Ekološki odnosi hridinskog (Paracentrotus lividus Lamarck, 1816) i crnog ježinca (Arbacia lixula Linnaeus, 1758) u infralitoralu Nacionalnog parka Mljet

Purple sea urchin Paracentrotus lividus and black sea urchin are the most common echinoderms of the shallow coastel sreas in the Adriatic Sea. The aim of this final thesis was to examine the relationship between the urchins Paracentrotus lividus, Arbacia lixula and predatory fish with respect to the exposure and inclination towards the seafloor, as well as the relationship between examined urchins. In the predator – prey relationship between fish and urchins, the main predator species are: Diplodus annularis, Diplodus sparus, Diplodus vulgaris, Sparus aurata, Coris julis, Thalassoma pavo. The field research using visual census method was conducted at the territory of the Mljet National Park in July and August 2017. The urchins were analysed at 17 stations within the Mljet National Park in the depth range from zero to two metres and in the depth range from four to eight metres while the fish were observed at eight stations in the depth range from four to eight metres. The research showed the small number of predator fish with species C. julis i T. pavo being the largest in number. In both depth ranges, the majority of urchins were exposed in both vertical and horizontal positions. The low level of obscurity of both species is the result of low predator pressure.