This very delicate species is easily distinguished from its more common southern Australian congener, Obelia dichotoma (Linnaeus, 1758), by its smaller size and almost invariably transverse diaphragm. The coenosarc of the hydrorhiza and stems and the tentacles of the specimens are packed with zooxanthellae.
Hydrorhiza tubular, reptant on substrate. Stems to 6 mm high, monosiphonic, lower stem smooth, unbranched, a series of annulations above base, upper stem region slightly sympodial, simple, dichotomously or trichotomously branched, internodes moderately long, tubular, straight, widening a little distally, nodes oblique, faint to absent, proximal part of internode deeply annulated.
Hydrothecal pedicels usually alternate, given off at an acute angle distal on internode, pedicels variable in length, annulated throughout or with a central smooth section. Hydrotheca gracefully campanulate, diaphragm transverse to hydrothecal axis, margin faintly everted.
Hydranth robust, number of tentacles could not be counted.
Perisarc smooth throughout, fairly thick on internodes, pedicels, and around upper body of hydrotheca.
Colour: lower stems shining brown, upper stems fading to white.
South America and Australia (New South Wales, Victoria, Queensland).
Subtidal, epizoic on barnacles, sponges, ascidians and other invertebrate substrates.