Electronic Flora of South Australia Species Fact Sheet

Plocamium mertensii (Greville) Harvey 1847: 122; 1855a: 553.

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Plocamiaceae

Selected citations: J. Agardh 1852: 401; 1876: 346. De Toni 1900: 599. Fuhrer et al. 1981: pl. 48. Guiler 1952: 90. Lucas 1909: 35; 1929a: 19; 1929b: 50. Lucas & Perrin 1947: 215, fig. 80. May 1965: 372. Reinbold 1897: 52. Sonder 1853: 682. Womersley 1950: 170; 1966: 147; 1971a: 15, figs 17–23.


Thamnophora mertensii Greville 1830: xlix. Sonder 1846: 193.

Thamnocarpus mertensii (Greville) Kützing 1849: 887; 1866: 19, pl. 55d-h.

Delesseria plocamium var. procerum C. Agardh 1822: 181; 1824: 251.

Thamnophora procera (C. Agardh) J. Agardh 1841: 10. Harvey 1844: 447.

Plocamium procerum (C. Agardh) Hooker & Harvey 1845: 542; 1847: 404. J. Agardh 1852: 400; 1876: 347. Harvey 1847: 122; 1855a: 553; 1859b: 318; 1862: pl. 223 (incl. var. mertensii); 1863, synop.: xxxix (incl. var. mertensii). Kützing 1849: 886; 1866: 19, pl. 54a-d? Levring 1946: 222. Lucas 1909: 35; 1929a: 19; 1929b: 50. Lucas & Perrin 1947: 214, fig. 79. May 1965: 372. Reinbold 1897: 52. Sonder 1853: 682.

Plocamium procerum var. nidificum Harvey 1863, synop.: xxxix (nomen nudum). Plocamium nidificum Harvey ex J. Agardh 1876: 346; 1894: 131. De Toni 1900: 599; 1924: 317. De Toni & Forti 1923: 32, pl. III figs 6, 7. Levring 1946: 222. Lucas 1909: 35; 1929b: 50. Lucas & Perrin 1947: 213. May 1965: 372. Reinbold 1897: 52; 1899: 45. Womersley 1950: 170.

Thallus (Fig. 128A, B) 10–50 cm high, much branched with spreading laterals. Erect axes (1–) 2–3 (–5) mm broad, thin and delicate in younger regions which are usually broader than older parts of axes. Ramuli in alternate pairs; upper ramulus divided, often developing into a lateral branch, lower ramulus subulate, (0.7–) 1–1.5 (–5) mm long and usually less than 0.25 mm broad at its base, entire or serrate in its upper part either on the abaxial edge only or on both edges; lower ramulus frequently becoming divided and often proliferating into dense clusters of sub-dichotomous pinnules which may occur commonly on older branches (Fig. 128B), or on occasional branches, or be totally absent in some plants (Fig. 128A).

Reproduction: Sexual thalli dioecious. Cystocarps (Fig. 128C) in axils of either of the two ramuli, 1–2 (–3) maturing, globular and pedicellate, 0.5–0.8 (–1) mm in diameter, arising on small branchlets in the axils. Spermatangia covering the surface of terete branchlets in dense axillary clusters (Fig. 128D).

Stichidia in dense hemispherical clusters arising from a pad of tissue in the axil of either ramulus or of laterals (Fig. 128E); individual stichidia (Fig. 128F) simple or branched basally only, linear, 0.25–0.5 (–1) mm long and (50–) 75–100 (–125) µm in diameter, becoming curved as they elongate; occasional old elongate stichidia become branched near their apex. Tetrasporangia in 1 or 2 series, ovoid, 50–75 µm long and 25–50 µm in diameter, zonately divided.

Type from "Novae Hollandiae" (Fraser); lectotype in Herb. Greville, E.

Selected specimens: Nichol Bay, W. Aust. (1881; MEL, 1006282). Salmon Bay, Rottnest I., W. Aust., drift (Parsons, 12.xi.1968; AD, A33397). Breaksea I., Albany, W. Aust., 20–25 m deep (Huisman, 3.iv.1993; Murdoch). Point Valliant, Two People Bay, W. Aust., drift (Womersley, 30.viii.1979; AD, A50958). Seamount off Cannan Reefs, S. Aust., 22–30 m deep (Branden, 22.i.1991; AD, A61121). Waldegrave I., S. Aust., 22 m deep (Shepherd, 23.x.1970; AD, A37390). Tiparra Reef, Spencer Gulf, S. Aust., 5–6 m deep (Shepherd, 24.vi.1970; AD, A35943). Daly Head, Yorke Pen., S. Aust., drift (Woelkerling, 22.iv.1969; AD, A34138 -"Marine Algae of southern Australia" No. 84). West I., S. Aust., 10 m deep (Shepherd, 13.vi.1970; AD, A35914). Seal Bay, Kangaroo I., S. Aust., drift (Womersley, 22.xi.1968; AD, A32966). Port MacDonnell, S. Aust., drift (Womersley, 15.iv.1959; AD, A22983). Queensc]iff, Vic., drift (Wollaston, 17.viii.1956; AD, A20566). Gabo I., Vic., 1–3 m deep (Shepherd, 15.ii.1973; AD, A43369). Mount Killicrankie, Hinders I., Bass Strait, 15 m deep (McCauley, 22.ii.1990; AD, A60462). Musselroe Bay, Tas. (Perrin, 1940; MEL, 1006365).

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

Distribution: From Nichol Bay, W. Aust., to Gabo I., Vic., and northern Tasmania. Found on coasts of strong to moderate wave action and known from shaded pools near low tide level to 50 m deep.

Taxonomic notes: The range of variation in this species is considerable though most specimens are readily recognised. The frequent presence of much-divided ramuli (the lower of each pair), forming conspicuous tufts, characterises many specimens which were described as a separate variety or species, nidificum. However, many plants are totally without such tufts, while others form them only occasionally or on certain branches. The type specimens of P. mertensii and of P. procerum are essentially without such tufts.

Apart from this variable feature, P. mertensii is characterised by its much branched, rather thin, thallus with relatively broad axes, pedicellate cystocarps and unbranched, linear or curved and relatively long and slender stichidia. The spermatangia are borne on much branched tufts in the axils of the upper ramulus of each pair, as distinct from the vegetative tufts developing from the lower ramulus. Cystocarps normally originate on only slightly branched axillary branchlets.

P. mertensii is most closely related to P. patagiatum; differences are discussed under the latter species.


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

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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.

Illustrations in Womersley Part IIIA, 1994: Plate 4 fig. 4; FIG. 128.

Plate 4 image

Plate 4   enlarge

PLATE 4 fig. 1. Tsengia comosa (AD, A61774). fig. 2. Sarcothalia radula (AD, A62995). fig. 3. Gigartina muelleriana at Evans Cave, Cape Lannes, S. Aust. (AD, A62993). fig. 4. Plocamium mertensii at Breaksea I., Albany, W. Aust. (in Murdoch). Photo: J.M. Huisman.

Figure 128 image

Figure 128   enlarge

Fig. 128. Plocamium mertensii (A, C, E, F, AD, A35914; B, AD, A35943; D, AD, A34138). A. Habit. B. Habit of form with much-branched lower ramuli. C. Branch with axillary cystocarps. D. Cluster of spermatangial branches. E. Branch with clusters of stichidia. F. Stichidia. [A–F as in Womersley 1971.]

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