Description: Capitate hydrozoans are a morphologically and ecologically diverse hydrozoan suborder, currently including about 200 species.Being grouped in two clades, Corynida and Zancleida, these hydrozoans still show a number of taxonomic uncertainties at the species,genus and family levels. Many Capitata species established symbiotic relationships with other benthic organisms, including bryozoans,other cnidarians, molluscs and poriferans, as well as with planktonic dinoflagellates for mixotrophic relationships and with bacteria forthiotrophic ectosymbioses. Our study aimed at providing an updated and comprehensive phylogeny reconstruction of the suborder, atmodelling the evolution of selected morphological and ecological characters, and at testing evolutionary relationships between the sym-biotic lifestyle and the other characters, by integrating taxonomic, ecological and evolutionary data. The phylogenetic hypotheses herepresented shed light on the evolutionary relationships within Capitata, with most families and genera being recovered as monophyletic.The genusZancleaand family Zancleidae, however, were divided into four divergent clades, requiring the establishment of the newgenusApatizancleaand the new combinations for species inZancleaandHalocorynegenera. The ancestral state reconstructionsrevealed that symbiosis arose multiple times in the evolutionary history of the Capitata, and that homoplasy is a common phenomenonin the group. Correlations were found between the evolution of symbiosis and morphological characters, such as the perisarc. Overall,our results highlighted that the use of genetic data and a complete knowledge of the life cycles are strongly needed to disentangle taxo-nomic and systematic issues in capitate hydrozoans. Finally, the colonization of tropical habitat appears to have influenced the evolu-tion of a symbiotic lifestyle, playing important roles in the evolution of the group.