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Ditria expleta Huisman 1994: 2, figs 1–13; 1997: 204; 2000: 164.

Phylum Rhodophyta – Family Rhodomelaceae – Tribe Herposiphonieae

Selected citations: Silva et al. 1996: 491.

Thallus (Fig. 136A) brown-red, entirely prostrate on the host, axes 2–6 mm long, dorsiventrally organised with pairs of simple, terete, determinate laterals 300–800 µm long, usually separated by a dorsal segment bearing only a single cell. Attachment by rhizoids from ventral pericentral cells; epiphytic on Lobophora variegata and Zonaria sp. Structure. Apical cells hemispherical, 14–18 µm in diameter, segmenting transversely to give short axial cells, subapical cells dividing obliquely to cut off in a spiral sequence initials of determinate laterals which lie in alternate pairs separated usually by a segment with only a single dorsal cell (Fig. 136B, C, arrows), occasionally lacking the latter. Pericentral cells 5, ecorticate. Axes compressed, (50–) 70–110 µm broad, segments L/D 0.6–1.0 (–1.5), determinate laterals 15–20 segments long, (40–) 50–85 µm in diameter, segments L/D 0.5–0.8. Lateral axes developed by continued growth of determinate laterals. Trichoblasts on occasional upper segments of determinate laterals, 400–1000 µm long, branched, basal cells 25–35 µm in diameter, L/D 2–5. Rhizoids cut off from pericentral cells, unicellular with multicellular haptera. Cells uninucleate; rhodoplasts discoid to elongate, often transversely orientated in axial cells.

Reproduction: Gametophytes dioecious (Huisman 1994). Procarps borne on the second cell of trichoblasts on apices of determinate lateral branches, with a 4-celled carpogonial branch and 2 sterile branches. Carposporophytes with a basal fusion cell and branched gonimoblast with clavate terminal carposporangia. Cystocarps terminal on determinate laterals, ovoid; pericarp ostiolate, ecorticate. Spermatangial organs arise on trichoblasts on determinate laterals, lanceolate, mostly 180–300 µm long and 35–45 µm in diameter, with 1–2 basal and 1–4 apical sterile cells.

Tetrasporangia (Fig. 136D, E) in linear series in swollen and often curved ends of determinate laterals, fertile segments 70–90 µm in diameter; 40–110 µm in diameter, with 3 cover cells.

Type from Goss Passage, Wallabi Group, Houtman Abolhos, W. Aust., on Lobophora variegata, 15 m deep (Huisman, 13.vii.1993; Murdoch HA313a).

Selected specimens: Cootes Hill, Yorke Pen., S. Aust., on Lobophora, 5 m deep (Edyvane, 30.x.1993; AD, A66541). Edithburg, S. Aust., on Lobophora, 3–4 m deep on jetty piles (Kay, 26.v.1975; AD, A46955). Port Noarlunga, S. Aust., on Lobophora, 1.5–3 m deep (Kraft 3015 & Womersley, 3.xii.1970; MELU and AD, A69075). Sellicks reef, S. Aust., on Zonaria, 5 m deep (O'Leary, 20.iii.1994; AD, A63383). Emu Bay, Kangaroo I., S. Aust., on Lobophora, upper sublittoral on jetty piles (Womersley, 10.ii.1956; AD, A20151). Rocky Point, Kangaroo I., S. Aust., on Lobophora, drift (Womersley, 4.vi.1947; AD, A5696).

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

Distribution: Houtman Abrolhos and Rottnest I., W. Aust., and from Cootes Hill, Sturt Bay, Yorke Pen., to Sellicks reef, and Rocky Point, Kangaroo I., S. Aust., but probably more widespread.

Taxonomic notes: D. expleta is an inconspicuous alga, being entirely prostrate on Lobophora and Zonaria, and is probably more widely distributed on southern Australian coasts.


HUISMAN, J.M. (1994). Ditria expleta (Rhodophyta: Rhodomelaceae) a new red algal species from Western Australia. Jpn. J. Phycol. (Sôrui) 42, 1–9.

HUISMAN, J.M. (1997). Marine Benthic Algae of the Houtman Abrolhos Islands, Western Australia. In Wells, F.E. (Ed.) The Marine Flora and Fauna of the Houtman Abrolhos Islands, Western Australia, pp. 177–237. (W. Aust. Museum: Perth.)

HUISMAN, J.M. (2000). Marine Plants of Australia. (Univ. W. Aust. Press, Nedlands, W. Aust. & ABRS, Canberra, A.C.T.)

SILVA, P.C., BASSON, P.W. & MOE, R.L. (1996). Catalogue of the Benthic Marine Algae of the Indian Ocean. (Univ. California Press: Berkeley.)

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

Publication: Womersley, H.B.S. (24 February, 2003)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIID. Ceramiales – Delesseriaceae, Sarcomeniaceae, Rhodomelaceae
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIID 2003, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.

Illustration in Womersley Part IIIA, 2003: FIG. 136.

Figure 136 image

Figure 136   enlarge

Fig. 136. Ditria expleta (AD, A46955). A. Part of tetrasporangial thallus showing branching pattern. B. Indeterminate branch with alternate pairs of determinate laterals usually separated by an unbranched segment (arrow) bearing a primordial cell. C. Apex showing branching pattern. D. Determinate laterals with tetrasporangia. E. Tetrasporangia in upper segments of determinate laterals.

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