Monecious or dioecious, annual or (in Victoria) perennial, marine herbs; stolons present, bearing a lateral branch (subtended by a hyaline scale) and 1–few unbranched roots at each node (subtended by a hyaline scale). Leaves in pairs, with (1–)2–3 small scales within the sheathing base, sessile or petiolate; lamina usually well-developed, with midvein and intramarginal vein connected by sub-parallel crossveins. Inflorescence 1-flowered, subtended by 2 sessile imbricate keeled floral bracts. Male flower subsessile initially, pedicel elongating at anthesis; the whole flower breaking off after anthers have opened; perianth segments 3; stamens 3, each with a sessile anther. Female flower usually sessile; hypanthium with 3 minute perianth parts; ovary unilocular, with numerous ovules; styles linear, 3 or 6. Fruit thin-walled, distally 'beaked' with persistent base of hypanthium.
About 10 species, widespread in warm temperate and tropical waters; 2 species occurring in Victoria.