Ecological relationships between purple sea urchin (Paracentrotus lividus Lamarck, 1816) and black sea urchin (Arbacia lixula Linnaeus, 1758) in the infralittoral zone of the Mljet National park

Abstract

Hridinski ježinac Paracentrotus lividus (Lamarck, 1816) i crni ježinac Arbacia lixula (Linnaeus, 1758) najčešći su bodljikaši u plitkom priobalnom području Jadranskog mora. Ciljevi ovog rada bili su istražiti odnos između ježinaca Paracentrotus lividus, Arbacia lixula i predatorskih riba s obzirom na izloženost i nagib spram podloge i utvrđivanje odnosa između istraživanih ježinca. U odnosu plijen – predator između riba i ježinaca glavne predatorske vrste su: Diplodus annularis, Diplodus sparus, Diplodus vulgaris, Sparus aurata, Coris julis, Thalassoma pavo. Na području Nacionalnog parka Mljet u srpnju i kolovozu 2017. godine provedeno je terensko istraživanje metodom vizualnog cenzusa. Ježinci su se istraživali na 17 postaj unutar Nacionalnog parka Mljet u dubinskom razredu od nula do dva metra i dubinskom razredu od četiri do osam metara, a ribe su istraživane na osam postaja u dubinskom razredu od četiri do osam metara. Nije uočena velika brojnost predatorskih riba. Najbrojnije vrste su bile C. julis i T. pavo. U oba dubinska razreda većina jedinki ježinaca je bila izložena i u vertikalnom i u horizontalnom položaju. Podaci o niskoj razini skrovitosti obje vrste rezultat su niskog predatorskog pritiska.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