2 gen., cosmop.

Submerged, marine, rhizomatous perennials. Leaves distichous, linear with an auriculate ligulate sheath. Flowers unisexual; the males and females in alternate rows forming a flat spike enclosed in a sheath near the base of a floral leaf. Perianth absent. Stamens 2. Carpel 1, with 1 ovule; stigma 2-fid. Fruit an achene.