PORT PHILLIP BAY


Hydroid 

Aglaophenia divaricata (Busk, 1852)

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.
Definition as for the family, gonothecae in rows along stem and branches, enclosed in a corbula armed with nematothecae.

Species identification.
Colonies usually large, comprising clusters of stems arising from a mass of hydrorhizal stolons. Lower stems polysiphonic, bare, upper stems profusely branched forming a feathery canopy. Hydrocladia alternate, moderately long, borne on front of stem. Hydrothecae cup-shaped, almost upright, one on each hydrocladial internode, a small intrathecal ridge projecting backwards into hydrotheca from base, connecting with one in hydrocladial internode. Margin of hydrotheca with three pairs of deep cusps and a sharp median anterior cusp. Median nematotheca upright, projecting above hydrotheca, often increasing in length along hydrocladium, twin lateral nematothecae tubular, upright, with a single orifice pointing forward at level of hydrothecal margin. Gonothecae protected by an open corbula of 15-20 pairs of alternate, arching pinnae armed with nematothecae. Colour: charcoal grey to almost black, corbula black, gonothecae cream to black. Up to 20 cm high.

Taxonomy

Phylum:
Cnidaria
Class:
Hydrozoa
Subclass:
Leptothecatae
Order:
Conica
Family:
Plumulariidae
Genus:
Aglaophenia
Species:
divaricata

General Description

Colony of individual polyps (hydranths) joined by root-like network of tubular stolons at the base. Colony shape is tree-like or bushy (arborescent). Colour: charcoal grey to almost black, corbula black, gonothecae cream to black. Up to 20 cm high.

Biology

This is one of the most spectacular and graceful hydroids in southern Australia. Their colonies grow most of year, and they are fertile in autumn and winter.

Habitat

Deep ocean reefs.

Reefs

Distribution guide

Southern Australia.

Species Group

Hydroids

Depth

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

Water Column

On or near sea floor

Max Size

20 cm

Diet

Plankton or Particles

Harmful

Generally not harmful but still able to sting bare skin.

Commercial Species

No

Species Code

MoV 3463

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, Aglaophenia divaricata, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 23 Oct 2017, http://portphillipmarinelife.net.au:8098/species/7091

Text: creative commons cc by licence