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Interthamnion attenuatum Gordon 1972: 137, fig. 44.

Phylum Rhodophyta – Order Ceramiales – Family Ceramiaceae – Tribe Spermothamnieae

Thallus (Fig. 103A, E) 1–2 mm high, with prostrate filaments producing erect, mostly unilaterally and abaxially (occasionally oppositely) branched, filaments, tapering considerably near their apices. Prostrate filaments attached by digitate haptera; epiphytic on Zonaria. Structure. Prostrate filaments 35–60 µm in diameter, cells L/D 3–9, erect filaments tapering from lower cells 20–60 µm in diameter and L/D 2–5 to upper cells 15–25 µm in diameter and L/D 3–7. Cells multinucleate; rhodoplasts discoid.

Reproduction: Gametophytes dioecious. Female axes (Fig. 103B) terminal, 5–7 cells long with the last 2 cells short, the hypogenous cell larger, the subapical cell bearing three periaxial cells, one sterile, one fertile (producing an auxiliary cell), and the supporting cell with a terminal sterile cell and lateral carpogonial branch. Fusion cell slight, involving both auxiliary cells, with short gonimoblast filaments bearing terminal ovoid to pyriform carposporangia (Fig. 103C) 30–35 µm in diameter. The sterile cells of the procarp produce post-fertilization, 3 small groups each of 3–5 cells and the hypogenous cell bears 4 cells at fertilization which later produce outer involucral filaments (Fig. 103C) around the carposporophyte. Spermatangial heads (Fig. 103D) borne unilaterally on erect branches, pedicellate, ovoid, 25–30 µm in diameter.

Tetrasporangia (Fig. 103E) pedicellate, on laterals of erect branches, with other small branches developing from the basal cell(s) forming a cluster, ovoid, 60–80 µm in diameter, tetrahedrally divided.

Type from Aldinga, S. Aust., on Zonaria spiralis in upper sublittoral pools (Womersley, 11.x.1965); holotype in AD, A29616.

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

Taxonomic notes: Distribution. Only known from the type specimen and from SE Tasmania.


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

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

Illustration in Womersley Part IIIA, 1998: FIG. 103.

Figure 103 image

Figure 103   enlarge

Fig. 103. Interthamnion attenuatum (AD, A29616). A. Thallus with prostrate and erect axes. B. Procarp just post-fertilization, with disconnected trichogyne. C. Mature carposporophyte, with carposporangia and involucral branches from hypogenous cell. D. Longitudinal section of spermatangial head. E. Erect axes bearing branchlets with tetrasporangia. (All as in Gordon 1972, courtesy of Aust. J. Bot.)

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