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Genus MEDEIOTHAMNION Pujals 1970: 200

Phylum Rhodophyta – Order Ceramiales – Family Ceramiaceae – Tribe Sphondylothamnieae

Reproduction: Gametophytes dioecious. Female axes several cells long with the last 2 cells small, the subapical cell bearing 2 sterile pericentral cells and a supporting cell bearing a terminal sterile cell and a lateral carpogonial branch, with the larger hypogenous cell usually bearing an adaxial, curved, involucral branchlet. One laterally elongate auxiliary cell per procarp, producing gonimolobes from each end, with short filaments producing terminal clavate carposporangia. Sterile procarp cells produce 3–4 inner involucral branchlets. Spermatangial heads subspherical to ovoid, borne on cells of the whorl-branchlets.

Tetrasporangia borne on reduced branches on lower cells of whorl-branchlets, tetrahedrally divided.

Type species: M. santacrucense Pujals 1970: 290.

Taxonomic notes: Thallus erect, usually with a prostrate base, branching of axes usually sparse, irregular to alternately subdistichous, whorl-branchlets 2–4 (–5) per axial cell, opposite or whorled, pinnately to subdichotomously branched, median cells 35–140 µm in diameter. Indeterminate branches arising in positions of whorl-branchlets. Cortication absent or by descending rhizoids from basal cells of whorl-branchlets, adhering by haptera with a terminal digitate disc. Cells multinucleate.

A genus of five species, the type from Argentina, one from New Zealand and 3 from southern Australia (Gordon 1972, p. 49).

The distinguishing features of Medeiothamnion are the comparatively elongate hypogenous cell of the female axis bearing (except in M. santacrucense) a single, adaxial, involucral branchlet, and the single auxiliary cell per procarp producing an elongate cell which cuts off gonimolobe cells at each end.

It is likely that M. lyallii (Harvey) Gordon (1972, p. 59, figs 16–18, 56A) also occurs in Victoria and Tasmania. [Gellibrand Pile Light, Port Phillip Bay, Vic., 5–8m deep (Kraft 5499 & O'Brien, 30.x.1975; MELU and AD, A67804) and Sarah I., Bathurst Channel, SW Tasmania, 2–5m deep (Edgar, 11.iii.1995; AD, A64258)]. The Port Phillip record (Lewis 1983, pp. 262, 264) may be of an adventive. Female specimens are required to substantiate these records. Records of Callithamnion flaccidum Hooker & Harvey from Victoria and Tasmania (Harvey 1859b, p. 334; 1863, synop.:1v. Sonder 1881, p. 11) and Tasmania (Sonder 1853, p. 673) are probably not of this Cape Horn species (see also Wollaston 1968, p. 300), but may also apply to M. lyallii.


GORDON, E.M. (1972). Comparative morphology and taxonomy of the Wrangelieae, Sphondylothamnieae and Spermothamnieae (Ceramiaceae, Rhodophyta). Aust. J. Bot. suppl. 4, 1–180.

HARVEY, W.H. (1859b). Algae. In Hooker, J.D., The Botany of the Antarctic Voyage. III. Flora Tasmaniae. Vol. II, pp. 282–343, Plates 185–196. (Reeve: London.)

LEWIS, J.A. (1983). Floristic composition and periodicity of subtidal algae on an artificial structure in Port Phillip Bay (Victoria, Australia). Aquatic Bot. 15, 257–274.

PUJALS, C. (1970). "Medeiothamnion" nuevo genero de Ceramiaceae. Revta Mus. Argent. Cienc. nat. Bernardino Rivadavia 3, 287–299.

SONDER, O.W. (1853). Plantae Muellerianae. Algae. Linnaea 25, 657–709.

SONDER, O.W. (1881). In Mueller, F., Fragmenta Phytographiae Australiae. Supplementum ad volumen undecinum: Algae Australianae hactenus cognitae, pp. 1–42, 105–107. (Melbourne.)

WOLLASTON, E.M. (1968).Morphology and taxonomy of southern Australian genera of Crouanieae Schmitz (Ceramiaceae, Rhodophyta). Aust. J. Bot. 16, 217–417.

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

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (24 December, 1998)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIC. Ceramiales – Ceramiaceae, Dasyaceae
©State Herbarium of South Australia, Government of South Australia


1. Main axes prostrate, producing erect branches 1–5 mm high; axial cells 400–1000 µm long; rhizoidal cortication absent; epiphytic on Cystophora spp.

M. repens

1. Main axes erect, 4–30 cm high; axial cells becoming 2–3 mm long; rhizoidal cortication present but slight; epiphytic on algae or seagrasses


2. Whorl-branchlets subdi- or trichotomous, inwardly curved, adjacent whorls imbricate

M. halurus

2. Whorl-branchlets pinnate, not markedly inwardly curved, adjacent whorls imbricate only near branch apices

M. protensum

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