Fls hermaphrodite and strongly protandrous, or unisexual with ♂ above ♀; solitary in lf axils, subtended by bracteoles. Calyx-lobes 4 or 0, free; petals 4 or 0, cucullate, glab.; stamens 8 or 4, anthers linear; ovary 4-celled, styles 4, short, stigmas papillose or plumose. Fr. small, separating into 2-4 1-seeded mericarps. Glab. herbs of water and wet places; lvs opp., alt., or whorled, entire, toothed, or pinnately divided, submerged lvs often with capillary segs. About 40 spp., cosmopolitan. Of the 5 N.Z. spp. 2 are endemic.

Annual or perennial herbs, usually glabrous, monoecious, dioecious, or fls ☿; stems rooting at nodes, prostrate to erect. Lvs usually whorled, sometimes opposite or alternate, usually dimorphic, the submerged lvs pectinate and finely divided, the aerial lvs ± entire, rarely all lvs of either form. Hydathodes often present near lf axils. Infl. a simple or rarely branched, terminal spike; fls usually solitary, in the axils of aerial lvs, occasionally in the axils of upper submerged lvs, each with 2 bracteoles; ♂ fls usually above ♀ within same infl. in monoecious spp. ♂ fls (2)-4-merous; sepals present; petals not clawed, ± hooded; stamens (1)-4 or 8; filaments short; anthers linear; pistil rudiments present or 0. ♀ fls (2)-4-merous; sepals and petals very reduced or 0; ovary (2)-4-celled; styles as many as cells of ovary; stigmas capitate, fimbriate; staminodes rudimentary. Fr. of 2-4, 1-seeded mericarps; mericarps usually smooth and rounded on dorsal face, lacking a differentiated pericarp, sessile or shortly pedunculate.