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Trophic plasticity in the sea urchin Paracentrotus lividus: herbivory, detritivory and omnivory as a function of resource availability and habitat features

Judith Camps-Castellà*, Javier Romero, Patricia Prado

*Corresponding author:

ABSTRACT: Factors controlling herbivory pressure have a central importance in shaping the seascape. In the Mediterranean, the sea urchin Paracentrotus lividus is considered as a keystone herbivore in seagrass meadows and macroalgal communities. Here we explore the trophic behavior of this sea urchin in a shallow seagrass habitat of Cymodocea nodosa mixed with Caulerpa prolifera and interspersed with sandy areas in the Alfacs Bay, Ebro Delta (NW Mediterranean). The seasonal pseudo-indigenous bryozoan Amathia verticillata is locally very abundant, and there is also an important population of Pinna nobilis, providing hard substrate and hides, thus being a unique environment for assessing the sea urchin trophic behavior. To this end, an ensemble of food preference and foraging experiments, stomach contents and stable isotope analyses were conducted. Our results showed that sea urchins strongly prefer A. verticillata over other local resources, and there was also an important presence of the bryozoan in stomach contents (ca. 44%), coupled with green and decayed seagrass leaves. Stable isotope analyses, evidenced that in the long-term, ca. 65% of the diet of P. lividus appears to be based on decayed seagrass leaves, followed by the bryozoan and green seagrass leaves (21.7 and 13.3%, respectively). The local availability of P. nobilis provides a preferred substrate for sea urchins which showed limited foraging movements into the surrounding seagrass beds, particularly when A. verticillata was attached to the pen shells. The apparently high contribution of animal and detrital food to P. lividus diet is unprecedented, and suggests an opportunistic feeding behavior in sea urchins in those habitats.