First record of the non-native jellyfish Chrysaora cf. achlyos (Cnidaria: Pelagiidae) in the Mediterranean Sea.
A single specimen of a Pelagiidae jellyfish (Scyphozoa) referable to the genus Chrysaora Péron and Lesueur, 1810 is reported from the port of Elefsina (Saronikos Gulf, Greece) on the basis of photographic evidence. Despite the absence of a voucher and the impossibility to check fine morphological features of diagnostic value, its recognizable features (coloration, umbrellar and oral arms morphology, number of tentacles, and squared shape of marginal lappets) are consistent with the group of the "Pacific" Chrysaora, and in particular with Chrysaora achlyos Martin, Gershwin, Burnett, Cargo and Bloom, 1997. The specimen is therefore identified here as Chrysaora cf. achlyos, a candidate as a new non-indigenous species in the Mediterranean Sea. Although this rare species has been often observed as single specimens even in its native range (northeastern Pacific Ocean), the occurrence of additional individuals cannot be excluded due to possible misidentifications with the native Chrysaora hysoscella (Linnaeus, 1767). Citizen science proved again to be a useful tool for NIS detection and monitoring, which overall calls for the necessity of focused and joint programmes along the whole Mediterranean coastline.