Thermohaline responses of the blooming non-indigenous Cnidostoma fallax (Cnidaria, Hydrozoa), from Patos Lagoon Estuary, Brazil, and adjacent coastal area

High-density populations of the non-indigenous hydromedusa Cnidostoma fallax Vanhöffen, 1911 have been recorded in the Southwest Atlantic, reaching maximum numbers of 11,000 ind.m −3 . Salinity and temperature conditions are key factors determining the survival and reproductive success of this species, as they trigger bloom events. We conducted experiments with immature medusae of C. fallax from the Patos Lagoon Estuary, Southern Brazil to evaluate mortality, medusae bud production, and growth ratio. Medusae were cultured for seven days under four experimental conditions: two temperatures (15 °C and 20 °C) and two salinities (10 and 30). High temperatures and salinities combined increased mortality and inhibited bud production ( p < 0.01), while low temperature in both salinity regimes (10 and 30) reduced mortality and increased bud production (p < 0.01). The maximum number of buds released was 0.52 ± 1.18 buds medusa −1 day −1 (mean of 0.15 ± 0.61) under 15 °C and 10 salinity. Growth ratio was significantly affected by both temperature and salinity ( p < 0.01). This study provides the first insights into the survival and reproduction conditions of C. fallax on different hydrological conditions. The species sensitivity to elevated temperature and salinity suggests a source-sink dynamic, with brackish estuarine conditions supporting populations while coastal and higher salinity waters act as sinks. These insights corroborate previous observations on the species population dynamics. Our results are crucial for predicting potential expansion regions of this exotic species, understanding the environmental conditions in which the species thrives, and assessing its potential mechanisms of invasion. • High temperatures and salinities increased mortality and inhibited bud production in C. fallax . • Low temperature and salinity reduced mortality and increased bud production. • The experiments allowed us to propose a source (estuary) and sink (coast) dynamic for the C. fallax population.