Habit and structure. Thallus dark red to chalky grey-red, 5–14 cm high, subdichotomously branched and clearly segmented, segments ovoid to elongate, rounded at both ends, 7–19 mm long, 3–5 mm broad. Holdfast discoid, 2–10 mm across; epilithic. Structure of gametophyte cortex three layered, the inner two layers of large, colourless cells that may fuse laterally, the outer layer of pigmented cells 20–30 µm in diameter. Cortex in tetrasporophyte of three layers: an inner layer of inflated, laterally adherent, colourless cells; a middle layer of smaller, rhodoplastic cells which supports the outer layer of densely rhodoplastic, hemispherical cells, usually 2 per supporting cell. Rhodoplasts campanulate with a central pyrenoid.
Reproduction. Sexual thalli dioecious. Carpogonial branches 3-celled, borne near apices in place of a cortical filament, with the hypogynous cell producing four large-celled branches and the basal cell 3–4 smaller-celled branches. Cystocarps 300–600 µm in diameter with gonimoblast filaments forming the cystocarp wall and producing inwardly directed obovoid carposporangia 35–45 µm long and 18–20 µm in diameter. Male conceptacles 270–400 µm in diameter with spermatangia (8–9 µm long and 5–6 µm in diameter) produced terminally and laterally from filaments projecting into the cavity. The tetrasporophyte with subspherical to ovoid, cruciately divided tetrasporangia, 30–35 µm long and 25–35 µm in diameter, borne in surface sori on the outer cortical layer. After spore release the initial elongates slightly and produces further tetrasporangia within the old sporangial wall, which remains as a collar around the initial.
Distribution. Tropics in general, occasionally in temperate areas. In Australia from Albany, W. Aust., around northern Australia to Lake Macquarie, N.S.W.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 112–113 (1994) added as Galaxaura obtusata]