PORT PHILLIP BAY


Hydroid 

Obelia dichotoma (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.
Stems branched, growing in thick lacy masses from a tubular hydrorhiza. Hydrothecae small, borne on short pedicels on stems and branches. Pedicels smooth, annulated at base and below hydrotheca. Hydrotheca conical, apertural margin circular, smooth. Gonotheca vase-shaped with a raised terminal collar containing many developing medusae, the oldest at top ready to escape. Medusa very small, saucer-shaped with a fringe of short tentacles, swimming jerkily inside-out. Colour: colonies dirty white, stems in older colonies brownish; medusae transparent white, stomach often golden brown. Often more than 6 cm long.

Taxonomy

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

General Description

Colony of individual polyps (hydranths) joined by root-like network of tubular stolons at the base. Colony shape is lacy. Colour: colonies dirty white, stems in older colonies brownish; medusae transparent white, stomach often golden brown. Often more than 6 cm long.

Biology

Colonies of this common southern Australian species grow and mature in winter.

Habitat

On hard surfaces and invertebrates in shallow water, often on jetty pylons.

Reefs

Distribution guide

Temperate waters worldwide. In southern Australia.

Species Group

Hydroids

Depth

Shallow (1-30 m)

Water Column

On or near sea floor

Max Size

6 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 dichotoma, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 18 Dec 2017, http://portphillipmarinelife.net.au:8098/species/7116

Text: creative commons cc by licence