Submersed marine or occasionally estuarine robust perennial herbs with creeping, monopodially branched, cylindrical to flattened rhizomes, the latter bearing a scale at each node and covered with fibrous remains of old sheaths. Roots branched, root hairs rare. Leaves alternate, distichously arranged on the rhizome, with distinct blade and sheathing base. Blade linear, flat, or biconvex to terete, with 3-20 or more longitudinal vascular bundles; squamules numerous; tannin cells numerous; stomata absent. Inflorescence racemose, spikelike on a long, flattened peduncle. Flowers actinomorphic, hermaphroditic, perianthless; stamens 3; anthers sessile, tetrasporangiate, pollen filiform; pollination hydrophilous; carpel 1 with a sessile ornate stigma; ovule 1. Fruit with spongy pericarp, dehiscent. Testa membranous; hypocotyl enlarged storing mainly starch. One genus of nine species, one in the Mediterranean, the other eight from the temperate to subtropical coasts of Australia.
In Australia Posidonia may occur in small clumps, or as vast underwater meadows which may be monospecific or consist of several Posidonia species, interspersed with other seagrasses, especially Amphibolis spp. They occur in estuaries, sheltered embayments, and nearshore, relatively protected oceanic regions; they are almost invariably subtidal, and if the water is clear, may occur down to 40 m. They are ecologically important because of their role in supporting food chains and stabilising sediments. There is some direct grazing by echinoderms and fish, but much of their importance lies in the production of nutrient-rich organic detritus which is used by filter feeding organisms. They support growth of epiphytic plants and animals and infauna which graze on epiphytes; they are important as a nursery area for fish, and provide shelter (Shepherd and Womersley 1981). Detached old leaves, with materials from other seagrasses and macroalgae, accumulate on beaches in wracks up to 1m high for up to 4 months each year, decomposing in the wracks or surf zone, where they are again important in food chains and nutrient cycling. Persistent fibres are rolled into balls by wave action, and such marine balls have been known since ancient times (Cannon 1979).
The distribution of the family is strikingly disjunct. One Posidonia species occurs in the Mediterranean, and the others are from temperate Australia. In that continent, P. australis occurs along the southern, southwestern and southeastern coastline, but other species are restricted to southern and southwestern Australia.
The main economic importance is indirect, in the support of various food chains which lead to commercially important animals such as fish and prawns; the loss of seagrass meadows after industrial or other developments is therefore viewed with concern. Another indirect effect lies in the ability of seagrass meadows to stabilise sediments and damp wave action. Large submerine deposits of persistent Posidonia fibres (e.g. in St. Vincent's Gulf, S Australia) have been viewed as potential sources of fibre for textile and paper manufacture, and for insulation (Winterbottom 1917; Kuo and Cambridge 1978).
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