Pelagia noctiluca is the most common jellyfish in the Western Mediterranean Sea, living in
oceanic waters with a holoplanktonic lifecycle. Frequent outbreaks have been well documented in coastal areas, yet little is known about their offshore distribution. In this study we
address the relationship between oceanographic structures and the distribution of P. noctiluca ephyrae along the central continental slope of the Western Mediterranean, covering a
wide latitudinal gradient, during July-August 2016. The region is characterized by a rich and
complex mesoscale surface circulation driven by the inflow of Atlantic Water into the Western Mediterranean through the Strait of Gibraltar. The results revealed a high variability in
the ephyrae spatial paterns related with different water masses and the resulting mesoscale
hydrographic features. Their horizontal distribution showed a clear latitudinal gradient with
high abundances in the south, associated with recent Atlantic Water, and low abundances
or absence in the north, in coincidence with the old Atlantic Water transported by the Northern Current. Ephyrae showed diel vertical migrations of short-extent in the first 50 m, with a
wide distribution above the thermocline and the Deep Chlorophyll Maximum during daytime,
being more concentrated towards the surface at night. The results suggest the population
connectivity of P. noctiluca between the Atlantic and the Mediterranean. In that case, the
abundance variability of the species in the Mediterranean could be modulated by its
entrance associated with the inflow of Atlantic Water through the Strait of Gibraltar.
