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Vertical migration of medusae in the open waters of south Adriatic Sea during 96 h sampling period (July 2003).

Understanding medusan roles in pelagic food webs requires knowledge of medusan bathymetric distributions and diel vertical migrations. This information is lacking in the Adriatic and, more generally, in the world’ s oceans and seas. Zooplankton were collected at single station in the Southern Adriatic Sea (42o 11’ N, 17o 42’ E ; 1275 m) at a location possessing a strong thermocline beginning at 12 m depth. Vertical hauls were made by a Nansen opening-closing net (200 µ m mesh, 113 cm diameter) at depth intervals: 0-12 (above thermocline), 12-50, 50-100, 100-200, 200-400, 400-600, 600-800 and 800-1200 m. In total, 19 sampling series (152 hauls) were performed starting on July 22, 2003. In total, twenty-four species of medusae were collected. The highest medusan abundance, dominated by of Rhopalonema velatum (maximum: 93 ind. 10 m-3), was recorded below the thermocline to 100 m. Several depth strata were identified based on the abundance and frequencies of the principal depth representative medusan species: Aglaura hemistoma (0-12 m) ; R. velatum (12-100 m) ; Rhabdoon singulare (100-200 m) ; Persa incolorata (200-400 m) ; Solmissus albescens (400-600 m) and Arctapodema australis (600-1200 m). Six medusae showed substantial diel vertical movement: S. albescens 50-1200 m, R. velatum 0-800 m, P. incolorata 100-800 m, Octophialucium funerarium 400-1200 m, A. australis 400-1200 m and Amphinema dinema 100-600 m. Vertical migrations of these medusae were significantly related to variations in microzooplankton and copepod abundances as well as light intensity.

Medusae; vertical migration; South Adriatic

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