Abstract
The species Echinometra lucunter, Echinometra viridis, Lytechinus variegatus, Tripneustes ventricosus, and Diadema antillarum are the most common sea urchins of littoral habitats in the Caribbean. T. ventricosus and L. variegatus 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 Echinometra lucunter, considered the most common sea urchin in the Caribbean region. The species T. ventricosus and D. antillarum 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 E. lucunter 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 (D. antillarum, E. lucunter, and E. viridis) 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 E. lucunter at all four localities found differences in food resources contribution. The algae D. menstrualis, C. crassa and B. triquetrum dominated in CGD; whereas C. nitens, Gracilaria spp., and D. caribaea represented the main contributor algae to the diet of E. lucunter 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 D. antillarum and T. ventricosus may hint towards a similar trophic level for these species, although T. ventricosus is widely accepted as an omnivore, while D. antillarum 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.
Highlights
Biological processes responsible for carbon and nitrogen movement in and out the ocean are critical in determining the natural abundance of these two elements in marine ecosystems (Nixon et al, 1996)
T. ventricosus and L. variegatus are associated with seagrass beds; whereas the other three generally inhabit hardground biotopes (Hendler, Miller, Pawson, & Kier, 1995)
Analysis of trophic relationships using stable isotopes is usually more complex, as primary producers and consumers are part of a complex trophic system in which changes in primary producers affect the isotopic signatures of herbivores (Fry & Sherr, 1984; Peterson, Howarth, & Garritt, 1985)
Summary
Biological processes responsible for carbon and nitrogen movement in and out the ocean are critical in determining the natural abundance of these two elements in marine ecosystems (Nixon et al, 1996). Analysis of trophic relationships using stable isotopes is usually more complex, as primary producers and consumers are part of a complex trophic system in which changes in primary producers affect the isotopic signatures of herbivores (Fry & Sherr, 1984; Peterson, Howarth, & Garritt, 1985) For this reason, stable isotope studies have become a fundamental tool for assessing trophic niche interactions (DeNiro & Epstein, 1981; Caut, Angulo, & Courchhamp, 2008; Parnell, Inger, Bearhop, & Jackson 2010; Boecklen, Yarnes, Cook, & James, 2011, Rodríguez-Barreras, Cuevas, Cabanillas-Terán, & Sabat, 2015). There is no other available information for sea urchins in the Caribbean related to their trophic interactions or diet assimilation of food resources using stable isotopes 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 Echinometra lucunter, considered one of the most common sea urchin in the Caribbean region
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