Electronic Flora of South Australia Genus Fact Sheet

Genus RHABDONIA Hooker & Harvey 1847: 408

Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Areschougiaceae

Thallus erect, irregularly radially branched, branches terete or constricted into clavate to elongate-ovoid segments with branching from their distal ends; holdfast discoid or fibrous. Structure uniaxial, with the axial filament cutting off a single periaxial cell on 3 radii, producing rhizoids and forming a laxly filamentous medulla with the axial filament becoming inconspicuous and a pseudoparenchymatous cortex of ovoid cells.

Reproduction: Sexual thalli monoecious or dioecious; non-procarpic. Carpogonial branches 3 (–4)-celled, borne on an inner cortical cell and rarely with a sterile cell on the basal cell, directed inwards with a reflexed trichogyne; fertilized carpogonium producing a single, non-septate, connecting filament. Auxiliary cell an inner cortical cell in a darkly staining complex, first producing a gonimoblast initial inwardly and later a large, irregular fusion cell bearing radially short chains of carposporangia, and with a basal fusion stalk connecting back to one of the axial filaments. Cystocarps with or without slight filamentous enveloping tissue, protruding slightly on one side of the branch, ostiolate. Spermatangia scattered near branch ends, cut off from outer cortical cells via initials.

Tetrasporangia scattered in the outer cortex, cut off laterally, zonately divided.

Life history triphasic with isomorphic gametophytes and tetrasporophytes.

Lectotype species: R. coccinea (Harvey) Hooker & Harvey 1847: 408

Taxonomic notes: A genus of 3 species, probably confined to southern Australia, described by Min-Thein & Womersley 1976; supposed species from elsewhere need re-investigation.

Rhabdonia is distinguished from other uniaxial genera of the Areschougiaceae by its terete or segmented branches, loss of a clear axial filament some distance below the apex, and lack of any nutritive cells associated with the developing carposporophyte.

References:

HOOKER, J.D. & HARVEY, W.H. (1847). Algae Tasmanicae. Lond. J. Bot. 6, 397–417.

MIN-THEIN, U. & WOMERSLEY, H.B.S. (1976). Studies on southern Australian taxa of Solieriaceae, Rhabdoniaceae and Rhodophyllidaceae (Rhodophyta). Aust. J. Bot. 24, 1–166.

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.

KEY TO SPECIES OF RHABDONIA

1. Thallus unsegmented, irregularly branched, upper branches 0.5–1.5 mm in diameter

R. coccinea

1. Thallus segmented, much branched from apices of clavate to elongate-ovoid segments (2–) 4–15 (–20) mm long, (0.5–) 1–2 (–3) mm in diameter

2

2. Upper segments 2–4 (–6) mm long, lower axes thickened and covered with short, unbranched, adventitious, branchlets of 1–3 segments; without internal refractive filaments

R. verticillata

2. Upper segments 5–20 mm long, lower axes only slightly thickened, without short adventitious branchlets; with long refractive filaments just internal to the cortex

R. clavigera


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