MEPS 189:105-115 (1999)  -  doi:10.3354/meps189105

ABSTRACT: The seasonal occurrence and vertical distribution of planktonic cnidarians (hydromedusae and scyphomedusae) were investigated over a 2 yr period in the Bay of Villefranche-sur-Mer. A total of 37 species were recorded, of which the meroplanktonic species were more specific and the holoplanktonic species were more abundant. The results suggest that although medusae were ever-present in the bay, the assemblage changed through time. These changes reflected species-specific responses to the environment and were linked to the thermal structure of the water column: the meroplanktonic species were most common during eutrophic periods of the year while the holoplanktonic species were present throughout the year. The dominant species of medusae partitioned vertical space and each occupied a characteristic depth. This segregation in the vertical dimension was maintained in the different seasons despite the fact that the exact species changed. It is suggested that the strong degree of structure to the medusoid assemblage in the Bay of Villefranche-sur-Mer might reflect the stability and age of the Mediterranean ecosystem.

KEY WORDS: Medusae · Vertical distribution · Seasonality · Mediterranean Sea · Assemblage structure