Coral reef framework cavities: is functional similarity reflected in composition of the cryptic macrofaunal community

Hard substratum surface area of framework cavities constitutes a major habitat in coral reefs. We studied the community composition and distribution of cryptic sessile macro-organisms in framework cavities in relation to abiotic parameters on a reef slope in Curaçao. Spatial characteristics were measured with a CaveCam (video) cave–explorer to investigate the macro-faunal community composition. Light intensity and water movement were measured. Bacterial densities were counted in- and outside the cavities over a year. Cover of the fauna and flora in cavities was about 95% of total hard surface area. Cavities harbored a distinctive macro-fauna. Species composition was very diverse, with a total of 88 species/taxa found. Diversity (H’) was high and evenness (V’) low, indicating the presence of dominant species. Community composition was related to abiotic parameters. Light intensity decreased with a factor of 10 from front to back of cavities, with a consequent decrease in crustose coralline algae in the same direction, but there was no other relation between light and distribution of organisms. Water motion and turbidity, generally less in cavities than on the open reef, were significantly related to biotic distribution. Inside cavities we found sponge and total suspension-feeder cover to decrease with increasing water movement and turbidity. There was an average depletion of bacteria of 40% in cavity water. In a functional sense reef framework cavities are a uniform trophodynamic environment characterized by high bacterioplankton removal rates and efflux of DIN and it is surprising to find each cavity having a different species composition and abundance.