The Role of the seagrass Posidonia oceanica on the sea urchin Paracentrotus lividus

In this research several aspects of the interaction between the seagrass Posidonia oceanica and the sea urchin Paracentrotus lividus were investigated. The 1st experiment was a descriptive study that had the aim to investigate whether protection has the same effect on the population structure of P. lividus occurring on rocky reef habitat and in P. oceanica. The 2nd is also a descriptive study that had the aim to estimate herbivory pressure of P. lividus on P. oceanica through indirect measures. Specifically, it was asked what portion of the seagrass leaf is attacked by the sea urchins, whether the abundance of grazing marks depend on the number of sea urchins and whether herbivory of P. lividus depends on shoot density of the seagrass. The 3rd experiment was done to estimate the daily migrations of P. lividus from a rocky habitat to P. oceanica patches and particularly if they depend on the size of sea urchins and the distance from the patch edge. It was postulated that P. lividus moves randomly to the seagrass patch and that the probability of reaching it could decrease with the distance. The 4th experiment had the purpose to investigate the role of P. oceanica structure on P. lividus distribution manipulating the seagrass structure and modifying the shoot density and leaf high. Also, in this experiment artificial (plastic) seagrass units mimicking natural seagrass have been used so that different combinations of shelter and food were provided.