Habit and structure. Thallus of numerous clumped axes attached by rhizoids, pale green or whitish from calcification, 2–6(–10) cm high, with few to numerous whorls of colourless (pale green when young) di- to quadri-chotomous hairs with 5–7 orders of branching, with perforations in the end walls, and a summit whorl of 11–18 green gametangial rays, free from each other and appearing clustered due to crowding. Gametangial rays clavate, 2–5 mm long and 0.5–1.0(–1.3) mm broad; superior corona of a single lobe bearing 1–3 hairs or scars (usually only one mature hair), inferior corona only a slight bulge; chloroplasts rounded, ovoid or lenticular, variable in size, 2–6 µm long, without pyrenoids.
Reproduction. With spherical cysts, (150–)200–350 µm in diameter, producing gametes. Cyst-like structures occasionally present within the stalk.
Distribution. From Shark Bay, W. Aust., around southern Australia (and Blackmans Bay, Tas.) to Newcastle, N.S.W.
Habitat. A. peniculus occurs in sheltered bays and inlets, uppermost sublittoral, on dead shells or limestone.
[After Womersley, Mar. Benthic Fl. Southern Australia I: 296–297 (1984), as Polyphysa peniculus]