Perennial herbs adapted to life in shallow sea water, with creeping rhizomes, monoecious or dioecious; rhizomes creeping or tuberous, nodes with intervaginal scales. Leaves distichous, linear or filiform; leaf sheath open or closed, often with stipuloid flanges, with an auriculate ligule at the junction with the blade; blade parallel-veined, sometimes with midrib. Inflorescence of 1 to several spadices, each enclosed in a spathe formed by the sheath of the next leaf; flowers small, sessile in two rows on a flattened axis; pollination by water; petals absent, stamen 1, sessile, dorsifixed, anther with longitudinal slits, the pollen filamentous, without exine, with the same density as sea water; pistillate flower protogynous, gynoecium 1, 1- locular with basally united styles and two stigmatic arms; ovule single, pendulous, orthotropous, bitegmic. Fruit a small irregular drupe, or irregularly dehiscing; endosperm absent.
Notes: Nowadays generally treated as a family of its own; but formerly this has been seen as part of Najadaceae (such as in F.T.A. and Fl. Cap.) or Potamogetonaceae (in Fl. Madag. and Sea-grasses of the World).
Perennial (rarely annual) herbs with creeping, monopodially branched rhizomes, wholly submerged, sometimes intertidal, marine, rarely estuarine, sometimes extending to brackish water habitats. Roots unbranched, with root hairs, growing on sand or mud substrates (Zostera, Heterozostera) or on rocks (Phyllospadix). Leaves alternate, distichous, with distinct blade and sheathing base; leaf sheath ligulate, auriculate; blade linear, flat to biconvex, leaf apex variable; stomata absent. Squamules present within each leaf sheath. Prophylls at the base of each shoot. Erect shoot annual, possibly sometimes perennial, and producing inflorescences. Flowers monoecious or dioecious, naked and arranged in 2 rows (in monoecious species of Zostera and Heterozostera, male and female flowers alternately) on 1 side of a flattened spike (spadix) enveloped within a modified leaf sheath (spathe). Retinacules present or absent. Male flowers of one stamen, consisting of 2 free, bilocular, lengthwise dehiscent, deciduous thecae connected by a ridgelike connective; pollen filiform. Female flowers of a unilocular ovary with a single short style and 2 long stigmas; uniovulate. Pollination hydrophilous. Fruit an achene.
A family consisting of 3 genera and 18 species from temperate waters in both hemispheres, some species extending to the subtropics.
Members of the family normally grow along shallow, low energy coastlines or in estuaries, and may be either subtidal or intertidal. Zostera extends into brackish water. The plants may occur in small clumps, may form large, mono-specific meadows, or may contribute (with other seagrass taxa) to meadows of different species. Both Zostera and Heterozostera are mostly associated with muddy or sandy substrates, but Phyllospadix is normally associated with rocky substrates both intertidally and subtidally. In the subartical circle, Zostera continues growth under frozen, icy surface water.
Large underwater meadows are visually striking and support a diverse population of marine flora and fauna including commercially important fisheries such as blue crabs. They are highly productive and support food webs either through direct grazing or through the production of large amounts of organic, nutrient-rich detritus.
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