Hydrozoan Phylogeny and Classification

The classification of the subclasses and orders used in the Hydrozoa Directory is based on the cladogram depicted above, while the taxa below the level of orders are based on Bouillon (1985), Petersen (1990), Bouillon et al (2006) and others (see references below). Some modifications, however, are based on personal opinions.

The phylogeny and classification of the Hydrozoa is currently investigated by molecular methods and the first results show that the classification will change profoundly in the next years (comp. Collins et al. 2008, Cartwright et al. 2008, Leclere et al. 2009). Unfortunately, a considerable number of important taxons remain unsampled and a comprehensive revision based on molecular results is still premature.

The Hydrozoa are clearly monophyletic and composed of two distinct clades, the Trachylinae and the Leptolinae (also named Hydroidolina or Hydoidomedusae). For clades with an * it is not certain whether they represent  holophyletic groups.

The cladogram  represents a synthesis derived primarily from molecular phylogenies provided by Collins (2002, 2002), Collins et al. (2006, 2008), Cartwright et al (2008) and to a lesser extent  phylogenetic analyses based on morphological characters given by  Marques & Collins (2004). It differs principally  from the one in Boero & Bouillon (2000) by placing the Limnomedusae in the Trachylinae and not Leptolinae, and by demoting the Laingoimedusae to a family of the Anthomedusae.

The class Laingiomedusae was created by Bouillon (1978). They are so far only known from their medusa phase. The Laingiomedusae have a lobed umbrella margin and tentacles with an origin shifted more or less to the exumbrella. They present thus some aspect otherwise found in the Narcomedusae. However, an identical situation is also found in Thecocodium quadratum (Anthomedusae, family Ptilocodiidae, see Jarms 1987). Furthermore, with their gonads on the manubrium and the fully developed radial canals they match typical Anthomedusae. Molecular analyses (Cartwright et al., 2008; Collins et al., 2008) show that Fabienna sphaerica, a putative member of the Laingiomedusae (see Boreo & Bouillon 2000), is closely related to the Proboscidactylidae. Therefore, the Laingiomedusae are here treated as a family of the Anthomedusae.

While the name Trachylinae Haeckel, 1879  is unanimously accepted, there are exist alternateves for Leptolinae Haeckel, 1879, namely Hydroidolina (Marques & Collins 2004) or Hydroidomedusae (sensu Boreo & Bouillon 2000). Usage of taxon names above the family level is not regulated by the code of zoological nomenclature and there are no rules of preference.

The intracellular parasite Polypodium hydriforme is here not included in the Hydrozoa as this has been done traditionally. Even its position among the Cnidaria itself is unclear (see Siddall et al. 1995).

Bouillon, J. 1978. Hydroméduses de la mer de Bismarck (Papouasie, Nouvelle-Guinée). II . Limnomedusa, Narcomedusa, Trachymedusa et Laingiomedusa (sous classe nov.). Cahiers de Biologie Marine 19: 473-483. 

Bouillon, J. 1985. Essai de classification des hydropolypes-hydroméduses (Hydrozoa-Cnidaria). Indo Malayan Zoology 2: 29-243. 

Bouillon, J., & Boero, F. 2000. Synopsis of the families and genera of the Hydromedusae of the world, with a list of the worldwide species. Thalassia Salentina 24: 47-296. 

Bouillon, J., Gravili, C., Pagès, F., Gili, J.-M., & Boero, F. 2006. An introduction to Hydrozoa. Mémoirs du Muséum national d'Histoire naturelle 194: 1-591.

Collins, A. G. 2000. Towards understanding the phylogenetic history of Hydrozoa: Hypothesis testing with 18S gene sequence data. Scientia Marina 64: 5-22. 

Collins, A. G. 2002. Phylogeny of Medusozoa and the evolution of cnidarian life cycles. Journal of Evolutionary Biology 15: 418-432. 

Cartwright, P., Evans, N. M., Dunn, C. W., Marques, A. C., Miglietta, M. P., Schuchert, P. & Collins, A. G. 2008. Phylogenetics of Hydroidolina (Hydrozoa: Cnidaria). Journal of the Marine Biological Association of the United Kingdom 88: 1663-1672

Collins, A.G., Winkelmann, S., Schierwater, B. 2005. An assessment of partial mitochondrial 16S rDNA sequences as indicators of Corynidae (Hydrozoa, Anthoathecata) phylogeny. Zoologica Scripta, 34, 91-99.

Collins, A. G., Schuchert, P., Marques, A. C., Jankowski, T., Medina, M. & Schierwater, B. 2006. Cnidarian Phylogeny and Character Evolution Clarified by New Large and Small Subunit rDNA Data and an Assessment of the Utility of Phylogenetic Mixture Models. Systematic Biology 55(1):97-115.

Collins, A. G., Bentlage, B., Alberto Lindner, A., Lindsay, D., Haddock, S.H. D., Jarms, G., Norenburg, J. L., Jankowski, T., & Paulyn Cartwright, P. 2008. Phylogenetics of Trachylina (Cnidaria: Hydrozoa) with new insights on the evolution of some problematical taxa. Journal of the Marine Biological Association of the United Kingdom 88(8): 1673-1685.

Jarms,G. 1987. Thecocodium quadratum (Werner 1965) redescribed, T. penicillatum sp. nov., and a method for rearing hydrozoans. Bouillon, J., Boero, F., Cicogna, F. & Cornelius, P.F.S. [Eds]. Modern trends in the systematics, ecology, and evolution of hydroids and hydromedusae. Clarendon Press, Oxford.

Kramp, P. L. 1961. Synopsis of the medusae of the world. Journal of the Marine Biological Association of the U. K. 40: 1-469. 

Leclere, L., Schuchert, P., Cruaud, C., Couloux, A., & Manuel, M. 2009. Molecular Phylogenetics of Thecata (Hydrozoa, Cnidaria) Reveals Long-Term Maintenance of Life History Traits despite High Frequency of Recent Character Changes. Systematic Biology 58: in press.

Marques, A., & Collins, A. 2004. Cladistic analysis of Medusozoa and cnidarian evolution. InvertebrateBiology 123: 23-42. 

Petersen, K. W. 1990. Evolution and taxonomy in capitate hydroids and medusae (Cnidaria: Hydrozoa). Zoological Journal of the Linnean Society 100: 101-231. 

Schuchert, P. 1993. Phylogenetic analysis of the Cnidaria. Zeitschrift für Zoologische Systematik und Evolutionsforschung 31: 161-173. 

Schuchert, P. 1996. The marine fauna of New Zealand: athecate hydroids and their medusae (Cnidaria: Hydrozoa). New Zealand Oceanographic Institute Memoir 106: 1-159. 

Schuchert, P. 2001b. Survey of the family Corynidae (Cnidaria, Hydrozoa). Revue Suisse de Zoologie 108: 739-878. 

Siddall, M. E., Martin, D.S., Bridge, D., Desser, S. S., & Cone, D. K. 1995. The demise of a phylum of protists: phylogeny of Myxozoa and other parasitic Cnidaria.Journal of Parasitology 81:961-967.

Werner, B. 1984. 4. Stamm Cnidaria, Nesseltiere. pp. 10-305. In H.-E. Gruner (ed). Wirbellose Tiere Gustav Fischer, Stuttgart.