Leaf partitioning of the seagrass Posidonia oceanica between two herbivores: is Sarpa salpa herbivory underestimated because of Paracentrotus lividus grazing?

In the Mediterranean region, herbivory appears to be a factor controlling the production of the endemic species Posidonia oceanica, which is mainly due to two main macroherbivores: the sea urchin Paracentrotus lividus, and the sparid fish Sarpa salpa. In this context the present study is a contribution in clarifying herbivory on P. oceanica by testing: 1) whether the abundance of grazing marks of the two herbivores is variable across different spatial scales, 2) whether spatial variation of P. lividus grazing marks and its density is consistent, 3) whether there is a dependence of the number of P. lividus grazing marks on its density, 4) whether the grazing of both macroherbivores, P. lividus and S. salpa, involves the entire leaf length, and 5) whether there is a dependence of the number of P. lividus grazing marks on the number of S. salpa grazing bites. With this aim, the density of Paracentrotus lividus was estimated at six different locations around the Gulf of Alghero (Northwestern Sardinia, Italy). At each location, two areas were considered and within each area the number of P. lividus individuals was counted in 10 plots of dimensions 1 × 1 m each. The abundance of P. lividus and Sarpa salpa grazing marks was estimated on two shoots harvested at random in each plot. The number of P. lividus individuals varied greatly among the locations and between areas within each location. A sigmoid function was found to better describe the relation between P. lividus density and the number of its grazing marks. The distance class-frequency distribution of P. lividus grazing marks from the leaf base showed that this herbivore affected the length of the Posidonia oceanica leaf for about 700 mm, and that the attacks were concentrated at the lower portion of the leaf near the base. Conversely, a larger number of the S. salpa grazing bites were more frequently seen on the higher portions of the leaves, although attacks were found along the entire length. The reported data suggest that the two herbivores share the seagrass by partitioning the portions of the leaf they consume.