Filaments single, growing amongst other algae, or in fine, unstructured mats on the benthos. Trichomes straight, not or slightly attenuated towards the ends, constricted at the cross walls. Vegetative cells cylindrical to barrel-shaped, 0.6–1.3 × longer than wide, 3.5–8.0 μm long × 4.3–5.5 (–6.5) μm wide, contents homogeneous, blue-green in colour; apical cells conical. Heterocytes intercalary, cylindrical 9.3–14.1 μm long × 5.9–7.6 μm wide. Akinetes cylindrical with bluntly rounded ends, adjacent to, or remote from heterocytes, solitary or up to five in series, 2–3 × longer than wide, 10–20 μm long × 5.0–8.5 μm wide, with smooth exospore.

Specimens examined:—Palmer R. at NESP Red.

Observations:—The material from north Queensland, as compared to the original description of A. willei from Puerto Rico, was slightly wider and had akinetes which lacked coloured exospores. The observed material formed fine, flat, mucilaginous mats on sandy and silty substrates in the littoral areas and backwaters of a tropical river.