Sargassum tristichum Sond.

Scientific Description

Habit and structure. Thallus medium to dark brown, 10–60 cm long, with a short, simple or branched, stipe 1–2 cm long, terete, 2–4 mm in diameter, with few, stubby, branch residues, bearing apically 1–4 primary branches. Holdfast discoid-conical, 5–12 mm across, usually with a single stipe; epilithic. Primary branches 10–60 cm long, axes moderately robust, triquetrous, bearing tristichously arranged but irregularly twisted, slightly retroflex, simple, laterals, becoming denuded below. Laterals (lower) 3–7(–10) cm long, 0.5–l.5 cm broad, smooth, margins entire; upper laterals distinctly smaller, 1–3 cm long, 1–5(–8) mm broad, entire or with odd spines, often lost from older fertile fronds. Vesicles absent or usually only a few per plant, occasionally frequent, petiolate, subspherical, 3–6 mm in diameter, with a small (occasionally compressed) mucro.

Reproduction. Thalli monoecious. Receptacles bisexual, in dense clusters on upper laterals, petiolate, simple, 2–5(–8) mm long, 1–1.5(–2) mm broad, strongly triquetrous with prominent spines along the ridges, with scattered ostioles on the receptacle surfaces. Conceptacles unisexual or bisexual; oogonia sessile, ovoid, 140–250(–320) µm long and 90–150(–200) µm in diameter; antheridia sessile or on branched paraphyses, ovoid, 12–16 µm long and 7–10 µm in diameter.

Distribution.From the Dampier Archipelago, W. Aust., to Port Noarlunga, S. Aust.

Habitat. S. tristichum is often common in rock pools and the upper sublittoral, usually extending to only a few metres deep.

[After Womersley, Mar. Benthic Fl. Southern Australia II: 436–438 (1987)]