Effect of sea urchin (Diadema setosum) density on algal composition and biomass in cage experiments

Abstract

This study examines the effects of the sea urchin Diadema setosum on algal composition, coverage and biomass on barren ground. In cage experiments, the effects of D. setosum density were examined at 5 levels over the range of 0–8 ind. m⁻². Algal coverage and number of species, densities and lengths of Sargassum spp. on experimental blocks in each cage were measured monthly. In the cage without D. setosum, algal coverage and biomass were higher than in cages with D. setosum. For D. setosum density of 1 ind. m⁻² and higher, decreased algal coverage and decreased biomass and density of Sargassum spp. were observed. D. setosum at a density higher than 2 ind. m⁻² had a negative effect on algal species numbers. Consequently, algae could grow when the D. setosum density was fewer than 2 ind. m⁻². This study revealed that grazing by D. setosum has a great effect on the seaweed bed ecosystems along the coast of central Japan.