Towards understanding the phylogenetic history of Hydrozoa: Hypothesis testing with 18S gene sequence data

Although systematic treatments of Hydrozoa have been notoriously difficult, a great deal of useful information on morphologies and life histories has steadily accumulated. From the assimilation of this information, numerous h ypotheses of the phylogenetic relationships of the major groups of Hydrozoa have been offered. Here I evaluate these hypotheses using the complete seq uence of the 18S gene for 35 hydrozoan species. New 18S sequences for 31 hy drozoans, 6 scyphozoans, one cubozoan, and one anthozoan are reported. Pars imony analyses of two datasets that include the new 18S sequences are used to assess the relative strengths and weaknesses of a list of phylogenetic h ypotheses that deal with Hydrozoa. Alternative measures of tree optimality, minimum evolution and maximum likelihood, are used to evaluate the reliabi lity of the parsimony analyses. Hydrozoa appears to be composed of two clad es, herein called Trachylina and Hydroidolina. Trachylina consists of Limno medusae, Narcomedusae, and Trachymedusae. Narcomedusae is not likely to be the basal group of Trachylina, but is instead derived directly from within Trachymedusae. This implies the secondary gain of a polyp stage. Hydroidoli na consists of Capitata, Filifera, Hydridae, Leptomedusae, and Siphonophora . "Anthomedusae" may not form a monophyletic grouping. However, the relatio nships among the groups are difficult to resolve with the present set of da ta. Finally, the monophyly of Hydrozoa is strongly supported.