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

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Abstract
Vertical distributions and diel migrations in the 0–1000 m water column at a position in the Northwestern Mediterranean Sea are given in detail for the main species of gelatinous macrozoo-plankton: three siphonophores (Abylopsis tetragona, Chelophyes appendiculata and Lensia conoidea), one hydromedusa (Solmissus albescens) and one pyrosomid (Pyrosoma atlanticum). Extensive diel vertical migration occurred in these five species, particularly in A.tetragona, S.albescens and P.atlanticum for which the bulk of the population migrated as a compact unit. Pyrosoma atlanticum underwent the largest migration, with a mean amplitude of 515 m. The migration pattern of L.conoidea was more complex. Its day-distribution was bimodal and at night part of the population ascended towards the surface, while the other part appeared to undergo a weak reverse migration of ∼50 m. Timing of the migrations was observed from horizontal hauls at a given depth around sunset and sunrise, and the migratory cycles presented a clear diurnal symmetry.

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