Diets and coexistence of the sea urchins Lytechinus variegatus and Arbacia punctulata (Echinodermata) along the central Florida gulf coast

Abstract

The basis for coexistence of similar species is fundamental in community ecology. One mechanism for coexistence is differentiation of diets. Lytechinus variegatus and Arbacia punctulata coexist in different microhabitats along the Florida gulf coast. Their great difference in morphology might affect their choice of microhabitats and diet. We analyzed diets of both species at 1 offshore and 1 nearshore site where both occurred in relatively equal numbers, an offshore site dominated by A. punctulata and an offshore site dominated by L. variegatus. Gut contents were analyzed to deter- mine the diet. A. punctulata prim. consumed sessile invertebrates except on dates when algal avail- ability was higher than normal. L. variegatus primarily consumed macroflora except on dates when macroflora was extremely limited. Electivity indices revealed no strong preferences for particular species of algae, although L. variegatus consumed many drift species. A. punctulata and L. variega- tus both fed in a random manner, although they avoided particular species of algae known to contain high concentrations of secondary metabolites. The diet of A. punctulata was correlated with algae only over rubble outcroppings at the offshore site with the highest biomass. Diets of offshore popula- tions were more similar to each other, regardless of the presence of conspecifics, than to those of populations at Caspersen Beach (nearshore site). As diets do not overlap, distribution of individuals at a location would not be affected by interspecific competition for food. However, intraspecific competition may be high due to low site productivity.