Gymnangium ilicistomum (Bale, 1882)

View scientific description and taxonomy

Scientific Details

Family level detail.
Erect branched or unbranched plumose stems. Stems and hydrocladia divided into segments (internodes). Hydrocladia alternate on stem internodes. Hydrotheca saccate, attached at base to hydrocladial internode, margin usually cuspate. Three nematothecae attached to each hydrotheca - one tubular median with a terminal orifice, fused to upper (anterior) side of hydrotheca and smaller twin laterals, one at each side of base of hydrotheca. Nematothecae similar to laterals on internodes of main stem. Gonophores fixed sporosacs containing eggs and sperm, protected by a gonotheca; accessory protective structure (corbula) in some families.

The most spectacular and graceful hydroids in southern Australia belong to the Aglaopheniidae. Preliminary field identification of many aglaophenian species can be made from observing colony size, structure and colour, however, precise identification usually requires microscopic examination. In the genus Aglaophenia the corbula is important in identification, including its size, the number and shape of leaflets (pinnae), whether the pinnae are free from one another (open corbula) or connected together by tissue (closed corbula), and the shape of the nematothecae on the pinnae. When fertile, species of the Aglaopheniidae can be identified from the presence, absence or development of the corbula.

Provisional identification is possible by plucking several hydrocladia from a specimen stem and laying them in a drop of water on a glass microscope slide, compressing them gently under a coverslip and examining them microscopically. The important structures of the hydrocladium, hydrotheca and nematotheca can usually be seen under low power magnification. Often however, the dense and strongly coloured internal tissue (coenosarc) obscures diagnostic structures. The coenosarc can be dissolved in domestic bleach (calcium hypocholorite solution) diluted in tap water. The specimen is soaked for a few minutes until the darker tissue begins to dissolve, then carefully transfer the specimen to fresh water for a few minutes to remove the bleach, leaving cleared perisarc behind. Specimens can then be mounted in a drop of water or glycerol under a coverslip on a microscope slide and examined under magnification. Examination is best done using a compound light microscope.

Genus level detail.
Plumose hydroids with the same basic morphology as the family Aglaopheniidae, gonothecae borne in a row along main stem; no accessory protective structures.

Species identification.
Stems arising from a creeping hydrorhiza. Colonies usually consisting of up to 10 lax, plumose, unbranched monosiphonic stems. Hydrocladia set densely along stem. Hydrothecae close-set along hydocladia, cup-shaped, facing obliquely upward, margin with a large incurved anterior cusp and six pairs of smaller cusps, some incurved, others everted. Median nematotheca long, curved over aperture of hydrotheca; lateral nematothecae small, saccate with a small forward-facing orifice. Gonothecae small triangular capsules crowded in a double row along stem. Colour: pale greyish. Up to 5 cm high.



General Description

Colony of individual polyps (hydranths) joined by root-like network of tubular stolons at the base. Colony shape is feather-like (pinnate). Colour: pale greyish. Up to 5 cm high.


This is one of the most spectacular and graceful hydroids in southern Australia. They are rarely seen because they often attach to the underside of ledges. Their colonies grow late summer to winter, and become fertile in winter.


Deep water reef, often under ledges.


Distribution guide

Southern Australia.

Species Group



Shallow (1-30 m)
Deep ( > 30 m)

Water Column

On or near sea floor

Max Size

5 cm


Plankton or Particles


Generally not harmful but still able to sting bare skin.

Commercial Species



Conservation Status

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


article author Watson, J.

Jan Watson is a consultant with expertise in hydroid taxonomy.


Cite this page as:
Watson, J., 2011, Hydroid, Gymnangium ilicistomum, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 15 Dec 2018, http://portphillipmarinelife.net.au:8098/species/7104

Text: creative commons cc by licence