Seagrass meadow structure alters interactions between the sea urchin Paracentrotus lividus and its predators

Predation on sea urchins, which are important structuring agents in seagrass communities, can be modulated by habitat structural complexity. Here we examine the importance of meadow structure (leaf length and presence of unburied root-rhizome layer) in determining predation pressure on the main invertebrate herbivore of the temperate seagrass Posidonia oceanica (L.) Delile, the sea urchin Paracentrotus lividus (Lamarck). The attributes of seagrass habitat complexity are subject to considerable spatial and temporal variability, thus affecting refuge availability. In the field, exposure of the root-rhizome layer depends on local sedimentary conditions, while changes in the leaf canopy are generally dictated by hydrodynamic regimes, light, nutrient availability as well as grazing activity. We reproduced 5 habitat conditions in the laboratory and 2 in the field by modifying leaf length and burial level of the root-rhizome layer, and we measured fish predatory efficiency under controlled conditions. In the laboratory we focused on juvenile sea urchins (0.2 to 1.2 cm test diameter, TD) while in the field we studied young adults (3 to 5 cm TD). Mortality of juveniles and young adults was significantly lower under long leaves, while the presence of an unburied root-rhizome layer appeared to provide a key refuge for juveniles, independent of leaf length. The presence of refuges thus appears to be a key factor in the interaction between sea urchins and predatory fish in seagrass meadows, and highlights the importance of local structural complexity (e.g. sedimentary regimes, fish herbivory pressure) on the regulation of predator-prey interactions.