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INTRODUCTION
Molecular data continue to enhance our understanding of the evolutionary history of life at all hierarchical levels. Nevertheless, many questions about cnidarian phylogenetic relationships remain unanswered (Daly et al., 2007). To address these questions, and thereby provide a robust phylogenetic framework for addressing biological questions involving cnidarians, is the primary aim of the Cnidarian Tree of Life project (http://cnidarian.info). To achieve this goal, this large, collaborative effort is compiling molecular data from both nuclear and mitochondrial markers from as many cnidarian species as possible. This paper summarizes progress in understanding the phylogeny of Trachylina and provides greater insight into the evolution of several problematical cnidarians.
Within Hydrozoa (Cnidaria), an ancient phylogenetic divergence appears to have given rise to two primary clades with extant representatives, Trachylina and Hydroidolina (Collins, 2002; Collins et al., 2006a). In terms of species richness, the vast majority of hydrozoan diversity is contained within Hydroidolina (=Leptolina sensu Schuchert, 2007). Trachylina encompasses roughly 150 valid species, whereas Hydroidolina contains more than 3000 (Schuchert, 1998). Nevertheless, because of lack of study and generally simple and plastic morphology, the true species richness of Trachylina remains obscure, and significant levels of crypsis are possible. Interestingly, the numbers of higher taxa classified within Trachylina and Hydroidolina are comparable. Limnomedusae, Narcomedusae, Trachymedusae and Actinulida (though it has not been sampled for molecular data until this study) are the main taxa, usually ranked as orders, within Trachylina (Figures 1-3). Hydroidolina (see Cartwright et al., this volume) contains three relatively diverse clades, Anthoathecata (athecate hydroids and anthomedusae), Leptothecata (thecate hydroids and leptomedusae) and Siphonophora.
Fig. 1.
Images of representative trachyline species. Note that the specimens depicted are not the ones DNA samples were obtained from. (A-C) Limnomedusae; (D-I) Trachymedusae; (J -L) Narcomedusae. (A) Craspedacusta sp.; (B) Olindias sambaquiensis; (C) Gonionemus vertens; (D) Tetrorchis erythrogaster; (E) Liriope tetraphylla; (F) Halicreas minimum; (G) Botrynema brucei; (H) Pantachogon 'white-red'; (I) Pantachogon 'orange'; (J) Solmissus incisa; (K) Aegina rosea; (L) Solmundella bitentaculata. A, B and E from A.E. Migotto (Centro de Biologia Marinha, Universidade de São Paulo); C from P. Schuchert (Muséum d'Histoire Naturelle de la Ville de Genève); D and L from S.H.D. Haddock...