Motile sexual stages and evolution of Leptomedusae (Cnidaria)

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Abstract
The production of free medusoids is considered to be a transition from the production of medusae to the abolition of this stage of the life cycle, within a generalized process of paedomorphosis occurring within hydromedusae, in which the hydroid stage is the larva and the medusa stage the adult. The discovery of a species of Haleciidae producing free medusoids allows the reconstruction of the phylogeny of this family according to the following sequence of genera: Campalecium (producing medusae); Hydrantbea producing free medusoids); Hydrodendron (with sporosacs and nematophores); Scoresbia (with sporosacs and nematothecae); Ophiodissa (with sporosacs and nematothecae, branched); Phylactotbeca (with sporosacs, differentiated nematothecae and hydrothecae, branched). The affinities of the medusa stages between Campalecium and Eucbeilota indicate that the Haleciidae originated by the evolution of hydroids of the family Campanulinidae s.l. The Haleciidae evolved further, giving rise to the Plumulariidae. The phylogeny of the Haleciidae is compared with that of the Campanulariidae, in which the reduction of the motile sexual stages is probably followed by the formation of the peculiar medusa of Obelia, considered here as deriving from fixed gonophores. Probably different processes can occur in the evolution of hydromedusae, according to both the gradualistic and the punctuated equilibria hypotheses.