Sargassum polycystum C.Agardh

Scientific Description

Habit and structure. Thallus medium brown, to 39 cm tall, with a discoid-conical, lobed or creeping holdfast to 30 mm wide, giving rise to 1–several terete primary axes (with radial scars) to 50 mm long, 1–3 mm diam. Primary branches terete to subterete, smooth to densely spinous, 0.6–2.0 mm diam.; spines simple or divided, with acute, obtuse or centrally depressed apices, 0.25–1.00 mm long. Primary laterals linear, ovate, lanceolate or botuliform, 2–85 mm long, 0.3–10 mm wide; margins smooth to serrulate; apices acute or obtuse. Secondary branches borne radially 1–5 mm apart, terete, 0.4–1.5 mm diam., smooth or with blunt or centrally depressed radial spines/protuberances that are 0.2–0.6 (–1.5) mm long. Secondary laterals linear to linear-lanceolate, 4–40 mm long, 0.5–4.5 mm wide; margins entire, serrulate or serrate; apices acute or obtuse; midrib ending just below the apex; cryptostomata often prominent, raised. Vesicles often clustered, with a short stalk to 1 mm long, spherical to slightly elongate toward the base, 0.6–2.0 (–3.0) mm diam. Inflorescences cymose or racemose clusters of up to 3 receptacles, always associated with vesicles and occasionally leaves. Receptacles usually dioecious, terete, smooth to verrucose, simple, 1–5 mm long, 0.4–1.0 mm diam., giving rise to other receptacles or sterile awns; apices rounded, flat and uneven, or acute. Conceptacles usually unisexual; oogonia sessile, oval, 140–230 µm long, 80–135 µm diam.; antheridia on short or long stalks, oval, 8.0–13.5 µm long, 6–9 µm wide.

Distribution. Known from N.W.A. and Qld; also in the Canary Islands, Africa, and in the tropical Indo-Pacific Ocean as far east as Tonga.

Habitat. Epilithic in the subtidal.

[After Dixon & Huisman, Algae of Australia: Mar. Benthic Algae of North-western Australia, 1. Green and Brown Algae 264 (2015)]