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

Abstract

Factors controlling herbivory pressure are of central importance in shaping the sea-scape. In the Mediterranean, the sea urchin <em>Paracentrotus lividus</em> is considered as a keystone her-bivore in seagrass meadows and macroalgal communities. Here we explored the trophic behaviorof this sea urchin in a shallow seagrass habitat of <em>Cymodocea nodosa</em> mixed with <em>Caulerpa prolif-</em><em>era</em> and interspersed with sandy areas in Alfacs Bay, Ebro Delta (NW Mediterranean). The sea-sonal pseudo-indigenous bryozoan <em>Amathia verticillata</em> is locally very abundant, and there is alsoan important population of pen shells<em> Pinna nobilis</em>, providing hard substrate and cover, thusbeing a unique environment for assessing sea urchin trophic behavior. To this end, we conductedan ensemble of food preference and foraging experiments and stomach content and stable isotopeanalyses. Our results showed that sea urchins strongly prefer <em>A. verticillata </em>over other localresources, 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 revealed thatover the long term, ca. 65% of the diet of P. lividus was based on decayed seagrass leaves, fol-lowed by the bryozoan and green seagrass leaves (21.7 and 13.3%, respectively). The localavailability of P. nobilis provides a preferred substrate for sea urchins, which showed limitedforaging movements into the surrounding seagrass beds, particularly when A. verticillata wasattached 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.