Pelagic cnidarians in the Boka Kotorska Bay, Montenegro (South Adriatic)

Abstract

          Planktonic cnidarians were investigated at six stations in the Boka Kotorska Bay from March 2009 to June 2010 by vertical hauls of plankton net from bottom to surface. In total, 12 species of hydromedusae and six species of siphonophores were found. With the exception of the instant blooms of Obelia spp. (341 ind. m^-3 in December), hydromedusae were generally less frequent and abundant: their average and median values rarely exceed 1 ind. m^-3. On the contrary, siphonophores were both frequent and abundant. The most numerous were Muggiaea kochi, Muggiaea atlantica, and Sphaeronectes gracilis. Their total number was highest during the spring-summer period with a maximum of 38 ind. m^-3 observed in May 2009 and April 2010. M. atlantica dominated in the more eutrophicated inner area, while M. kochi was more numerous in the outer area, highly influenced by open sea waters. This study confirms a shift of dominant species within the coastal calycophores in the Adriatic Sea observed from 1996: autochthonous M. kochi is progressively being replaced by allochthonous M. atlantica in the coastal waters, especially in the eutrophicated areas. This study provides a detailed report on the composition and abundance of the planktonic cnidarians community in this region, and should be considered as a baseline for future studies on gelatinous zooplankton.