Habit and leaf form. Marine herbs. Perennial; rhizomatous (rhizomes sympodial, much-branched, lignified, with 1 or 2 roots at each node, producing numerous erect lignified branches). Hydrophytic; marine; rooted. Leaves submerged; alternate (in a terminal tuft); sessile; sheathing (with auricles present). Leaf sheaths with free margins. Leaves simple; epulvinate. Leaf blades entire; 8–12 -nerved; parallel-veined. Leaves ligulate (at junction of sheath and blade). Axillary scales present. Leaves with a persistent basal meristem, and basipetal development. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants dioecious. Plants viviparous. Pollinated by water.
Inflorescence and flower features. Flowers solitary; terminal (on short lateral branches, enclosed by leaves); bracteate (the female flower of A. antarctica enveloped in a scarious involucre), or ebracteate (A. griffithii without involucre); small. Perianth absent. Fertile stamens present, or absent (female plants). Androecium 2. Androecial members coherent (the two dorsally united, the anthers paired on a common filament, attached at the same height). Androecium exclusively of fertile stamens. Stamens 2. Anthers unilocular, or bilocular; tetrasporangiate; 2–3- appendaged. The anther appendages apical (branched). Pollen shed as single grains (filamentous). Pollen grains lacking exine, and dispersed in the sea as long filaments. Fertile gynoecium present, or absent (male plants). Gynoecium 2 carpelled; apocarpous; eu-apocarpous; superior. Carpel stylate; apically stigmatic; 1 ovuled. Placentation apical. Styles 3-branched in the distal half. Ovules pendulous; non-arillate; orthotropous.
Fruit and seed features. Fruit fleshy, or non-fleshy; an aggregate. The fruiting carpel indehiscent; nucular (surrounded at the base by 4 large, pectinate, upturned pericarpic lobes, known as the ‘comb’; comb persistent at the base of the seedlings after they are shed from the parent plant). Seeds non-endospermic. Cotyledons 1.
Geography, cytology, number of species. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, South Australia, Victoria, and Tasmania. South-West Botanical Province.