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Electronic Flora of South Australia Genus Fact Sheet

Genus ANTITHAMNION Nägeli 1847: 202

Phylum Rhodophyta – Order Ceramiales – Family Ceramiaceae – Tribe Antithamnieae

Thallus usually 2–20 mm high, delicate, with prostrate and occasionally erect indeterminate axes, completely ecorticate, bearing determinate whorl-branchlets (often as pinnae) in opposite pairs, each with a small unbranched basal cell and other rachis cells bearing opposite, alternate or secund pinnules; gland-cells sessile on short 2–4-celled branchlets on the pinnules; attachment of prostrate axes by rhizoids with haptera; usually epiphytic. Cells uninucleate.

Reproduction: Gametophytes usually dioecious. Procarps on the basal cells of whorl-branchlets near apices of indeterminate branches, with series of (4–) 8–20 developed successively; the basal cell acts as the supporting cell and bears a 4-celled carpogonial branch, and only a single carposporophyte matures per branch. Post-fertilization the auxiliary cell is cut off the supporting cell and divides to form a lower foot cell and upper gonimoblast cell which forms one or a few rounded groups of carposporangia; no involucral branchlets occur. Spermatangia occur in branched clusters on cells of the pinnules.

Tetrasporangia occur on cells of the pinnules, usually sessile, cruciately divided.

Type species: A. cruciatum (C. Agardh) Nägeli (1847, p. 202), from Europe.

Taxonomic notes: Antithamnion is a large genus with species in all oceans of the world. Wollaston (1968) monographed the 7 then known southern Australian species and Athanasiadis has added 3 more.


NÄGELI, C. (1847). Die neueren Algensysteme und Versuch zur Begründung eines eigenen Systems der Algen und Florideen. Neue Denkschr allg. schweiz. Ges. Naturwiss. 9, 1–275, Plates 1–10.

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: E.M. Wollaston & 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. Whorl-branchlets (pinnae) distichously arranged along axes


1. Whorl-branchlets decussately arranged along axes (irregularly decussately to distichously in A. uniramosum), usually slightly curved and facing the axes


2. Thallus branching not entirely complanate, pinnules lax and often diverging, rachis cells L/D 1.5–2.5

A. gracilentum

2. Thallus branching complanate, pinnae usually closely branched; rachis cells L/D mostly 0.5–1.5 (2–4 in A. pectinatum)


3. Pinnae with only adaxial pinnules

A. armatum

3. Pinnae with adaxial and abaxial pinnules


4. Pinnae with alternate pinnules

A. verticale

4. Pinnae with opposite pinnules


5. Mature axial cells 50–90 µm in diameter and L/D 2–4, pinnae rachis cells 25–40 µm in diameter and L/D 2–4

A. pectinatum

5. Mature axial cells 90–250 µm in diameter and L/D 1.5–2, pinnae rachis cells mostly 30–55 µm in diameter and L/D 1–1.7


6. Axial cells 120–250 µm in diameter, pinnae rachis cells mostly 35–55 µm in diameter, gland cells with markedly angled walls to 2 pinnule cells

A. biarmatum

6. Axial cells 90–120 µm in diameter, pinnae rachis cells 30–45 µm in diameter, walls of pinnule cells alongside gland cells not markedly angled

A. pinnafolium

7. Pinnae rigid, cells L/D 1–2, terminal cells mucronate

A. hanovioides

7. Pinnae (whorl-branchlets) relatively lax, cells L/D (1–) 2–4, terminal cells with rounded ends


8. Whorl-branchlets pinnate, complanately branched with distichous pinnules, tetrasporangia sessile


8. Whorl-branchlets not distinctly pinnate, pinnules separated on rachis, lax; tetrasporangia pedicellate


9. Pinnae 200–580 µm long, upwardly curved and shorter on prostrate axes and lower parts of erect lateral branches; pinnules opposite below for 1–3 pairs, alternate above, with 1 to several simple branches

A. diminuatum

9. Pinnae mostly 600–1200 µm long; pinnules alternate (or lowest 1–3 in opposite pairs), mostly unbranched

A. delicatulum

10. Whorl-branchlets laxly branched, single above and in opposite pairs below, bearing only opposite pairs of short, simple, 1–2-celled pinnules; axial cells 20–80 µm in diameter and L/D 4–8, whorl-branchlet rachis cells 20–25 µm in diameter and L/D 4–6

A. uniramosum

10. Whorl-branchlets densely branched but lax, with opposite pinnules below, often unilateral above; axial cells 60–110 µm in diameter and L/D 2–4, whorl-branchlet rachis cells 25–35 µm in diameter and L/D 2–3

A. cruciatum

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