Heterocladia australis Decne.

Scientific Description

Habit and structure. Thallus erect, medium to dark red-brown, (10–)20–50 cm high, with a prominent, terete to slightly compressed main axis 1–3(–5) mm broad, bearing irregularly but usually densely lateral branches 2–15 cm long with compressed lower parts 0.5–3 cm long and 2–6(–14) mm broad, surmounted by terete, irregularly branched, upper parts 4–14 cm long, often profusely covered with trichoblasts. Holdfast discoid, 3–6 mm across, often fibrous; epilithic or on calcareous algae. Structure. Growth apical, monopodial, apical cell dome-shaped 30–40 µm in diameter (with thick walls), axial cells with 4 pericentral cells, rapidly and extensively corticated with rhizoidal filaments from the pericentral and inner cells, pericentral cells soon obscured, inner cortical cells large, outermost small. Trichoblasts from each segment below apices, 1–3 mm long, branched from each lower cell with the walls adherent at each dichotomy, lower cells 30–55 µm in diameter and L/D 2–3, rhodoplastic. Cells multinucleate; rhodoplasts discoid, chained in larger cells.

Reproduction. Gametophytes dioecious. Procarps usually on the second cell of trichoblasts, the supporting cell bearing a 4-celled carpogonial branch and a sterile 3-celled group. Carposporophyte with a basal fusion cell and a short, branched, gonimoblast with clavate terminal carposporangia 80–150 µm in diameter. Cystocarps stalked, ovoid, 1–2.5 mm in diameter; pericarp ostiolate, corticated, 4–5 cells thick. Spermatangial organs on branches of trichoblasts, elongate-ovoid, 50–70 µm in diameter and 150–200 µm long, with 4 pericentral cells and a 3–4-celled sterile apex. Tetrasporangia borne in lesser, simple or compound, corticated, branches, bearing abundant trichoblasts, spirally arranged and single per tier, 150–70 µm in diameter.

Distribution. Geraldton, W. Aust., to Gulf St Vincent, S. Aust.

Habitat. H. australis is a rare, deep-water alga.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 282–284 (2003)]