Electronic Flora of South Australia Species Fact Sheet

Dictyota dichotoma (Hudson) Lamouroux. Harvey 1847: pl. 103.

Phylum Phaeophyta – Order Dictyotales – Family Dictyotaceae – Tribe Dictyoteae

Selected citations: Womersley 1967: 208.


D. apiculata J. Agardh 1894a: 67. Womersley 1967: 208.

Thallus (Fig. 65A,B) usually medium brown, darker near the base, 2–20 cm long, fairly regularly dichotomously branched at intervals of 0.5–1 (–3) cm, branches (2–) 3–7 (–10) mm broad (down to 1 mm near apices of var. intricata), usually more or less linear and tapering only slightly from lower to upper parts of the thallus, rarely spirally twisted, axils more or less rounded, apices of branches rounded and usually 3–4 mm broad just below the apices (narrower in var. intricata), with occasional specimens tapering in upper 0.5–1 cm to apiculate apices; proliferations usually absent except as a response to damage, occasionally present marginally. Holdfast of rhizoids; mainly epilithic. Cortical cells 14–20 µm across, L/B 1–2 (–4). Hair tufts scattered, hairs 10–18 µ in diameter.

Reproduction: Tetrasporangia (Fig. 64H,I) 70–110 µm in diameter, usually densely scattered (singly or in small groups) over the thallus except close to the margins. Oogonia (Fig. 64J,K) in scattered, irregularly ovate sori 150–400 µm long and 120–250 µm across; oogonia 60–90 µm long and 30–50 in diameter. Antheridia (Fig. 64L,M) in scattered, ovate to irregularly elongate sori (150–) 250–600 µm long and 150–250 µm across; antheridia 60–80 µm long and 20–30 µm in diameter.

Type from England; probably lost.

Selected specimens: Whitford Beach, Perth, W. Aust., 4 m deep (Cook, 20.viii.1979; ADU, A50561). Waterloo Bay (Elliston), S. Aust., 1 m deep (Johansen, 28.x.1981; ADU, A52740, female). Coffin Bay, S. Aust., 1–2 m deep (P. Womersley, 1.xii.1975; ADU, A46797). N. Spencer Gulf, S. Aust., 10 m deep (Shepherd, 6.ix.1973; ADU, A44328). Glenelg, S. Aust., 18 m deep on artificial reef (Branden, 2.xii.1975; ADU, A46746, female). Port Noarlunga, S. Aust., 21 m deep, 5 km offshore (Ottaway, 8.xii.1980; ADU, A52120, female). E. side Ballast Head, Kangaroo I., S. Aust., upper sublittoral (Womersley, 31.x.1947; ADU, A6102). Robe, S. Aust., upper sublittoral pools (Womersley, 25.viii.1960; ADU, A24478). Apollo Bay, Vic., 0–0.5 m deep on pipeline in dock (Owen, 1.ix.1971; ADU, A39489). Crawfish Rock, Westernport Bay, Vic., 0–3 m deep (Watson, 15.ix.1968; ADU, A32753). Bluestone Bay, Freycinet Pen., Tas., 6 m deep (Brown, McGeary & Womersley, 4.xi.1982; ADU, A56274). Watson Bay, Sydney, N.S.W., uppermost sublittoral (Womersley, 27.xii.1962; A26409, sporangial, male and female).

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

Distribution: Widespread in temperate waters and extending into both subtropical and colder waters.

In Australia, from Whitford Beach, W. Aust. around southern Australia and Tasmania to southern Queensland; probably further north on both eastern and western coasts.

Taxonomic notes: var. intricata (C.Agardh) Greville 1830: 58. Venus Bay, S. Aust., reef pools (Womersley, 17.i.1951; ADU, A15169). Port Clinton, S. Aust., low eulittoral (Womersley, 18.ix.1981;; ADU, A52912) American R. inlet, Kangaroo I., S. Aust., in Posidonia beds off Strawbridge Point (Womersley, 26.i.1946; ADU, A3142). Pig I., American R. inlet, Kangaroo I., S. Aust., upper sublittoral ( Womersley, 30.xii.1948; ADU, A10431).

Most of the above southern Australian specimens agree well with specimens from England. In sheltered areas such as American R. inlet on Kangaroo Island, Venus Bay and Port Clinton, plants are narrower than normal, with long interbranch segments, and are placed in var. intricata (C. Agardh) Greville; to what extent this variation is ecological is not established. One such specimen (ADU, A3142) also shows distinctive, thick, inclusions on the end walls of scattered medullary cells, as seen in surface view of the branches. Specimens from Crawfish Rock (e.g. A32753) show more small marginal proliferations than usual.

D. apiculata J. Agardh (1894a, p. 67), from Port Phillip, Vic. (Wilson), lectotype in Herb. Agardh, LD (48813), has the general dimensions and habit of D. dichotoma, with scattered sporangia, but tapers over the upper 0.5–1 cm to apiculate apices. A few other such specimens are known from Port Phillip (e.g. Macpherson, 12.xi.1961; ADU, A28550), and are interpreted as growth forms of D. dichotoma, as suggested previously / (Womersley 1967, p. 208). Further studies on variation in species of Dictyota within Port Phillip are clearly desirable.


AGARDH, J.G. (1894a). Analecta Algologica. Cont. I. Acta Univ. lund. 29, 1–144, Plates 1, 2.

GREVILLE, R.K. (1830). Algae Britannicae. (Edinburgh.)

HARVEY, W.H. (1847). Phycologia Britannica. Plates 73–144. (Reeve: London.)

WOMERSLEY, H.B.S. (1967). A critical survey of the marine algae of southern Australia. II. Phaeophyta. Aust. J. Bot. 15, 189–270.

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

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (14 December, 1987)
The Marine Benthic Flora of Southern Australia
Part II
©Board of the Botanic Gardens and State Herbarium, Government of South Australia

Illustrations in Womersley Part II, 1997: FIGS 64 H–M, 65A,B.

Figure 64 image

Figure 64   enlarge

Fig. 64. A–D. Dictyota prolifera (A,B, ADU, A19576; C,D, ADU, A28588). A. Thallus with sporangial sori on opposite sides in concave undulations, with small proliferations. B. Transverse section of thallus with undivided sporangia. C. Thallus with male sori on opposite sides. D. Transverse section of male sorus with antheridia and paraphyses, and a young hair tuft from opposite surface. E. Dictyota naevosa (ADU, A13602). Transverse section of thallus with undivided sporangia. F. Dictyota diemensis, broad form (ADU, A35622). Transverse section of thallus with undivided sporangia. G. Dictyota diemensis, narrow form (ADU, A56304). Transverse section of thallus with undivided sporangia. H–M. Dictyota dichotoma (ADU, A26409). H. Surface view of branch with scattered sporangia and hair groups. I. Transverse section of thallus with sporangia. J. Surface view of branch with oogonial sori. K. Transverse section of thallus with an oogonial sorus. L. Surface view of an antheridial sorus with sterile peripheral paraphyses. M. Transverse section of thallus with an antheridial sorus.

Figure 65 image

Figure 65   enlarge

Fig. 65. A. Dictyota dichotoma (ADU, A52740). B. Dictyota dichotoma var. intricata (ADU, A10431). C. Dictyota furcellata (ADU, A8384). D. Dictyota alternifida (ADU, A56812). E. Dictyota fenestrata (ADU, A35640).

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