The invasive freshwater jellyfish Craspedacusta sowerbii: silent invasion of an unknown species?

Biological invasions are a major driver of biodiversity loss, yet inconspicuous or “cryptic” species often escape public and scientific attention. The freshwater jellyfish Craspedacusta sowerbii, likely native to China and now present on six continents, exemplifies such silent invasions. Despite its broad distribution, the species’ ecology and societal perception remain poorly understood. We conducted a 24-month, multilingual online survey across 17 European countries (n = 1,388) to assess public awareness, perceptions, and management views regarding C. sowerbii. Results reveal limited knowledge: over 80% of respondents did not know its scientific name, 49% misidentified its freshwater habitat as marine, and only 33% recognized it as non-native. While its aesthetic qualities were often praised, mass occurrences provoked caution, and direct contact was rare and typically harmless. Bayesian Network analysis indicated that direct observation—rather than demographic factors or ecological knowledge—was the strongest predictor of concern and support for increased public discussion. Although most respondents did not perceive C. sowerbii as a major local threat, many recognized potential ecological implications and supported greater awareness and governance measures. This study underscores how visual subtlety and low media coverage limit detection and prioritization of cryptic invasive species, with implications for early warning systems and policy. Leveraging citizen science to enhance recognition and reporting could improve detection of such species, complementing formal monitoring networks. We advocate integrating cryptic invasions into invasive alien species management frameworks, emphasizing experiential engagement alongside conventional ecological data to address overlooked yet potentially impactful species.