Ceccherelli, Giulia and Pinna, Stefania and Sechi, Nicola (2009) Evaluating the effects of protection on Paracentrotus lividus distribution in two contrasting habitats. Estuarine, Coastal and Shelf Science, Vol. 81 (1), p. 59-64. ISSN 0272-7714. Article.
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The sea urchin Paracentrotus lividus is common in the Mediterranean in shallow subtidal rocky habitats and in Posidonia oceanica beds. The aim of this study is to investigate whether protection has the same effect on the population structure of P. lividus occurring in rocky reef habitats and in P. oceanica beds. These results are important to generate hypotheses about the influence of human harvesting, predatory pressure and migration processes on P. lividus in the two habitats. Paracentrotus lividus was sampled at seven locations within the Gulf of Alghero (North West Sardinia) where the Capo Caccia–Isola Piana MPA (Marine Protected Area) is sited: 1 location was sited in Zone A, where no harvesting of P. lividus is allowed (NH), 3 locations were sited in Zone B, where harvesting is restricted (RH), and the other 3 were located outside the MPA where no restrictions apply to sea urchin harvesting (UH). For each combination of habitat × location, P. lividus density was assessed in 10 replicates using quadrats of 1 × 1 m and the size of 20 individuals (test diameter without spines) was measured. Finally, the specimens were grouped into size-classes to examine frequency distributions at each location. Sampling was performed at the end of the sea urchin harvesting period (April–May 2006). Analyses of data have highlighted significant variability among locations for both response variables. In both habitats, no differences were found in Paracentrotus lividus abundance among levels of protection (NH vs. RH vs. UH), while a significantly higher size was found in NH rather than in RH and UH locations. Differential direct and indirect effects of protection on P. lividus size is discussed. Also, P. lividus size seemed dependent on the habitat being quite larger in Posidonia oceanica than in the rocky reefs. This finding suggests that settlement and recruitment could be more highly successful events in rocky habitats, and that in P. oceanica meadows large-sized immigrants coming from the rocky habitat contribute to the population structure. The need to define the role of the P. oceanica habitat is also discussed.
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