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Plocamium dilatatum J. Agardh 1876: 347.

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Plocamiaceae

Selected citations: De Toni 1900: 601. Guiler 1952: 90. Lucas 1909: 35; 1929a: 19; 1929b: 50. Lucas & Perrin 1947: 215. May 1965: 372. Reinbold 1898: 46. Womersley 1971a: 20, figs 24–28.

Thallus (Fig. 129A) erect, robust, much branched, 10–25 cm high from the prostrate base. Erect axes (1.5–) 2–2.5 (–3.5) mm broad, centrally thickened and becoming costate below. Ramuli in alternate pairs; lower ramulus robust, (1.5–) 2–3 (–4) mm long and about 1 mm broad at its base, with the adaxial side usually straight and the abaxial side convex with coarse, short serrations.

Reproduction: Sexual thalli dioecious. Cystocarps (Fig. 129B) arising from small axillary branchlets, appearing pedicellate, globular and smooth surfaced. Spermatangia (observed in one specimen only) covering the surface of small but normal branchlets arising in the axils of upper ramuli.

Stichidia in clusters (Fig. 129C) 1–1.5 mm high in axils of ramuli, basally branched only; individual stichidia (Fig. 129D) clavate, with a relatively slender pedicel, 0.75–1 mm long and 200–300 µm in diameter. Tetrasporangia in two distinct series, 90–150 µm long and 60–80 µm in diameter, zonately divided.

Lectotype from Tasmania; in Herb. Agardh, LD (see Womersley 1971, p. 20).

Selected specimens: Whale Point, West I., S. Aust., 20 m deep (Shepherd, June 1966; AD, A30590). Stinky Bay, Nora Creina, S. Aust., drift (Womersley, 25.viii.1960; AD, A24471). 1.3 km off Cape Northumberland, S. Aust., 15 m deep (Shepherd, 26.vi.1976; AD, A55180). Point Roadknight, Vic., drift (Womersley, 6.vi.1953; AD, A18789). Cape Woolamai, Vic., 15–18 m deep (Goldsworthy & Berthold, 16.ii.1990; AD, A60189). Musselroe Bay, Tas. (G. & F. Perrin, March 1937; AD, A8517). Marion Bay, Tas., 6 m deep (Shepherd, 13.ii.1970; AD, A35637). Lady Bay, Southport, Tas., drift (Wollaston & Mitchell, 27.ii.1964; AD, A27528).

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

Distribution: West I., S. Aust. to Port Phillip Bay, Vic., and around Tasmania. Found on coasts of strong to moderate wave action and known from depths of 3–22 m.

Taxonomic notes: P. dilatatum is characterised by the massive stichidia borne on slender pedicels in axillary clusters. It is superficially similar to P. costatum (which has slenderer, branched stichidia) but the thallus of P. dilatatum is more robust with somewhat larger ramuli. South & Adams (1979, p. 126) place New Zealand specimens, credited to P. dilatatum, as P. costatum.


AGARDH, J.G. (1876). Species Genera et Ordines Algarum. Vol. 3, Part 1 - Epicrisis systematis Floridearum, pp. i-vii, 1–724. (Weigel: Leipzig.)

DE TONI, G.B. (1900). Sylloge Algarum omnium hucusque Cognitarum. Vol. 4. Florideae. Sect. 2, pp. 387–776. (Padua.)

GUILER, E.R. (1952). The marine algae of Tasmania. Check-list with localities. Pap. Proc. R. Soc. Tasm. 86, 71–106.

LUCAS, A.H.S. & PERRIN, F. (1947). The Seaweeds of South Australia. Part 2. The Red Seaweeds. (Govt Printer: Adelaide.)

LUCAS, A.H.S. (1909). Revised list of the Fucoideae and Florideae of Australia. Proc. Linn. Soc. N.S.W. 34, 9–60.

LUCAS, A.H.S. (1929a). The marine algae of Tasmania. Pap. Proc. R. Soc. Tasm. 1928, 6–27.

LUCAS, A.H.S. (1929b). A census of the marine algae of South Australia. Trans. R. Soc. S. Aust. 53, 45–53.

MAY, V. (1965). A census and key to the species of Rhodophyceae (red algae) recorded from Australia. Contr. N.S.W. natn. Herb. 3, 349–429.

REINBOLD, T. (1898). Die Algen der Lacepede und Guichen Bay (Siid Australien) und deren naherer Umgebung, gesammelt von Dr. A. Engelhart-Kingston. II. Nuova Notarisia 9, 33–54.

SOUTH, G.R. & ADAMS, N.M. (1979). A revision of the genus Plocamium Lamouroux (Rhodophyta, Gigartinales) in New Zealand. Phycologia 18, 120–132.

WOMERSLEY, H.B.S. (1971a). The genus Plocamium (Rhodophyta) in southern Australia. Trans. R. Soc. S. Aust. 95, 9–27.

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

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (14 January, 1994)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIA, Bangiophyceae and Florideophyceae (to Gigartinales)
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIIA 1994, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.

Illustration in Womersley Part IIIA, 1994: FIG. 129.

Figure 129 image

Figure 129   enlarge

Fig. 129. Plocamium dilatatum (A, AD, A27528; B, AD, A18789; C, D, AD, A35637). A. Habit. B. Branch with axillary cystocarps. C. Branch with clusters of stichidia. D. Stichidia. [A–D as in Womersley 1971.]

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