The diel migrations and vertical distributions of zooplankton and micronekton in the Northwestern Mediterranean Sea. 2. Siphonophores, hydromedusae and pyrosomids

Vertical distributions and diel migrations in the 0–1000m water column at a position in the Northwestern MediterraneanSea are given in detail for the main species of gelatinous macrozoo-plankton:three siphonophores (Abylopsis tetragona, Chelophyes appendiculataand Lensia conoidea), one hydromedusa (Solmissus albescens)and one pyrosomid (Pyrosoma atlanticum). Extensive diel verticalmigration occurred in these five species, particularly in A.tetragona,S.albescens and P.atlanticum for which the bulk of the populationmigrated as a compact unit. Pyrosoma atlanticum underwent thelargest migration, with a mean amplitude of 515 m. The migrationpattern of L.conoidea was more complex. Its day-distributionwas bimodal and at night part of the population ascended towardsthe surface, while the other part appeared to undergo a weakreverse migration of