Understanding trophic relationships among Caribbean sea urchins

The species <em>Echinometra lucunter</em>,<em> Echinometra viridis</em>,<em> Lytechinus variegatus</em>, <em>Tripneustes ventricosus,</em> and <em>Diadema antillarum</em> are the most common sea urchins of littoral habitats in the Caribbean. <em>T. ventricosus</em> and <em>L. variegatus</em> are associated with seagrass beds, while the other three species usually inhabit hardground substrates. Food preferences of these species are well documented and they are commonly accepted as being primarily herbivorous-omnivorous; nevertheless, few of them have previously been characterized isotopically. We used this approach for assessing the isotopic characterization of five echinoids. We established the trophic position of two groups of co-occurring species and quantified the contribution of food resources in the diet of <em>Echinometra lucunter</em>, considered the most common sea urchin in the Caribbean region. The species <em>T. ventricosus</em> and <em>D. antillarum</em> showed the highest values of δ¹⁵N. Sea urchins exhibited similar values of δ¹³C varying from -11.6 ± 0.63 to -10.4 ± 0.99%. The echinoid <em>E. lucunter</em> displayed the lowest values of carbon, from -15.40 ± 0.76%. Significant differences among species were found for δ¹⁵N and δ¹³C. Seaweed communities exhibited no differences among sites for overall δ¹⁵N (F= 1.300, df= 3, p= 0.301), but we found spatial differences for δ¹³C (F= 7.410, df= 3, p= 0.001). The ellipse-based metrics of niche width analysis found that the hardground biotope species (<em>D. antillarum</em>, <em>E. lucunter</em>, and <em>E. viridis</em>) did not overlap each other. Similar results were obtained for the co-occurring species of the seagrass biotope; however, the distance between these species was closer than that of the hardground biotope species. The Bayesian mixing models run for <em>E. lucunter</em> at all four localities found differences in food resources contribution. The algae <em>D. menstrualis</em>, <em>C. crassa</em> and <em>B. triquetrum</em> dominated in CGD; whereas <em>C. nitens</em>, <em>Gracilaria</em> spp., and <em>D. caribaea</em> represented the main contributor algae to the diet of <em>E. lucunter</em> at LQY. In Culebra Island, no dominance of any particular algae was detected in TMD, where six of the eight species exhibited a similar contribution. Similarities in δ¹⁵N between <em>D. antillarum</em> and <em>T. ventricosus</em> may hint towards a similar trophic level for these species, although <em>T. ventricosus</em> is widely accepted as an omnivore, while <em>D. antillarum </em>is considered a generalist herbivore. The lack of overlap among species in the two biotopes seems to indicate a resource partitioning strategy to avoid niche competition among co-occurring species.