA FANS (<i>GENUS GORGONIA</i>)

Abstract

1. Measurements were made of the orientation to points of the compass of the plane of 189 sea fans from five patches on shoal reefs in the upper Florida Keys. Small fans were observed to be randomly oriented, whereas large fans showed a high degree of preferred orientation within each patch. Microscopic examination of the axial skeleton of some large fans revealed progressive changes in orientation that had taken place during growth. A passive mechanism of orientation is suggested. Due to the high velocity and short period of surf and surge and the observed motions of fans on the reef, surf and surge are judged to be the most important components of water movements controlling fan orientation. 2. Colonial cnidarians have evolved two different composite systems of effective support against surge on the shoal reefs. (a) Branched stony corals have rigid, highly mineralized skeletons that present their smallest projected area to the current. (b) Alcyonarian (soft) corals have highly deformable, fibrous-organic axial skeletons that expose maximum area to the current. 3. We are just beginning to understand the relationships between biological building materials and the functional supportive systems in which they are found.