Introduction to introduced hydrozoa (benthic polyp stage)
The Hydrozoa is a class of Cnidaria. They are characterized by having a life-cycle involving sexual and asexual reproduction. Most hydrozoans have a benthic, colonial polyp stage, which reproduces asexually by budding. Many have free swimming, sexually reproducing medusae (see Introduction to Ctenophores (and Cnidarian medusae)). Others have attached gonophores, which will produce eggs or sperm. The systematics of this group is highly unstable and phylogenetic relationships are still under study. This has caused many name changes and synonymizations, and probably this will continue for many years to come. Hence lists of synonyms and references to primary taxonomic literature are important even in management related publications and databases. See here for recent information on systematics (click on "browse" to get to classification).
Drawing of thecate and athecate hydroid to show terminology
In Nordic waters only one species, Cordylophora caspia is an established introduced species. However, a number of species have been found in neighbouring waters in the southern and/or western North Sea, and some have been recorded a few times in Nordic waters.
Rasmussen, E. 1973. Systematics and ecology of the Isefjord marine fauna (Denmark). Ophelia 11: 1-507.
Schuchert, P. 2004. Revision of the European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Families Oceanidae and Pachycordylidae. Revue Suisse de Zoologie 111(2): 315-369.
Schuchert, P. 2007. The European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Filifera Part 2. Revue Suisse de Zoologie 114(2): 195-396.
Verwoort, W. 1964 Note on the distribution of Garveia fransiscana (Torrey, 1902) and Cordylophora caspia (Pallas, 1771) in the Netherlands. Zoologische Mededelingen 39: 125-146.
Wolff, W.J. 2005. Non-indigenous marine and estuarine species in The Netherlands. Zoologische Mededelingen 79(1): 1-116.