Autotrophy, Heterotrophy, and Resource Partitioning in Caribbean Reef-Building Corals

Abstract

Arguments are developed from the biology and geometry of reef corals to show that species having large polyps and a low S/V [surface/volume] ratio are appropriately shaped for zooplankton capture; species with small polyps and a high S/V ratio are more appropriately shaped for light capture. This follows from the fact that in plankton capture only 1 plane is needed to remove all zooplankton impinging on an area, whereas in light capture sufficient energy is contained in shallow-water radiation to require more than 1 plane to remove this energy effectively. Surface area is directly related to reproductive potential, causing high S/V ratios to be selected for where there is sufficient incident energy to support the additional tissue. A cline of morphological specialization in the Caribbean reef corals exists such that those species with a high S/V ratio do have small polyps and are specialized for light capture, and those species with a low adult S/V ratio have larger polyps and are specialized for zooplankton capture. Niche separation for these energy resources appears to be occurring among the Caribbean corals in that no 2 spp. occupy the same place on a graph of their autotrophic and heterotrophic abilities. In general, the distance of this separation increases with geological time but only within well-defined boundaries set by the familial zone. Constructional and energetic constraints probably prevent occupancy of the extreme corners of this graph, and sediment-rejection and antipredatory needs will also influence the morphology of these corals.