Habit and structure. Thallus medium red, 4–21 cm high, with an axis 2–5 mm broad producing laterals 2–8 mm in diameter bearing slender tapering branches for 3 or 4 orders, soft and mucilaginous. Holdfast small, discoid; epilithic. Structure uniaxial with each axial cell bearing 4 periaxial cells (and sometimes adventitious filaments) which produce lateral filaments with several orders of cells, the outer forming a lax cortex and with many inner medullary cells producing descending rhizoids.
Reproduction. Sexual thalli dioecious or monoecious. Carpogonial branches developing from periaxial cells or from second or third order cells of the lateral filaments, replacing one of the vegetative filaments on their bearing cell, 9–10 cells long with the carpogonium sharply reflexed towards cells 4 and 5 and a long and usually basally coiled trichogyne. One to four cells nearest the base of the carpogonial branch produce no laterals but the next 5–6 cells form distichous lateral branches 2–4 cells long, which may further cut off 1–2-celled side branches towards the recurved carpogonium; the cell below the carpogonium does not cut off lateral cells. Post-fertilization fusion occurs between the carpogonium and terminal cells of these lateral branches, usually those from cells 4 and 5 of the carpogonial branch, while the cell below the carpogonium remains densely cytoplasmic. Following this fusion, several connecting filaments with basal pit-connections develop. Auxiliary cell branches develop in similar positions to the carpogonial branches, and are 5–9 cells long without lateral cells and with a larger, subspherical, terminal auxiliary cell, with which a connecting filament fuses and initiates gonimoblast cells; each connecting filament usually continues to further auxiliary cells. Carposporophytes globular, scattered in the outer medulla, 80–100 µm in diameter, with nearly all cells becoming sub-spherical carposporangia 12–20 µm in diameter. Male gametophytes (Hawaiian plants) smaller than female gametophytes, with small clusters of spermatangia on outer cortical cells. Tetrasporophytes unknown.
Distribution. Oahu, Hawaiian I. In Australia, known from isolated records from Rottnest I., W. Aust., Coffs Harbour, N.S.W., Qld., and Lord Howe I.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 220–222 (1994)]