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.
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.