Stranding of Porpita porpita (Cnidaria, Hydrozoa) in Ligurian coast: sampling bias or evidence of a warming sea?

On September 1st, 2023 three specimens of the blue button Porpita porpita (Linnaeus, 1758) (Cnidaria, Hydrozoa) were found stranded along the store of the Genoa metropolitan area (northwest Mediterranean Sea, Ligurian Sea) as a result of a citizen science contribution. This record represents the second observation of the species in this area after the first one, occurred in 2019. The general increase in observations of P. porpita in the Mediterranean basin, from 2010 on, may suggest a higher abundance of specimens in the sea, likely due to climate change, an intensified effort in reporting sightings of citizens scientist (whose data accounts for more than 85% of total available records), or a combination of both factors. The analysis of environmental data related to this latest stranding event suggests that this occurred under specific meteorological conditions. Since monitoring the spread of non-indigenous species is fundamental to study global warming effects on marine biodiversity, the understanding of the precise environmental settings under which they may occur might reduce observational biases and contribute to the acquisition of sound data less linked to fortuitous observations.