Structure of sea urchin assemblages in reef environments of an oceanic island
Channel: Encontro Recifal Brasileiro
Date: 2020-07-01

Luisa Martins Fagundes; Cesar Augusto Marcelino Mendes Cordeiro; Alberto Lindner

Several biotic and abiotic interactions influence the structure of sea urchin assemblages in reef environments, for instance, substrate complexity, wave action, competition, and predation. Among these, predation is considered the main top-down effect controlling the abundance of sea urchins. Our objective was to evaluate spatial distribution patterns and influencing factors of the structure of sea urchin assemblages in reef environments of an oceanic island. We applied ecological surveys to address two main questions (1) what is the effect of depth, benthic coverage, and structural complexity of reefs on species distribution? (2) which species are the main predators of sea urchins in a resource-limited environment? Samplings were carried out at five sites at Trindade Island, South Atlantic, where sea urchins were identified and counted on transects along a vertical gradient. Benthic cover was classified by functional groups through photoquadrats and the structural complexity of reefs was evaluated by image analysis and tridimensional reconstruction. Predation assays were developed in which sea urchins were tethered, offered as prey, and their interaction with predators recorded. Four sea urchins species were observed: Diadema antillarum, Echinometra lucunter – dominant species, Eucidaris tribuloides, and Tripneustes ventricosus. Rugosity was a determinant factor affecting the abundance of D. antillarum and E. lucunter, as it offers shelter from predation, however, relationships are species-specific. E. lucunter is negatively influenced by depth, being concentrated in shallower areas, whereas D. antillarum was widely distributed in all sites and depths. Sea urchins were preyed mainly by triggerfishes and wrasses, especially by the species Melichthys niger, Balistes vetula, and Halichoeres brasiliensis. Finally, our results indicate the relevant role of complexity and possibly of predation pressure in structuring sea urchin assemblages of reef environments under limited resource conditions at oceanic islands.