@ARTICLE{TreeBASE2Ref18459, author = {Casey W Dunn and Philip R Pugh and Steven H. D. Haddock}, title = {Molecular phylogenetics of the Siphonophora (Cnidaria), with implications for the evolution of functional specialization}, year = {2005}, keywords = {}, doi = {10.1080/10635150500354837}, url = {}, pmid = {}, journal = {Systematic Biology}, volume = {54}, number = {6}, pages = {916--935}, abstract = {Siphonophores are a group of pelagic colonial hydrozoans (Cnidaria) that have long been of general interest because of the division of labor between the polyps and medusae that make up these superorganisms . These polyps and medusae are each homologous to free living forms of other Cnidaria but are generated by an incomplete asexual budding process that leaves them physiologically integrated. They are specialized for various tasks such as locomotion, feeding, reproduction, excretion, defense, and prey capture, and are precisely organized within each colony. The degree of functional specialization varies across taxa, and different authors have used this character to construct phylogenies polarized in opposite directions depending on whether they thought siphonophore evolution proceeded by a reduction or an increase in functional specialization. We have collected taxa across all major groups of siphonophores, many of which are found exclusively in the deep-sea, using remotely operated underwater vehicles (ROVs) and by SCUBA diving from ships in the open ocean. We have used these specimens, and several collected by manned submersible, to estimate the siphonophore phylogeny with molecular data from the nuclear small subunit ribosomal RNA gene (18S) and the mitochondrial large subunit ribosomal RNA gene (16S). We have found, based on these 52 siphonophores and 4 outgroup taxa, that there are no consistent trends towards increased or decreased functional specialization in siphonophores: functional classes of polyps and medusae have been gained and lost across the tree. These data also indicate that the common ancestor of siphonophores was dioecious, and are consistent with a single transition to monoecy. The present analysis also bears on several long-standing questions about siphonophore systematics. It indicates that the cystonects are sister to all other siphonophores, a group that we call the Codonophora. It is also found that the Calycophorae are nested within the physonects, and that the Brachystelia, a historically recognized grouping of short-stemmed taxa, are polyphyletic.} }

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