Marine, perennial, submerged herbs, monoecious (Victorian genera); rhizomes well differentiated, rooting at nodes; branches slender. Leaves alternate, distichous, sessile; lamina linear, base sheathing with ligule and auricle present; scales present in each leaf axil. Inflorescence with flowers borne on a flattened spike-like axis enclosed within a modified leaf-sheath. Flowers small, unisexual; perianth absent, male and female 'flowers' arranged alternately in 2 longitudinal rows on axis (Victorian genera). Male 'flowers' with 2 free stamens which are ± enclosed by bract-like outgrowths of the flattened axis; anthers joined at their bases by a narrow connective, pollen filamentous. Female 'flowers' with 1 carpel; ovary ± cresent-shaped (in outline) with a short simple style terminating in 2 stigmas. Fruit small, drupe-like, 1-seeded; seed without endosperm.
3 genera with c. 21 species world-wide; Heterozostera and Zostera occur in Victoria.
Tomlinson (1982) discusses the inflorescence and the two main interpretations of the 'flowers'. This treatment follows that of den Hartog (1970). Pollination occurs within the water. The filamentous pollen floats through the water to the stigmas of the female 'flowers'.