Venom and cnidome ontogeny of the cubomedusae Chironex fleckeri

► First study to explore venom and cnidome variation of individual cubomedusae. ► As medusae mature proportion of mastigophores in the cnidome increases. ► As medusae mature proportion of the vertebrate toxic fraction in the venom profile increases. ► The venom is dose dependent and toxic specifically to vertebrate cardiac cells. ► The ontogenetic cnidome and venom variation is correlated with the feeding ecology of the species.