Electronic Flora of South Australia Genus Fact Sheet
Phylum Magnoliophyta – Subphylum Seagrasses – Class Liliopsida – Subclass Alismatidae – Order Hydrocharitales – Family Hydrocharitaceae
Monoecious or dioecious; annual or perennial, stoloniferous, marine herbs. Stolons bearing 2 scales with a lateral shoot and 1- few roots at each node. Roots unbranched with abundant root hairs. Scales usually hyaline, sub-opposite, one associated with the root(s) and clasping the stolon, the other embracing a lateral shoot (frequently very reduced) which bears the leaves. Leaves in pairs (in southern Australian species), sessile or more frequently petiolate, usually with a distinct blade, linear to ovate, with a midvein and an intramarginal vein connected by more or less parallel crossveins. Squamules (1–) 2–3, linear to lanceolate, attenuate at the base, about 1 mm long (in southern Australian species). Tannin cells absent. Inflorescence usually of a solitary unisexual flower, spathe of 2 sessile imbricate keeled bracts. Male flower subsessile at first, pedicel elongating considerably at time of anthesis; tepals 3; stamens 3, anthers sessile, the whole male flower caducous after dehiscence of the anthers. Pollen grains ellipsoid, united in moniliform chains when released but readily breaking up in contact with water; pollination hydrophilous. Female flower usually sessile; hypanthium bearing 3 minute tepals at its summit; ovary inferior, ovoid to ellipsoid, unilocular, placentation parietal, ovules numerous; styles 3 or 6, linear. Fruit thin walled, ellipsoid to ovoid or globose, rostrate due to the persistent hypanthium.
Type species: H. madagascariensis Steudel [= H. ovalis (R. Br.) J.D. Hooker]
Taxonomic notes: (see Doty & Stone 1967).
A genus of about 10 species, widely distributed in tropical and warm temperate waters. Some species are well defined and restricted in distribution but H. ovalis sensu lato* is widespread in the Indo-West Pacific and has great ecological tolerance. Den Hartog (1970, p. 250) referred to H. ovalis as a "collective species" in which a number of closely allied taxa are united. He recognized the populations growing in extra-tropical Australia as being relatively distinctive and having all parts generally larger, but still considered them as a subspecies, H. ovalis subsp. australis (Doty & Stone) den Hartog, within his broad concept of H. ovalis.
As usual with seagrasses, most herbarium collections are of sterile material as fertile material is frequently difficult to obtain. However, for the common southern Australian species of Halophila, good collections of fertile material are now available. The distinguishing features include a constant difference in style number and position of the female flower. The consistent characters of 6 styles and the green, foliose nature of the scales on the erect flowering branch (an extended lateral shoot), together with the generally more robust nature of the plant and the narrower leaves [L/B 3–4 (–6)] appear sufficient to warrant species status. The name H. australis is recognized for this taxon and applied here to most of the material from southern Australia.
However, specimens from the Perth region in Western Australia fall within H. ovalis, having female flowers with 3 styles, borne on the horizontal stolons. Although no material with six styles has been found from the west coast of Western Australia, the vegetative characters of some collections suggest that H. australis probably extends at least as far north as Dongara on the Western Australian coast and that there is a region of overlap of H. ovalis and H. australis from approximately Cowaramup Bay in the south to Dongara in the north. On the eastern coast of Australia overlap probably occurs in the Sydney region in New South Wales, and good fertile collections would clarify the limits of the two 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).
DOTY, M.S. & STONE, B.C. (1967). Typification for the generic name Halophila Thouars. Taxon 16, 414–418.
THOUARS, L.M.A. Du Petit- (1806). Genera nova Madagascariensia secundum methodum Jussiaeanam disposita. (Paris.)
The Marine Benthic Flora of Southern Australia Part I complete list of references.
Womersley, H.B.S. (31 May, 1984)
The Marine Benthic Flora of Southern Australia
©Board of the Botanic Gardens and State Herbarium, Government of South Australia
KEY TO SPECIES OF HALOPHILA
1. Plants monoecious; leaf margins always finely serrulate; glabrous or hairy to a variable extent 1
1. Plants dioecious; leaf margins entire; blade always glabrous
2. Female flowers borne on the prostrate horizontal stolons, styles 3; scales all hyaline, sessile; leaf blade oblong to obovate, mostly rounded at the base, L/B mostly
2. Female flowers borne on erect lateral branches, styles 6; scales at the first node of the fertile female branch usually green and foliose, subsessile or shortly petiolate; leaf blade more or less elliptic, narrowed towards the base, L/B mostly
State Herbarium of South Australia