Temporal persistence in the vertical structure of the assemblage of planktonic medusae in the NW Mediterranean Sea

The seasonal occurrence and Vertical distribution of planktonic cnidarians (hydromedusae and scyphomedusae) were investigated over a 2 yr period in th e Bay of Villefranche-sur-Mer. A total of 37 species were recorded, of whic h the meroplanktonic species were more specific and the holoplanktonic spec ies were more abundant. The results suggest that although medusae were ever -present in the bay, the assemblage changed through time. These changes ref lected species-specific responses to the environment and were linked to the thermal structure of the water column: the meroplanktonic species were mos t common during eutrophic periods of the year while the holoplanktonic spec ies were present throughout the year. The dominant species of medusae parti tioned Vertical space and each occupied a characteristic depth. This segreg ation in the vertical dimension was maintained in the different seasons des pite the fact that the exact species changed. It is suggested that the stro ng degree of structure to the medusoid assemblage in the Bay of Villefranch e-sur-Mer might reflect the stability and age of the Mediterranean ecosyste m.