Rhabdonia coccinea (Harv.) Hook.f. & Harv.

Scientific Description

Habit and structure. Thallus medium to dark red, erect, 5–25 cm high, much branched irregularly with terete, linear, gently tapering branches 200–400 µm in diameter above, 0.5–1.5 mm in central parts, and in dendroid specimens 2–3 mm thick in lower axes. Holdfast discoid, 2–8 mm across with one to several axes and becoming stoloniferous from their bases; epilithic. Structure uniaxial, with periaxial initials single per axial cell and on three radii, developing rhizoids and forming a laxly to moderately filamentous medulla in which the original axial filament becomes inconspicuous; occasional filaments become thickened and filled with yellow refractive material; cortex at first 2–5 cells thick (later 5–8 cells thick), inner cells ovoid, multinucleate, 20–80 µm in diameter, outer cells isodiametric, (6–)10–15 µm in diameter; bases of older axes thickened from a cortical meristem, becoming perennial with new apical growth. Rhodoplasts discoid to elongate, many per cell.

Reproduction. Sexual thalli monoecious; non-procarpic. Carpogonial branches 3(–4)-celled, borne on inner cortical cells, directed inwards with a reflexed trichogyne, rarely with a sterile cell on the basal cell. Connecting filament single, nonseptate, unbranched, uniting with the auxiliary (inner cortical) cell in a darkly staining complex (probably so before connection). Gonimoblast initial first inwards, later with a prominent fusion cell (connected to an enlarged medullary filament) with radial gonimoblasts producing short chains (the terminal one mature) of ovoid carposporangia 50–90 µm in diameter. Cystocarps swelling one side of lesser branches, without filamentous enveloping tissue but with a pericarp of thickened cortex, ostiolate. Spermatangia scattered near branch apices, with surface cortical cells cutting off 3–6 initials, each of which produces 2–3 ovoid spermatangia, 2–3 µm in diameter. Tetrasporangia scattered in the cortex of younger branches, cut off laterally, ovoid, 80–100 µm long and 60–80 µm in diameter, zonately divided.

Distribution.Champion Bay, W. Aust., to the Snowy River mouth, Vic., and around Tas.

Habitat. R. coccinea is a deeper water species on rough-water coasts or with strong tidal currents.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 345–347 (1994)]