Sargassum fallax Sond.

Scientific Description

Habit and structure. Thallus medium to dark brown, 20–100 cm long, with stipes 1–4(–7) cm long, terete, simple, 2–4 mm in diameter, relatively smooth, bearing apically 1–4 primary branches. Holdfast discoid-conical, 5–20 mm across, with a single stipe; epilithic. Primary branches 10–50(–90) cm long, with robust, triquetrous, axes bearing tristichously arranged laterals below, and much branched laterals above. Laterals (lower) basally retroflex, simple, 4–10(–25) cm long, 0.5–1.5(–2) cm broad, costate, more or less flat and smooth with entire margins, rarely with age becoming slightly to moderately spinous; intermediate laterals 2–10 mm broad, margins entire to incised; upper laterals grading, often abruptly (especially on older branches) from lower laterals, 1–3 cm long and much branched above with compressed to terete, filiform, branches 0.3–1 mm broad, usually lost as the branch ages. Vesicles usually abundant on upper laterals, petiolate, subspherical, 5–10 mm in diameter, mucronate or with a terminal leaflet (to 1.5 cm long).

Reproduction. Thalli monoecious. Receptacles bisexual or unisexual, in dense racemose clusters on upper, subterete, laterals, each 1–2.5(–4) mm long and 300–900 µmbroad, petiolate, simple or occasionally with a branch and often bearing 1(–2) sterile, lateral awns, terete and short-lanceolate or occasionally slightly compressed above, smooth when young, slightly verrucose when older, usually without spines but eastern plants often with one or two spines on the upper part, with scattered ostioles. Conceptacles usually unisexual, occasionally bisexual; oogonia sessile, ovoid, 150–250 µmlong and 110–140 µm in diameter, few per conceptacle; antheridia sessile or on short branched paraphyses, ovoid, 14–20 µmlong and 10–14 µm in diameter.

Distribution.From the Houtman Abrolhos Is., W. Aust., around southern Australia and Tas., to Ballina, N.S.W.

Habitat. S. fallax is a common species on southern Australian coasts, in rock pools or the uppermost sublittoral but extending to 48 m deep.

[After Womersley, Mar. Benthic Fl. Southern Australia II: 432–434 (1987)]