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Class HYDROZOA Owen, 1843

Hydroids, Hydromedusae, Hydrozoans, Milleporine Corals, Solitary Hydroids

Compiler and date details

June 2017 - Dr Narissa Bax

Updated March 2015 - Dr Lisa-Ann Gershwin, Hobart


The class Hydrozoa is a subgroup of the phylum Cnidaria. The taxa are predominantly marine (some are freshwater), formed from asexual polyps and free swimming sexual medusae (epidermal gonads). Hydrozoans are characterised by their complex life cycle, in which three main stages can be identified:
1) the growth of medusae from buds,
2) the presence of a velum inside the bell of the medusa,
3) the production of gametes from ectodermal tissue.

Colonies are mostly benthic filter-feeders, but some, are pelagic floaters (e.g., siphonophores).

Common hydrozoans include the Portuguese man-o-war (Physalia physalis), freshwater genus Hydra and fire coral (Milleporidae).

Hydrozoans are best represented in cool temperate southern Australian seas, where they are known from the intertidal zone to the deep-sea (Watson, 1982), also described from the Bass Strait (Watson, 1994) and Western Australia (Watson, 1996). About 2,700 hydrozoan species have been described worldwide (Watson, 1996). Approximately 600 species have been described from the waters around the Australian mainland (Watson, 1996). Although, based on literature searches to date (as of June, 2017), knowledge on the taxonomy, biology and ecology of Australian hydroids remains limited, with many collections, especially those from deeper waters and the Australian Antarctic territory undescribed, or only described to higher taxonomic levels (e.g., family) due to a lack of available taxonomic expertise in the Southern Hemisphere (Bax, pers.obs).


History of changes

Note that this list may be incomplete for dates prior to September 2013.
Published As part of group Action Date Action Type Compiler(s)
03-Sep-2018 HYDROZOA Owen, 1843 17-Jul-2018 MODIFIED Dr Narissa Bax
05-Aug-2022 13-Aug-2013 MODIFIED
12-Feb-2010 (import)