Potential omnivory in the sea urchin Diadema antillarum?

Sea urchins are one of the most important herbivores of shallow-water reefs in the Western Atlantic, capable of controlling seaweed communities. One of them, Diadema antillarum Philippi, 1845 has been target of several studies due to its capacity to control algal communities, and it has been considered a generalist herbivore species. Nevertheless, the assimilation process in D. antillarum using stable isotopes remains unclear. Our main goals were to characterize isotopically the sea urchin D. antillarum with the surrounding macroalgae communities, determine its trophic position, and calculate the contribution of the primary producers using stable isotopes. We collected samples of sea urchin and their potential food resources at four shallow water reefs of Puerto Rico Archipelago during February and October of 2013. The δ13C isotopic ratio of D. antillarum did not vary on site and in time, but it varied spatial and temporally in term of δ15N. The Trophic level (TL) ranged from 2.35 ±0.11 to 3.24 ±0.17, and exhibited spatial and temporal significant differences. The lack of congruent polygons found in this study, and the high TL values support the conclusion that D. antillarum produced isotopic ratios not adequately explained by the algal data. Our findings reinforce the importance of benthic invertebrates as potential nitrogen sources for D. antillarum. Further studies including benthic invertebrates may help to quantify the contribution of those species to the diet of D. antillarum.