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Aquatic fern-like herbs up to 40 cm tall. Leaves submerged, basal, 10-40 cm long, 2-3 pinnatisect, covered all over with warty to ligulate excrescences; petiole and rhachis flattened, sometimes winged; pinnae sub-opposite, up to c. 3 cm long, numerous, spreading (somewhat resembling a fox-tail), pinnules much ramified, irregular, filamentous (the so-called 'gill-tufts'). Spikes emergent, erect, dioecious; male and female sub-similar, up to 30 cm long, arising from the base of the plant; scape verrucose or rarely smooth, stout; male flowers with the imbricate bracts broadly obovate, verruculose near the obtuse apex, margin recurved, locules joined only at the base; female flowers with conchiform bracts, c. 3 mm long, apex lip-like, central nerve decurrent below. Capsule hidden inside the bract, compressed-obovoid, smooth; seeds minute, orange.
Pinnae in c. 25–70 pairs, alternate to subopposite, 7.5–50 mm long, tapering-cylindric to ellipsoid in outline, those of at least some leaves differentiated into an upper and lower surface, with scale-like emergences all around the pinna axis; emergences simple, entire, ± closely spaced on the upper surface, and variously lobed, laciniate or ± filiform-branched on the lower surface, or all pinna emergences simple and scale-like; pinnules (pinna branches) sometimes present on the lower surface, 4–10 mm long, with linear to elliptic emergences.
Male inflorescences 3.5–22 cm long; peduncles 1.5–15 cm long, covered with emergences; bracts 1.5–2 ? 1.5–2.0 mm, irregularly shaped, with several rows of tubercles often ± fused into ridges, apical lip entire and rounded or sometimes recurved, the base decurrent. Anther thecae c. 0.5 mm long, elliptic to rounded, joined at base only, divergent.
Bracts of female inflorescence 2–4 ? 1.5–2 mm, conchiform, rounded to broadly elliptic in outline, smooth to verruculose, with 1–4 rows of tubercles towards the apex sometimes fused into ridges; the apical lip flange-like, rounded, entire, recurved; the central nerve decurrent below, lateral nerves sometimes prominent.
Leaves fern-like in appearance, few to numerous from the rootstock apex, trailing, submerged, up to 70 cm long, narrowly elliptic to lanceolate in outline, often 2–3-pinnate, usually with a differentiated upper and lower surface.
Petiole and rhachis ± flattened, bearing numerous small ovate to ligulate scale-like emergences; emergences all similar in shape but varying in size and largest on the rhachis, the largest up to 3 ? 2 mm; petiole 4–25 cm long.
Female inflorescences 6–30 cm long; peduncles 3–17 cm long, covered with tiny emergences, (sometimes flaking off on dried specimens to give smooth surface);spikes 150–200-flowered.
Inflorescences emergent, few–numerous (up to c. 50), scapose, spicate above, pendulous in bud, stiffly erect at anthesis.
Ovary 1.5–2 ? 1–1.5 mm, ellipsoid, glabrous, usually with a bundle of hairs on each side; styles 1–2.0 mm long, glabrous.
Aquatic caespitose perennial herb from a ± globose woody base, firmly attached to rocks by stiff radiating roots.
Fruit 2–2.5 ? 0.75–1 mm, elliptic, glabrous.
Seeds ovoid to oblong.
