Electronic Flora of South Australia Species Fact Sheet

Posidonia australis J. D. Hooker 1858: 43.

Phylum Magnoliophyta – Subphylum Seagrasses – Class Liliopsida – Subclass Alismatidae – Order Potamogetonales – Family Posidoniaceae

Selected citations: Aston 1973: 270. Beadle, Evans & Carolin 1972: 522. Blackall & Grieve 1954: 9. Cambridge & Kuo 1979: 317. Green 1981: 5. den Hartog 1970: 132 (in part). Jessop 1978: 77. Ostenfeld 1916: 31. Sainty & Jacobs 1981: 366. Willis 1970: 67.


Caulinia oceanica sensu R. Brown (non DC.) 1810: 339.

Rhizomes 5–15 mm thick, laterally compressed, internodes usually 1–4 cm long. Sterns (Pl. 15 fig. 4) erect, short, sparingly branched; older stems and rhizomes both covered with senescent fibrous remains of leaf sheaths (Figs 24B, 26C). Leaves 2–3 (–4) per shoot; sheaths (Fig. 27A) 6–10 cm long, margins overlapping for about 1/3 of their length, persistent, finally disintegrating into a mass of shaggy pale-yellow to grey, hair-like fibres; ligule 1–1.5 mm long, auricles 3–4 mm long, usually dark-brown; blade (Figs 27B–D) linear, flat, frequently stiffly curved, (15–) 20–45 cm long, (6–) 10–15 (–20) mm broad, with 14–20 longitudinal veins uniting near the apex, epidermal cells (Fig. 27C) in surface view more or less isodiametric, L/B 0.5–1, with smooth margins; apices (Fig. 27D) rounded to truncate. Squamules (Fig. 27E) linear to lanceolate, 0.5–1 mm long. Inflorescence (Figs 26C,D, 27F) about the same height as, or extended above, the canopy of leaves, on a flattened peduncle 15–45 (–60) cm tall and 3–5 mm broad; 2–5 spikes subtended by 2 unequal bracts, the longer one up to 40 cm long; each spike terminating in an acuminate spur up to 1.5 cm long. Flowers (2–) 4 (–6) per spike; anthers (Fig. 27G,H) dark-brown before dehiscence, connective (Fig. 27I,J) 5–7.5 mm tall, lanceolate, usually strongly keeled, keel entire, serrate or with a coarse hook, sometimes winged, apex acute; stigma lobes (Fig. 27K) 3, irregularly papillose, frequently spurred; ovary about 2 mm long. Fruit (Fig. 27L,M) 2–3 cm long, oblong-ellipsoid, asymmetric, somewhat laterally compressed.

Lectotype from Georgetown, Tas. (Gunn 1347); in K (see Cambridge & Kuo 1979, p. 317).

Selected specimens: Strickland Bay, Rottnest I., W. Aust., 0–1.5 m deep (Engler & Clarke, 5.ix.1979; ADU, A51108). Spalding Cove, Port Lincoln, S. Aust., 0.5 m deep (Womerskv, 10.i.1976; ADU, A46919). Proper Bay, Port Lincoln, S. Aust., 3 m deep (Shepherd, 26.viii.1975; ADU, A46587). Red Cliff Point, Spencer Gulf, S. Aust., 0.4 m deep (Johnson, 18.ix.1974; ADU, A45924). Mangrove Creek, Spencer Gulf, S. Aust., 1 m deep (Shepherd, 12.ix.1973; ADU, A45287). Goose I., Spencer Gulf, S. Aust., 0.5 m deep (Shepherd, 20.xii.1971; ADU, A41271). Barker Rocks, Yorke Peninsula, S. Aust., 1.5–3 m deep (Lipkin, 20.ix.1981; ADU, A52645). Aldinga, S. Aust., S end of reef, 0–1 m deep (Womersley, 19.v.1977; ADU, A48004). Eastern Cove, Deal I., Bass Strait, Tas., 15 m deep (Shepherd, 23.iii.1981; ADU, A52049). Towra Point, Botany Bay, N.S.W., 1.5 m deep (Steffenson, 23.viii.1981; ADU, A52353).

Distribution map based
on current data relating to
specimens held in the
State Herbarium of SA

Distribution: Widespread from Shark Bay, W. Aust., southwards and eastwards around southern Australia, and north and east Tas., to Lake Macquarie, N.S.W., with some reports from localities further north on both the west and east coasts of Australia. It occurs from just below low water mark, where the tops of the leaves are emergent at low tide (Pl. 8 fig. 2) to about 15 m deep and grows sympatrically with both P. angustifolia and P. sinuosa in the shallower part of their range. The rhizomes of P. australis are usually not deeply buried but creep more or less horizontally just below the surface of the substrate at a depth of about 5–15 cm with the roots frequently penetrating to about 50 cm deep.

Taxonomic notes: P. australis is the commonest and most widespread species of Posidonia. Prior to the work of Cambridge & Kuo (1979) all flat-leaved material of Posidonia in Australian waters was referred to P. australis, although the existence of variants was recognised. Detailed ecological, morphological and anatomical studies led to the recognition of 3 species within the P. australis group, two narrow-leaved species (P. sinuosa and P. angustifolia) and the broader-leaved P. australis.

The cellulose-rich fibres of the persistent leaf sheaths of P. australis are very resistant to decay. As the plants die, sediment is trapped amongst the foliage and accumulates with the fibrous remains, gradually raising the level of the sea floor. Huge deposits, several metres thick, of fibre and sediment have built up in the shallow coastal waters of the South Australian gulfs. These deposits near Port Broughton were mined for several years from 1908–1914 and the fibre extracted. It had a number of industrial uses principally in the textile trade and as an insulation material. The venture proved uneconomic and was abandoned following the collapse of the European market in 1914 (Winterbottom 1917).


ASTON, H. (1973). Aquatic plants of Australia. (Melbourne University Press: Melbourne.)

BEADLE, N.C.W., EVANS, O.D. & CAROLIN, R.C. (1972). Flora of the Sydney region. 2nd edn. (Reed: Sydney.)

BLACKALL, W.E. & GRIEVE, B.J. (1954). How to know Western Australian Wildflowers. Part I. (University of Western Australia Press: Perth.)

CAMBRIDGE, M.L. & KUO, J. (1979). Two new species of seagrasses from Australia, Posidonia sinuosa and P. angustifolia (Posidoniaceae). Aquat. Bot, 6, 307–328.

DEN HARTOG, C. (1970). The seagrasses of the World. Verh. k. ned. Akad. Wet. Afd. Natuurk., ser. 2, 59(1), 1–275 (-1–31 Plates).

GREEN, J.W. (1981). Census of the Vascular Plants of Western Australia. (Western Australian Herbarium: South Perth.)

HOOKER, J.D. (1858). The Botany of the Antarctic voyage of H.M. Discovery Ships Erebus and Terror, in the years 1839–1843. III. Flora Tasmaniae. Vol. 2. (Monocotyledones). (Reeve: London.)

JESSOP, J.P. (1978). J.M. Black's Flora of South Australia. 3rd edn. Part I. (Govt. Printer: Adelaide.)

OSTENFELD, C.H. (1916). Contributions to West Australian Botany. I. The Seagrasses of West Australia. Dansk. bot. Arkiv. 2(6), 1–44.

SAINTY, G.R. & JACOBS, S.W.L. (1981). Water Plants of New South Wales. (Water Resources Commission of New South Wales: Sydney.)

WILLIS, J.H. (1970). A handbook to plants in Victoria. Vol. 1. 2nd edn. (Melbourne University Press: Melbourne.)

WINTERBOTTOM, D.C. (1917). Marine fibre. Bulletin No. 4. Department of Chemistry. (Govt Printer: Adelaide.)

The Marine Benthic Flora of Southern Australia Part I complete list of references.

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
Part I
©Board of the Botanic Gardens and State Herbarium, Government of South Australia

Illustrations in Womersley Part I, 1984: PLATES 8 fig. 2, 15 fig. 4, 16 figs 2, 3; FIGS 24B, 26C, D, 27 A–M.

Plate 8 image

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PLATE 8 Fig. 1. The lower eulittoral in American River inlet, Kangaroo L, S. Aust., with the calm-water form of Hormosira banksii and old, yellow mats of Chaetomorpha billardierii;
Fig. 2. A bed of Posidonia australis at Aldinga, S. Aust., with the stiff leaves just projecting above the water level at low tide.

Figure 24 image

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Fig. 24. A. Posidonia sinuosa (ADU, A53018). Habit B. Posidonia australis (ADU, A46919). Habit. C. Posidonia angustifolia (ADU, A53021). Habit.

Figure 26 image

Figure 26   enlarge

Fig. 26. A, B. Posidonia sinuosa (ADU, A50860). A. Plant showing inflorescence shorter than foliage leaves. B. Inflorescence with fruits. C, D. Posidonia australis. C. Plant showing inflorescence taller than foliage leaves (ADU, A45924). D. Inflorescence (ADU, A52645).

Figure 27 image

Figure 27   enlarge

Fig. 27 A–M Posidonia australis. A. Portion of leaf (adaxial view) showing sheath (sh.) margin overlap, ligule (I.) and auricles (a.). B. T.S. mid portion of leaf blade showing vascular bundles (v.b.). C. Epidermis of leaf blade (surface view) showing isobilateral cells. D. Leaf apex showing longitudinal veins (tv.), transverse veins (t.v.) and tannin cells (t.c.). E. Squamules showing shape and size variation. F. Inflorescence of 2 spikes showing terminal spur (sp.). G. Stamen showing 2 young anthers (an.), line of dehiscence (co.) and elongate connective with keel (k.). H. Stamen showing 2 dehisced anthers and connective with hooked keel. I1,I2. Connectives (abaxial view) showing scar (sc.) after anther fall. J1,J2. Connectives (side view) showing well-developed keel. K1-K4. Ovaries and stigmas showing variation in lobing. L. Fruit showing persistent anther connectives (c.) and stigma remnant (st.r.). M. T.S. fruit showing pericarp (p.c.) and seed (se.). (A–G, J1,J2, K2-K4 from ADU, A45924. H, II,12, K1 from ADU, A48004. L, M from ADU, A41271.)
N–Y Posidonia angustifolia. N. Portion of leaf (adaxial view) showing sheath (sh.) margin overlap, ligule (l) and auricles (a.) 0. T.S. mid portion of leaf blade showing vascular bundles (vb.), P. Epidermis of leaf blade (surface view) showing elongate cells. Q. Leaf apex showing longitudinal veins (l.v.), transverse veins (t.v.) and tannin cells (t.c.) R. Leaf apices, variation in shape. S. Squamules showing shape and size variation. T. Inflorescence of 3 spikes showing terminal spur (sp.). U. Flower showing dehisced anthers (an.), elongate connectives (c.), ovary and lobed stigma (st.). V1-V5. Connectives after fall of anthers showing scars (sc.) and shallow keel (k.). W1-W3. Ovaries and stigmas, showing variation in stigma lobing. X. Fruit showing terminal spur (sp.), persistent connective (c.) and stigma remnant (st.r.). Y. T.S. Fruit showing pericarp (p.c.) and seed (se.). (N–Q, S, U from ADU, A48844. R from ADU, A50924. T,V1-V5,W1-W3 from ADU, A46474. X, Y from ADU, A46466.)

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