Halymenia plana Zanardini

Scientific Description

Habit and structure. Thallus medium red, foliose, 10–30 cm high, becoming irregularly divided into broad lobes (2–)4–6(–10) cm across, 250–400(–600) µm thick, with rounded axils and apices, surface smooth and mottled with darker red, irregularly shaped, areas 1–5 mm across, often aggregated into patches 4–8 mm across, margin smooth to slightly irregular, stipe 2–10 mm long, narrow-cuneate, with one to several fronds. Holdfast discoid, 2–6 mm across; epilithic. Structure of a cortex 3–6 cells broad with a single (–2) outer layer of isodiametric cells 3–4(–7) µm across and L/D 1–2 in thallus section, inner cells closely packed and pseudoparenchymatous, 15–22 µm across, mostly not becoming stellate; medulla lax to moderately dense with mostly irregular filaments (many transverse in young parts), becoming dense in older parts with no or few rhizoids, and with prominent refractive ganglionic cells. Rhodoplasts discoid to elongate and lobed, becoming ribbon shaped and branched in inner cells.

Reproduction. Sexual thalli probably dioecious. Carpogonial branch ampullae with 3–5 secondary (and occasional tertiary) filaments, converging above and extending almost to thallus surface, and a 2-celled carpogonial branch. Auxiliary cell ampullae with several long secondary and tertiary filaments, converging above in the outer cortex, with a prominent, large, auxiliary cell. Carposporophyte lying within the medulla, 80–160 µm across with dense, ovoid, carposporangia 8–12 µm across. Involucre cells cut off short lateral chains around the small ostiole. Spermatangia not observed. Tetrasporangia scattered, on mid cortical cells, 15–25 µm long and 10–12 µm in diameter, cruciately divided.

Distribution. 16 km E of Eucla, S. Aust., to Walkerville, Vic., and Tas.

Habitat. H. plana occurs from 2–3 m depth on open coasts, under heavily shaded ledges at shallow depths.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 195 (1994)]