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The freshwater jellyfish Craspedacusta sowerbii (Lankester, 1880) is a global invader of lakes. Empirical and experimental evidence indicate that the environmental factors that govern the global expansion of Craspedacusta and the development of jellyfish blooms are temperature, meso-eutrophic character and food availability in lakes. In newly colonised lakes, the medusa stage of C. sowerbii feeds preferentially on medium-sized (0.2-2.0 mm in diameter) microcrustaceans, for example Daphnids and copepods. Clonal populations of this invader have recently been recorded in several Andean lakes of northern Patagonia (40°S). Here, we report for the first-time synchronous blooms of clonal populations of C. sowerbii in two nordpatagonian lakes with similar habitat and water quality but with different types of zooplankton as food supply for jellyfish. Jellyfish populations in these lakes showed notable differences in body size, as well as in the colour of the gonads. We speculate that the observed differences of the medusa populations reflect phenotypic plasticity and could be mediated by the quality of the diet, which was the main environmental factor that differed between the studied lakes. Both lakes are inhabited by microcrustaceans, but in one lake, the highly coloured symbiotic ciliate Stentor amethystinus was also present, amounting to almost 52% of total zooplankton biomass. This type of food source for jellyfish blooms of C. sowerbii is for the first time reported in the world. The invasion process of clonal jellyfish in north Patagonian lakes represents a unique opportunity to study the potential role of diet quality in inducing plasticity responses to environmental stress derived from climate change.

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