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NOBANIS - European Network on Invasive Species
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).
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.