Electronic Flora of South Australia
Electronic Flora of South Australia
Census of SA Plants, Algae & Fungi
Identification tools

Electronic Flora of South Australia Species Fact Sheet

Halydictyon arachnoideum (Harvey) Harvey 1858: pl. 37A.

Phylum Rhodophyta – Family Rhodomelaceae – Tribe Uncertain

Selected citations: De Toni 1903: 1246. Falkenberg 1901: 693. Guiler 1952: 100. Lucas 1912: 159. Lucas & Perrin 1947: 322. May 1965: 360. Shepherd & Womersley 1971: 166.


Hanowia arachnoidea Harvey 1855a: 558. J. Agardh 1863: 1254. Sonder 1880: 31. Tisdall 1898: 512. Wilson 1892: 164.

Hanowia robusta Harvey 1855a: 558. J. Agardh 1863: 1254. Sonder 1880: 31. Tisdall 1898: 512. Wilson 1892: 164.

Halodictyon robustum (Harvey) Harvey 1858: pl. 37B. De Toni 1903: 1245. Falkenberg 1901: 693. Huisman & Walker 1990: 434. Lucas 1912: 159. Lucas & Perrin 1947: 323. May 1965: 360. Silva et al. 1996: 495.

Halodictyon velatum Reinbold 1897: 58; 1898: 48. De Toni 1903: 1246. Lucas 1912: 159. Lucas & Perrin 1947: 322. May 1965: 360.

Thallus (Fig. 224A, F, G) forming an irregularly expanding net 1–12 cm high and broad, with more-or-less compressed lobes or branches 2–10 mm broad. Usually epiphytic on other algae, attached by small adherent branchlets or long rhizoidal branches. Structure. Growth from plate-like areas of small meristematic cells at the end of branches, the cells regularly positioned and producing laterals which join to adjacent filaments at their cross walls thus forming 4 cells (rarely 3 or 5) around square interstices. Young cells 20–60 µm in diameter and L/D 1–2, expanding to 80–200 µm in diameter and L/D (2–) 4–8 (–10), with the largest cells in the lower thallus 120–240 (–400) µm in diameter and L/D (3–) 4–10. Cells multinucleate; rhodoplasts discoid, chained in larger cells.

Reproduction: Female plants unknown. Spermatangial organs (Fig. 224B, C) ovate, compressed, 100–200 µm broad and 140–400 long with a single sterile basal cell and sterile marginal cells (Fig. 224C) 20–25 µm in diameter and L/D 1–2 (–3) surrounding spermatangial cells. Axial cells 4–7, each (except the terminal one) cutting off a lateral pericentral cell which produces a sterile marginal cell and a transverse cell forming a plate of initials each cell of which cuts off 3–4 spermatangia.

Tetrasporangial stichidia (Fig. 224D, E) compressed, 120–180 µm broad, when first fertile 200–300 µm long, later extending to 800–1000 µm long. Axial cells cutting off 4 pericentral cells, the transverse ones remaining undivided and the lateral cells (Fig. 224E) each cutting off a marginal cell, two lateral cells and a tetrasporangium which becomes 40–50 µm in diameter; cover cells uncertain, doubtfully present.

Type from King George Sound, W. Aust. (Harvey); lectotype in Herb. Harvey, TCD (Trav. Set 52).

Selected specimens: Cottesloe, W. Aust., drift (Smith A427, June 1964; AD, A28997). Fremantle, W. Aust. (Harvey, Alg. Aust. Exsicc. 342, TCD-type of H. robusta) and W. Aust. (Clifton; AD, A18257). Waterloo Bay (Elliston), S. Aust., 10 m deep (Shepherd, 2.v.1982; AD, A53134). Investigator Strait, S. Aust., 43 m deep (Watson, 27.i.1971; AD, A38160). Off Outer Harbor, S. Aust., 16 m deep (Loan, 8.viii.1963; AD, A26655). Off Grange, S. Aust., on artificial reef, 20 m deep (Branden, 4.ix.1985; AD, A56771). Kingscote, Kangaroo I., S. Aust., 6 m deep at jetty (Lavers, 7.vii.1997; A67768). American R. inlet, Kangaroo I., S. Aust., drift (Womersley, 31.x.1966; AD, A30826) and (L. Clark, 20.viii.1954; AD, A19785). Muston, Kangaroo I., S. Aust., 0–3 m deep (McKelvey, 18.iv.1999; AD, A68247). Rocky Point beach, Kangaroo I., S. Aust., drift (Womersley, 21.viii.1948; AD, A8612). Stanley Beach, S coast Kangaroo I., S. Aust., drift (Womersley, 7.ii.1956; AD, A20078). Margaret Brock Reef, S. Aust., 3–4 m deep (Ball, 8.xii.1973; AD, A44589). Lacepede Bay, S. Aust. (Engelhart; AD, A1425, probably isotype of H. velatum Reinbold). West Cove, Erith I., Kent Group, Bass Strait, 20 m deep (Shepherd & R. Lewis, 10.v.1974; AD, A45257). East Cove, Deal I., Kent Group, Bass Strait, 10 m deep (Shepherd & R. Lewis, 7.v.1974; AD, A45100).

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

Distribution: Cottesloe, W. Aust., to Deal I., Bass Strait and Coles Bay, Tasmania (Guiler 1952, p. 100).

Taxonomic notes: Both Halydictyon arachnoideum and H. robustum (type from Fremantle, W. Aust., Trav. Set 342 in Herb. Harvey, TCD) have been recognised on southern Australian coasts since Harvey (1855a) described them in the same publication. However the cell dimensions as the cells mature from the branch apices cover a wide range, and both species come within the ranges given above. Hence they are here combined under H. arachnoideum as the most suitable species name. H. velatum Reinbold [type from Lacepede Bay, S. Aust. (Engelhart), in Herb. Reinbold, M; isotype in AD, A1425] also comes within the species circumscription.

Apparently female plants have never been found, but one male plant (AD, A56771) is known.


AGARDH, J.G. (1863). Species Genera et Ordines Algarum. Vol. 2, Part 3, pp. 787–1291. (Gleerup: Lund.)

DE TONI, G.B. (1903). Sylloge Algarum omnium hucusque Cognitarum. Vol. 4. Florideae. Sect. 3. pp. 775–1521 + 1523–1525. (Padua.)

FALKENBERG, P. (1901). Die Rhodomelaceen des Golfes von Neapel und der angrenzenden Meeres-abschnitte. Fauna und Flora des Golfes von Neapel. Monogr. 26. (Friedländer: Berlin.)

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

HARVEY, W.H. (1855a). Some account of the marine botany of the colony of Western Australia. Trans. R. Jr. Acad. 22, 525–566.

HARVEY, W.H. (1858). Phycologia Australica. Vol. 1, Plates 1–60. (Reeve: London.)

HUISMAN, J.M. & WALKER, D.I. (1990). A catalogue of the marine plants of Rottnest Island, Western Australia, with notes on their distribution and biogeography. Kingia 1, 349–459.

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

LUCAS, A.H.S. (1912). Supplementary list of the marine algae of Australia. Proc. Linn. Soc. N.S.W. 37, 157–171.

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

REINBOLD, T. (1897). Die Algen der Lacepede und Guichen Bay und deren náherer Umgebung (Slid Australien), gesammelt von Dr. A. Engelhart-Kingston. Nuova Notarisia 8, 41–62.

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

SHEPHERD, S.A. & WOMERSLEY, H.B.S. (1971). Pearson Island Expedition 1969.-7. The subtidal ecology of benthic algae. Trans. R. Soc. S. Aust. 95(3), 155–167.

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

SONDER, O.W. (1880). In Mueller, F., Fragmenta Phytographiae Australiae. Supplementum ad volumen undecinum: Algae Australianae hactenus cognitae, pp. 1–42, 105–107. (Melbourne.)

TISDALL, H.T. (1898). The algae of Victoria. Rep. 7th Meet. Aust. Ass. Adv. Sci., Sydney, 1898, pp. 493–516.

WILSON, J.B. (1892). Catalogue of algae collected at or near Port Phillip Heads and Western Port. Proc. R. Soc. Viet. 4, 157–190.

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. 224.

Figure 224 image

Figure 224   enlarge

Fig. 224. A–E. Halydictyon arachnoideum (A, AD, A45257; B, C, AD, A56771; D, E, AD, A26655; F, AD, A1425; G, AD, A38160). A. Habit. B. Branches with spermatangial organs. C. Compressed spermatangial organs with sterile-cell margins. D. Branches with stichidia. E. Compressed stichidia with paired tetrasporangia. F. Habit of "robustum" form. G. Enlargement of part of robust thallus.

Disclaimer Copyright Disclaimer Copyright Email Contact:
State Herbarium of South Australia
Government of South Australia Government of South Australia Government of South Australia Department for Environment and Water