A global prospective on life cycle, colony form and skeleton evolution in the Hydractiniidae (Phylum Cnidaria, Class Hydrozoa), with a taxonomic revision of the family

The cnidarian class Hydrozoa is one of the metazoan groups with the widest arrays of life cycle outcomes. In this group the adult medusa has been lost or reduced several times independently through heterochrony. The family Hydractiniidae shows nearly the entire spectrum of life cycles of the Hydrozoa, with species that show various degrees of heterochrony. Moreover, in the Hydractiniidae another heterochronic event can be identified at the colony level, with colonies that vary from a reticulate to an encrusting form. The Hydractiniidae also represent one of only three families of Hydrozoa in which a calcified skeleton has evolved. The presence of only a few calcified species within the Hydractiniidae makes this family an excellent group to investigate the transition from non-calcified to calcified taxa. Moreover, one species of Hydractiniidae builds a crystalline mat containing calcium carbonate granules, showing that species in this group can produce calcium carbonate without showing a true skeleton. Finally, the Hydractiniidae, despite being one of the most intensively studied families of Hydrozoa, still does not have a settled generic and species taxonomy. The lack of suitable taxonomic characters led many experts to give up any attempt of generic subdivision and to merge all the previously recognized genera in the oldest one (Hydractinia).;In the present work I focus on the Hydractiniidae to investigate the evolution of life cycle and colony form using phylogenetic hypothesis based on mitochondrial and nuclear DNA sequences obtained from a world-wide collection of samples.;I also used the phylogenetic hypothesis to investigate the evolution of skeleton within the Hydractiniidae, the distribution of the calcium carbonate granules in the family, and to investigate how this distribution relates to the evolution of the true skeleton (Chapter II).;The phylogenetic hypothesis was then used to test the monophyly of the nominal genera. In the light of the molecular and morphological analyses, a generic revision of the family Hydractiniidae is here proposed (Chapter III). The generic revision will be implemented after its proper publication as suggested by the International Code of Zoological Nomenclature.;Finally the description of a semi-calcified species of Hydractiniidae new to science is given in chapter IV.