Electronic Flora of South Australia Species Fact Sheet

Thalassodendron pachyrhizum den Hartog 1970: 194.

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

Selected citations: Green 1981: 5.

Dioecious; rhizomatous perennial (Fig. 29F). Rhizomes robust, (3–) 4–6 mm in diameter, becoming lignified with age. Roots arising in pairs, 3–5 mm thick, usually unbranched, becoming lignified, black when dry. Leaf sheath (Fig. 29G) compressed, (2–) 3–4 (–5) cm long, cuneate at the base, margins not overlapping; ligule obtuse, about 1 mm high; auricles obtuse; blade linear, falcate, 7–15 (–20) cm long, (6–) 8–10 (–12) mm broad, midrib prominent, 7–9 inconspicuous, parallel veins on either side of the midrib, connected by oblique crossveins; ligule obtuse, 1 mm high; auricles obtuse; apex (Fig. 29H) rounded, bearing numerous square or trapezoid sclerenchymatous denticulations 0.1–0.2 mm long which continue at less frequent intervals down the margin. Squamules (Fig. 291, J) linear, 1–4 mm long in 2 groups of 8–10 on either side of the node. Male flower not seen. Female flower enclosed by 4 leafy bracts; ovary ellipsoid, 2 mm long, style up to 1 mm long, 2 stigmas about 20 mm long. Fruit a one-seeded nut with a hard pericarp and surrounded by the fleshy innermost of the 4 floral bracts.

Type from Leighton, W. Aust., drift (Baird, 30.vii.1941); in UWA.

Selected specimens: Port Denison, W. Aust., drift (Gordon 8.xi.1968; ADU, A51965). Whitfords Beach, W. Aust., 4–5 m deep (Lipkin, 2.ii.1982; ADU, A52947). Sorrento Beach, Perth, W. Aust. (Royce 3063, 7.viii.1949; PERTH, s.n.). Cottesloe, W. Aust. (Glauert, Nov. 1922; PERTH, s.n.). Cowaramup Bay, W. Aust., 2–3 m deep (Clarke & Engler, 1.ix.1979; ADU, A50665). South coast, E. of Augusta, W. Aust. (Royce 3233, 26.vi.1950; PERTH, s.n.).

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

Distribution: Restricted to the temperate coast of W. Aust. from Geraldton southward to Cape Leeuwin and eastward to Bremer Bay. T. pachyrhizum commonly grows on rocks at the edge of limestone and granite reefs at depths up to 6 m. It has been found in regions of extreme turbulence at greater depths, to 40 m. It is a poor competitor and is not normally found growing in association with other seagrasses.

Taxonomic notes: Kirkman (pers. comm.) has monitored a stand of T. pachyrhizum for a twelve month period and has recently made numerous collections of both male and female flowering material. His findings will help to enlarge our knowledge of this species.


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.)

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: FIGS 28A, 29 F–J.

Figure 28 image

Figure 28   enlarge

Fig. 28 A. Thalassodendron pachyrhizum (ADU, A52947). Habit. B. Amphibolis antarctica (ADU, A52563). Habit. C. Amphibolis griffithii (ADU, A35725). Habit.

Figure 29 image

Figure 29   enlarge

Fig. 29 A–C. Syringodium isoetifolium (ADU, A52953). A. Portion of male inflorescence. B. T.S. Leaf blade showing air canals (a.c.) and vascular bundles (v.b.). C. Habit. D, E. Halodule uninervis (ADU, A51311). D. Habit. E. Leaf apex. F–J. Thalassodendron pachyrhizum. F. Habit. Note leaf scars (l.sc.). G. Blade-sheath junction showing venation, ligule (l.) and auricles (a.). H. Leaf apex. I. Squamules (sq.) in position at node. J. A group of squamules from one side of node. (F, G, I, J from ADU, A52947. H from ADU, A51965.)

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