Herbs, submerged marine. Stems slender, easily breaking, creeping, branched, rooting at nodes, with 2 scales at each node. Leaves usually opposite or binate, usually petiolate, margin entire or serrulate. Plants dioecious or monoecious; flowers unisexual. Spathes 2-valved, sessile, usually 1-flowered. Male flowers pedicellate; perianth segments 3; stamens 3, alternate with perianth segments, filament absent; anthers 2-4-thecous, extrorse; pollen moniliform. Female flowers sessile or subsessile; ovary with an apical beak; beak expanded at apex into 3 minute perianth lobes; ovules 2 to numerous; styles (2 or)3-5, threadlike. Fruit with beak at apex; pericarp membranous. Seeds few to many, globose or subglobose.

About nine species: along the coasts of the W Indian and S Pacific Oceans; four species in China.