PORT PHILLIP BAY


Hydroid 

Obelia geniculata (Linnaeus, 1758)

View scientific description and taxonomy

Scientific Details

Family level detail.
Hydroids with a simple bell-shaped (campanulate) hydrotheca without operculum. Apertural rim of hydrotheca may be smooth, toothed, crenulated or cuspate. Gonophores protected by a gonotheca. Hydroids of the family Campanulariidae are common throughout the world. Many species form large, spreading colonies comprising many simple stems arising from a creeping network of stolons. Other species from large, multi-branched colonies, usually lacy in appearance. Medusae range in structure from primitive eumedusoids that swim only feebly to fully formed free-swimming medusae.

Genus level detail.
Colony erect, flexuose, usually branched, no subhydrothecal spherule on pedicel, hydrotheca campanulate, apertural rim cusped or smooth, gonotheca conical, gonophore a free medusa. Obelia is often presented in text-books to illustrate the typical hydroid life cycle. There are many species in the genus, often world-wide in distribution. Species release free swimming medusa into the plankton, so over geological time they have colonised habitats around the globe. Obelia is highly opportunistic and some probably have extended their ranges around the world by hitching rides on ships' hulls.

Species identification.
Colonies consisting of many stems attached to a hydrorhiza creeping on algal fronds. Stems unbranched, internodes zigzag, each bearing a short pedicel. One hydrotheca at end of pedicel, conical, margin circular. Gonothecae borne on stem below hydrotheca, urn-shaped with small tubular terminal neck containing many developing medusae. Medusa at release small, saucer-shaped with a fringe of short tentacles; swimming jerkily inside out. Colour: colourless to yellowish white, medusa colourless. At night the colonies bioluminesce blue. Up to 10 mm high.

Taxonomy

Phylum:
Cnidaria
Class:
Hydrozoa
Subclass:
Leptothecatae
Order:
Proboscoida
Family:
Campanulariidae
Genus:
Obelia
Species:
geniculata

General Description

Colony of individual polyps (hydranths) joined by root-like network of tubular stolons at the base. Colony shape is single stems. Colour: colourless to yellowish white, medusa colourless. At night the colonies bioluminesce blue. Up to 10 mm high.

Biology

Colonies of this common southern Australian species grow and become fertile in summer.

Habitat

On fronds of the brown kelps.

Reefs

Distribution guide

Worldwide. Southern Australia.

Species Group

Hydroids

Depth

Shallow (1-30 m)

Water Column

On or near sea floor

Max Size

10 cm

Diet

Plankton or Particles

Harmful

Generally not harmful but still able to sting bare skin.

Commercial Species

No

Global Dispersal

Recorded in Australia

Identify

Conservation Status

  • DSE Advisory List : Not listed
  • EPBC Act 1999 : Not listed
  • IUCN Red List : Not listed

Author

article author Watson, J.

Jan Watson is a consultant with expertise in hydroid taxonomy.

citation

Cite this page as:
Watson, J., 2011, Hydroid, Obelia geniculata, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 25 Feb 2025, http://136.154.202.208:8098/species/7115

Text: creative commons cc by licence