The Class Hydrozoa is the most diverse group of cnidarians. Although marine species vastly outnumber freshwater ones, the genus Hydra may nevertheless be the best known hydrozoan. It appears in almost all textbooks on invertebrate zoology, but this simple animal is not at all representative of the Hydrozoa as a whole, and may actually give a false impression of the complexity of this animal group. For an in depth introduction to the morphology and classification of the Hydrozoa see Bouillon et al. (2006).
The World Hydrozoa Database includes all extant hydrozoan species and is intended to act as a tool for anyone needing taxonomic information, both specialists and non-specialists alike. The data are based on a species inventory initially compiled by Wim Vervoort and Peter Schuchert (see Schuchert, 1989) and later completed with the help of the Zoological Record. The list of the Siphonophorae was provided by Gill Mapstone who also manages the species of this Order. Likewise, the Stylasteridae are treated by Stephen Cairns. New hydrozoan species will be added continuously by the editors.
The number of known hydrozoan species is about 3,800, although the status of many nominal species is currently unclear. A considerable number of names also represent synonyms. The list of currently accepted names is at least 95% complete, but the taxonomy of many species still needs revision. There is currently no broad consensus available for the higher classification of the Hydrozoa and several systems and alternative names are in use. Here, the classification given in Cartwright & Collins (2007) is used, although some minor spelling changes were made. Hence, the classification used in this database is somewhat provisional. The results of ongoing molecular phylogenetic investigations will certainly help to stabilise the taxonomic system in the near future.
No guarantee for the correctness of the data can be given. Please inform the editor responsible for the group of any omissions, errors or typos you come across, or if you need to confirm some crucial data. If you disagree with a synonym, genus assignment or other taxonomic decision, please, send in your reasoned argument to the editors. We will evaluate each contribution and corrections will be incorporated quickly.