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Abstract Cnidarians display a wide diversity of life cycles. Among the main cnidarian clades, only Medusozoa possesses a swimming life cycle stage called the medusa, alternating with a benthic polyp stage. The medusa stage was repeatedly lost during medusozoan evolution, notably in the most diverse medusozoan class, Hydrozoa. Here, we show that the presence of the homeobox geneTlxin Cnidaria is correlated with the presence of the medusa stage, the gene having been lost in clades that ancestrally lack a medusa (anthozoans, endocnidozoans) and in medusozoans that secondarily lost the medusa stage. Our characterization ofTlxexpression indicate an upregulation ofTlxduring medusa development in three distantly related medusozoans, and spatially restricted expression patterns in developing medusae in two distantly related species, the hydrozoanPodocoryna carneaand the scyphozoanPelagia noctiluca. These results suggest thatTlxplays a key role in medusa development and that the loss of this gene is likely linked to the repeated loss of the medusa life cycle stage in the evolution of Hydrozoa. 

Craspedacusta sowerbii is an invasive hydrozoan found globally in freshwater habitats. C. sowerbii has a complex life cycle that includes benthic, pelagic, dispersal and dormant stages. The distribution of the medusa (jellyfish) stage has been well documented, but little is known about the other life cycle stages, which are suggested to be more widespread. In addition, the conditions required for growth, reproduction and dispersal of the different life cycle stages, as well as the environmental cues that regulate life cycle transitions, are not fully understood. The aim of this study was to determine laboratory conditions for growth of, and transition to, different life cycle stages in order to improve our ability to culture all life cycle stages of C. sowerbii. In addition, insight into the environmental triggers that promote life cycle transitions will enable us to better predict the potential negative effects C. sowerbii could impose on freshwater ecosystems.