Distribution of Pelagia noctiluca (Cnidaria, Scyphozoa) in the Ligurian Sea (NW Mediterranean Sea)

The scyphozoan Pelagia noctiluca reproduces by direct development without a benthic stage. Typically, this jellyfish is found offshore with a holoplanktonic lifecycle, vertical migration and feeding behaviours. Frequent outbreaks have been well documented on the Mediterranean shores since the 19th century; however, the offshore distribution of this species remains mostly unknown. In this study, we performed a bimonthly monitoring of P. noctiluca surface density, at high resolution, from a sailboat, along a 35-km coastal to offshore transect in the Ligurian Sea, between February and October 2011. During daylight, P. noctiluca was rarely seen. At night, offshore, P. noctiluca was always present, while within 5 km of the coast, P. noctiluca was rarely observed. Pelagia noctiluca aggregations were most abundant within the Northern Current of the Ligurian Sea. Our findings suggest that P. noctiluca outbreaks observed on Mediterranean shores may result from the transport of the permanent offshore population inshore by specific hydrodynamic conditions.