The freshwater jellyfish Craspedacusta sowerbii Lankester 1880 is a cryptic cosmopolitan invasive species, which occurs in all continents except Antarctica. Recent molecular studies suggest the existence of at least three very different genetic lineages of Craspedacusta: the “sowerbii”, the “kiatingi”, and the “sinensis” lineages. We report the presence of both medusae and polyps of this alien taxon in the Large Lake of Monticolo / Montiggl, a meso-eutrophic natural lake in the Province of Bolzano / Bozen in Northern Italy. Molecular analyses of mitochondrial 16S sequences showed that this population belongs to a different lineage than that recently described for Sicily (Southern Italy). Therefore, there are two different genetic lineages of C. sowerbii in Italy. In the Large Lake of Monticolo / Montiggl medusae were observed in 6 consecutive summers (2015–2020), from July to September. All the examined medusae were males. The stomach content analyses showed that zooplanktonic copepods and cladocerans with size range between 0.3 and 0.8 mm were the preferred prey of medusae. Polyps of C. sowerbii were recorded in the lake on the zebra mussel Dreissena polymorpha in shallow water and on the underside of artificial substrates. The analyses of zebra mussels would therefore be a simple method to check for the presence of the polyp stage of C. sowerbii in various aquatic environments.

Olindiid freshwater jellyfishes of the genus Craspedacusta Lankester, 1880 are native to eastern Asia; however, some species within the genus have been introduced worldwide and are nowadays present in all continents except Antarctica. To date, there is no consensus regarding the taxonomy within the genus Craspedacusta due to the morphological plasticity of the medusa stages. The species Craspedacusta sowerbii Lankester, 1880 was first recorded in Italy in 1946, and until 2017, sightings of the jellyfish Craspedacusta were reported for 40 water bodies. Here, we shed new light on the presence of the freshwater jellyfishes belonging to the genus Craspedacusta across the Italian peninsula, Sardinia, and Sicily. First, we report 21 new observations of this non-native taxon, of which eighteen refer to medusae sightings, two to environmental DNA sequencing, and one to the finding of polyps. Then, we investigate the molecular diversity of collected Craspedacusta specimens, using a Bayesian analysis of sequences of the mitochondrial gene encoding for Cytochrome c Oxidase Subunit I (mtDNA COI). Our molecular analysis shows the presence of two distinctive genetic lineages: (i) a group that comprises sequences obtained from populations ranging from central to northern Italy; (ii) a group that comprises three populations from northern Italy—i.e., those from the Lake Levico, the Lake Santo of Monte Terlago, and the Lake Endine—and the single known Sicilian population. We also report for the first time a mtDNA COI sequence obtained from a Craspedacusta medusa collected in Spain.

Biological invasions are a major driver of biodiversity loss, yet inconspicuous or “cryptic” species often escape detection and public awareness, limiting management responses. We investigated the freshwater jellyfish Craspedacusta sowerbii , likely native to China and now present on six continents, through a 22‐month multilingual online survey across 17 European countries ( n = 1388), to assess public knowledge, perceptions and views on its management. Results revealed low public awareness: over 80% of respondents did not know the scientific name, 49% misidentified its freshwater habitat as marine and only 33% recognized it as non‐native. Aesthetic perceptions were largely positive, but mass occurrences triggered caution and direct contact was rare and usually harmless. Bayesian Network analysis showed that direct observation—rather than demographic factors or scientific knowledge—was the strongest predictor of concern and support for public discussion. While C. sowerbii was not widely perceived as a local threat, many respondents acknowledged its potential ecological implications and supported greater public awareness and management actions. Our findings highlight how visual subtlety and limited media coverage reduce detection and prioritization of cryptic invasive alien species (IAS), hindering early warning systems and policy development. Leveraging citizen science and experiential engagement can improve recognition and reporting, complement formal monitoring and support more inclusive management strategies. We recommend integrating cryptic invasions like that of C. sowerbii into IAS frameworks and policies, emphasizing public participation and awareness to address overlooked, but potentially impactful species. Read the free Plain Language Summary for this article on the Journal blog.