The two species of sea nymph, Amphibolis antarctica (Labill.) Sonder & Aschers, ex Aschers. and A. grifithii (J. M. Black) Den Hartog, are endemic to Australia. They have distinctive leaf morphology, and stem and rhizome anatomy. Environmental and ecological factors that limit the distribution of both species in southern Australia are considered. Both species support a wide range of algal and other epiphytes, and a list of the common epiphytic biota is presented. Penetration of the host tissue by epiphytes is seldom observed.