Widespread mixotrophy in reef-inhabiting soft corals: the influence of depth, and colony expansion and contraction on photosynthesis

Abstract

Primary production, respiration, and bathymetric distribution were investigated in the 10 most common zooxanthellate soft coral, (Octocorallia: Alcyonaceae) genera of mid-shelf reefs of the central Great Barrier Reef, Australia. In most taxa, photosynthetic carbon acquisition was lower than respiratory carbon loss under irradiance conditions of cloudless summer days at 20 m depth, and only slightly higher than respiration at five m depth. Hence, most if not all zooxanthellate taxa were also dependent upon heterotrophic nutrition to cover their carbon demand. Colony contraction significantly reduced photosynthetic productivity in soft coral taxa possessing large polyps. The moderate dependence of soft corals on phototrophy was reflected in their bathymetric distribution at Davies Reef. Highest soft coral densities were found at 20m depth. -from Authors