Class HYDROZOA Owen, 1843

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Introduction

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).

General References

Watson, J.E. 1982. Hydroids (Class Hydrozoa). pp. 77-115 in Shepherd, S.A. & Thomas, I.M. (eds). Marine Invertebrates of Southern Australia. Handbook of the Flora and Fauna of South Australia Adelaide : Government Printer Part 1 491 pp.

Watson, J.E. 1994. Shallow water hydroids from eastern Bass Strait. Victorian Naturalist 111: 65-69

Watson, J.E. 1996. Distribution and biogeographic relationships of the hydroid fauna of the Australian west coast: A preliminary account. Scientia Marina 60(1): 75-83

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