The hydroid Hydractinia: a versatile, informative cnidarian representative

The Cnidaria represent the most ancient eumetazoan phylum. Members of this group possess typical animal cells and tissues such as sensory cells, nerve cells, muscle cells and epithelia. Due to their unique phylogenetic positi on, cnidarians have traditionally been used as a reference group in various comparative studies. We propose the colonial marine hydroid, Hydractinia, as a convenient, versatile platform for basic and applied research in devel opmental biology, reproduction, immunology, environmental studies and more. In addition to being a typical cnidarian representative, Hydractinia offer s many practical and theoretical advantages: studies that are feasible in H ydra like regeneration, pattern regulation, and cell renewal from stem cell s, can be supplemented by genetic analyses and classical embryology in Hydr actinia. Metamorphosis of the planula larva of Hydractinia can be used as a model for cell activation and communication and the presence of a genetica lly controlled allorecognition system makes it a suitable model for compara tive immunology. Most importantly, Hydractinia may be manipulated at most a spects of its (short) life cycle. It has already been the subject of many s tudies in various disciplines, some of which are discussed in this essay. ( C) 2001 John Wiley & Sons, Inc.