Grazing on fleshy seaweeds by sea urchins facilitates sponge Cliona viridis growth

We studied possible interactions among invertebrate species and algal assemblages in a shallow sublittoral community of the Mediterranean Sea using a 2-step approach. First, we analysed the general pattern by correlation analysis. Thereafter, we experimentally assessed cause–effect relationships between the species or assemblages that were clearly related in the correlation study. The abundance of Cliona viridis (Schmidt 1862) was positively correlated with that of sea urchins and negatively correlated with fleshy algal cover. These relationships were confirmed by field experiments: treatments without fleshy algae, owing to both natural and simulated sea urchin grazing, promoted C. viridis growth significantly more than controls. Our results support the hypothesis that sea urchin grazing on seaweeds increases light availability at the basal stratum, which favours the primary production of the symbiotic zooxanthellae and thus C. viridis growth through nutrient transfer. We found a network of interactions with several signs, directions and strengths: a strong positive indirect interaction (‘facilitation’) between sea urchins and C. viridis, a negative direct interaction between sea urchins and seaweeds (‘predation’), a negative direct interaction between seaweeds and C. viridis (‘shading-interference’) and a weaker but significant direct interaction between C. viridis and Pione vastifica (‘space competition’). The multiple interactions observed suggest a cascade that involves 4 trophic levels: primary producers, herbivorous, carnivorous, and filterfeeders. This cascading process can have negative cryptic implications on the environment, as the excavating sponge C. viridis is a bio-eroder that may strongly impact the shallow sublittoral landscape by producing substrate weakening and instability. The knowledge acquired on the sign and strength of these multi-species interactions is useful to model and predict the responses of shallow benthic communities to anthropogenic disturbances (i.e. overfishing and eutrophication).