The new Checklist of the Italian Fauna: Hydrozoa (Cnidaria)

The hydrozoan fauna of the Mediterranean Sea is considered as the best-known fauna of this class in the world, and the last monograph covers 457 species representing about 12% of the 3,702 currently valid species reported in the last world assessment of hydrozoan diversity. In this paper the checklist of the hydrozoan marine species is reported for the nine Italian marine biogeographical units, updating the one previously published in the series ‘Checklist delle Specie della Fauna d'Italia’ in 1995 that reported 319 hydrozoans on 463 cnidarian taxa. This note describes the state of the art of the Italian Hydrozoa checklist data set until June 2024. In detail, the updated checklist includes 340 hydrozoan species (128 Anthoathecata; 121 Leptothecata; 53 Siphonophorae; 6 Limnomedusae; 3 Actinulida; 14 Narcomedusae; 15 Trachymedusae), representing 74% of Mediterranean hydrozoan species. In detail in the current Italian Hydrozoa checklist, 40 species were added (increase of 12%) compared to the previous checklist with 14 strictly endemic species (4% of the total) for the Italian waters, 55 (16%) subendemic ones, and 20 (6%) are alien species. On the other hand, 19 species reported in the previous checklist were removed because considered dubious, synonyms of older ones or without detailed distributions for our territory. In addition, 69 species (20%) expanded their biogeographic distribution. The checklist data set will be dynamically updated with new records, and it will be freely available from Lifewatch Italy at https://www.lifewatchitaly.eu/en/initiatives/checklist-fauna-italia-en/checklist. This note describes the state of the art of the Hydrozoan checklist data set until June 2024.