In the Adriatic Sea there are 20 recorded species of sea urchins, the most abundant of which is the purple sea urchin Paracentrotus lividus (Lamarck, 1816) and the black sea urchin Arbacia lixula (Linnaeus, 1758). Their habitat is a shallow rocky bottom overgrown by algae, which is the primary food source for the urchins. Sea urchins are among the most important herbivores, and thus have a major role in shaping benthic algae communities. The ecology of these communities can be disrupted if sea urchin feeding rates surpass the new colonization and growth of the algae. This scenario can lead to a complete replacement of habitats, from photophilic communities rich in algae, to habitats with dense populations of sea urchins called urchin barrens, a habitat widely spread across the Adriatic. Urchin populations have become unstable due to a release from predation pressure thanks to a decline in their predators, and this can drive this regime shift and dominance in urchin barrens. This project aims to map patterns in urchin density, size structure and diversity around Silba island, to identify sites covering a gradient of overall urchin pressure. These sites will be used to assess benthic community structure, with a particular focus on macroalgal cover, to quantify the impacts of urchin densities on benthic communities. Students could also survey predator species to determine whether there is evidence for a cascade effect, where predator declines have stimulated booms in urchin numbers and subsequent loss of macroalgal coverage.

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