Assessing the influence of macroalgae and micropredators on the early life success of the echinoid Diadema africanum

The sea urchin Diadema africanum is an herbivore that has a significant influence on subtidal rocky reef communities on the Canary Islands, facilitating transitions between macroalgal beds and barren grounds. Barren grounds in the archipelago are characterized by a dominance of crustose coralline algae and a lack of invertebrates and fishes compared to macroalgal beds, which are mainly dominated by brown algae. To assess the impact of these benthic community states on the early life success of D. africanum, a manipulative study was conducted in September 2021 at four sites on the eastern coast of Tenerife Island, characterized by the availability of historical settlement rates and adult data for this species. For this purpose, experimental larval collectors modified with a hollow algae-container were employed to examine sea urchin settlement rates in response to two dominant algal species, Lobophora schneideri, in macroalgal beds and crustose coralline algae (CCA) in barren grounds. Statistical analysis revealed significant spatial differences in sea urchin settlement and the interaction between site and treatment response. Specifically, the site with higher sea urchin settlement exhibited lower numbers of D. africanum settlers in treatments containing L. schneideri than in the Control and CCA treatments. Three decapod species (the pistol shrimp Alpheus macrocheles, and the crabs Lophozozimus incisus, and Pachygrapsus marmoratus) negatively correlated with D. africanum settlers. These results suggest that larval supply, L. schneideri beds, and micropredator abundance play a role in the early life success of this species. This may imply a community stability mechanism that helps to prevent shifts between alternative stable states.