Bathymetric distribution of medusae in the open waters of the middle and south Adriatic Sea during spring 2002

Twenty-eight species of hydromedusae and one sgphomedusa were collected from the middle and south Adriatic in Spring 2002. Haliscera bigelowi was recorded for the first time in the Adriatic Sea and Dicodonium adriaticum for the southern Adriatic. The cosmopolitan species Rhopalonema velatum, Aglaura hemistoma and Persa incolorata averaged 88% of total medusan abundance. The Mediterranean species Rhabdoon singulare and Solmissus albescens were numerous at the deeper stations. These species were found either in the southern or in middle Adriatic, and their abundance is greater than in other regions of the Mediterranean Sea. Species diversly and abundance generally were highest in the upper 200 m and decreased with depth. Diel vertical migration patterns included some species that nocturnally sought shallower or deeper layers, and some species migrated in both directions. A few species did not show any vertical migration. Although light likely triggers vertical movements, data suggest that foraging strategies also influence the vertical migration of medusae.