Bushy, deep crimson fronds, to 200 mm long, from basal disc. Axis prominent, cylindrical, diminishing in diam. from base to apex. Branching repeatedly distichously pinnate, branches spreading , alternate. Ultimate ramuli bifid or multifid, monosiphonous, sharply pointed. Main axis of one central siphon, 9 pericentral siphons and outer layer of corticating cells in which rings of elongated cells alternate regularly with rings of short cells.

Original publication: Batters, E.A.L. (1902). A catalogue of the British marine algae. Journal of Botany, British and Foreign 40(Supplement): 1-107.

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 Heterosiphonia plumosa is a red to deep crimson seaweed which appears black when dried. This species has a flattened, fern-like appearance with a hairy thallus growing from a discoid holdfast. The fronds are flat or slightly cylindrical up to 20 cm in length and 0.5 cm in diameter at the base, tapering towards the apex. The primary branching from the main frond occurs in a single plane, and is alternately, yet irregularly spaced with up to 1 cm between each branch. Each branch is bare at the base, with the rest bearing an irregular and alternately arranged series of smaller secondary branches. The secondary branches are progressively shorter towards the apex, and each branch bears numerous pointed branchlets giving an overall tufted and feather-like appearance.

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Maggs, C.A. & Hommersand, M.H. (1993). Seaweeds of the British Isles. Volume 1. Rhodophyta. Part 3A. Ceramiales. pp. xv + 444, 129 figs, map. London: HMSO.

Basionym: Conferva plumosa J.Ellis