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 tall, comprising clusters of polysiphonic stems arising from a root-like hydrorhiza. Stems branched and rebranched many times giving a bushy appearance to the colony. Hydrocladia moderately long, held out stiffly from front of branch. Hydrothecae close-set along the hydrocladia, cup-shaped, standing almost upright, a small intrathecal ridge projecting backwards into hydrotheca from base and connecting with one in hydrocladial internode. Margin of hydrotheca with four pairs of sharp cusps. Median nematotheca upright, projecting above hydrotheca, sometimes with a bifid terminal orifice, twin laterals tubular, upright, each with a single orifice pointing forward; a row of large cyst-shaped nematothecae along axis of stems and branches. Gonothecae protected by a long open corbula of 15-20 pairs of arching pinnae armed with nematothecae. Colour: honey brown, corbula brown, gonothecae cream. Up to 15 cm high.