Macroalgae contribution to the diet of two sea urchins in Sargassum Beds: Tripneustes depressus (Camarodonta: Toxopneustidae) and Eucidaris thouarsii (Cidaroide: Cidaridae)

Macroalgae availability is thought to be a major driver of the diet of marine herbivores. In subtropical regions, macroalgal biomass is highly seasonal and the diet of herbivores usually varies accordingly. In Sargassum beds in the Gulf of California, Tripneustes depressus and Eucidaris thouarsii dominate the invertebrate fauna and feed on macroalgae, though they represent different trophic levels. The objective of this study was to relate the availability of macroalgae with their potential contribution to the diet of both sea urchins. Macroalgae biomass and composition in Sargassum beds were analyzed in contrasting seasons (winter and summer) at four sites. Also, the food sources and the percent contribution of selected sources were determined by a mixing model analysis using stable isotope values (13C and 15N). In winter and summer, brown macroalgae attained the highest biomass, followed by red and green macroalgae. Red macroalgae showed the highest number of taxa and brown macroalgae the highest biomass. In T. depressus, the red (Gracilaria pachydermatica, Laurencia pedrochei) and brown (D. flabellata and Sargassum horridum) macroalgae made the highest contribution to their diet, consistent with the higher availability of these macroalgae in the local flora throughout the year. In E. thouarsii, mixing model indicate that the red algae Polysiphonia pacifica and G. pachydermatica were the macroalgae species with the greatest contribution to its diet.