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Order Ceramiales

Family Rhodomelaceae

Osmundaria serrata (Suhr) R.E.Norris 1991: 23, 29, figs 34-42

Plants red to brownish-red or yellowish-red, erect, up to 40 cm high, stiff to cartilaginous, with spirally twisted ribbon-like branches bearing regularly-spaced small teeth along margins; holdfast conical, prominent, woody. Proximal axes cylindrical, woody; distal axes flattened, up to 5 mm wide, sometimes with faint midrib; indeterminate branching sparse, at intervals of 0.5 – 6.0 cm; branches regularly twisted, margins with regular alternate serrations (determinate laterals) about 1 mm apart and 1 mm long, often with serrulations at the tip; apices inrolled. Axes comprising central filament of axial cells, 5 pericentral cells and 2 pseudopericentral cells on the dorsal side; determinate laterals arising alternately on every second or third segment; thallus corticated with 2-3 layers of cells. Reproductive structures borne on adventitious branchlets, usually clustered distally on serrations but sometimes on midrib. Tetrasporangia in fusiform stichidia that are about 1 mm long, in two rows, about 90 µm in diameter. Male plants with ovoid antheridial stands in clusters on adventitious branchlets that are borne abaxially near tips of marginal teeth, ca 80 µm in diameter with monosiphonous stalk cells. Female plants with fertile branchlets clustered on abaxial sides of marginal teeth; branchlets with curved apices and rudimentary trichblasts,usually one cystocarp per branchlet, ovoid, ca 650 µm in diameter, small terminal ostiole.

Collections, ecology and regional distribution

Recorded from Port Alfred eastward to Mission Rocks in northern Kwazulu-Natal (39-54). Found from the lower eulittoral zone to at least 40 m depth.

Worldwide distribution: Also reported from the Maldives (Silva et al. 1996).

Type locality: Mtentu, east of Port St Johns, Eastern Cape, South Africa (Silva et al. 1996).

Note: Norris (1991) points out that this species can be distinguished from the superficially similar Dictyomenia stephensonii by the absence of inrolled branch tips in the latter. Osmundaria serrata was formerly known as Vidalia serrata (Suhr) J. Agardh. See Norris (1991) for details of this and other Osmundaria species. O. serrata is the only South African species in this genus with fertile branchlets borne on marginal teeth.


Osmundaria serrata.

Osmundaria serrata, showing characterisitic twisting and a branch (re-drawn from Norris 1991).

Osmundaria serrata, tetrasporangial stichidia (re-drawn from Norris 1991).


References Osmundaria

Norris, R.E. 1991. The structure, reproduction and taxonomy of Vidalia and Osmundaria (Rhodophyta, Rhodomelaceae). Journal of the Linnean Society of London, Botany 106: 1-40, 60 figs.

Silva, P.C., Basson, P.W. & Moe, R.L. 1996. Catalogue of the benthic marine algae of the Indian Ocean. University of California Publications in Botany 79: 1-1259.


Cite this record as:

Anderson RJ, Stegenga H, Bolton JJ. 2016. Seaweeds of the South African South Coast.
World Wide Web electronic publication, University of Cape Town,; Accessed on 02 March 2024.