Submerged, aquatic, monoecious herbs without roots. Stipules 0. Leaves in whorls, filiform, regularly forked into linear segments, often rigid or brittle, with spiny teeth on the margin; apical segments truncate, 2-spined at the apex with a reddish glandular projection between the spines (basal segments sometimes parasitised and becoming swollen and sac-like).Flowers actinomorphic, hypogynous, axillary, 1-several at each node; male and female flowers at separate nodes, ± sessile. Perianth lobes 6-13, united at base; margin often with a single lateral spine or ± lacerate; apex as leaf apex. Male flowers with numerous stamens. Female flowers: staminodes 0; ovary superior, tapering into a long style; ovule 1. Fruit a nut.