Electronic Flora of South Australia Species Fact Sheet
Phylum Rhodophyta – Class Florideophyceae – Order Gigartinales – Family Acrotylaceae
Rhabdonia racemosa J. Agardh 1892: 116. Kylin 1932: 21, pl. 7 fig. 14.
Thallus (Fig. 122D) dark red-brown, 5–15 cm high, relatively stiff, with one to a few percurrent axes or main branches, bearing densely 1–2 orders of irregularly radial, elongateclavate, terete laterals, main laterals 1–4 cm long and 2–3 mm broad near their rounded tips, lesser laterals 0.5–2 cm long and 2–3 mm broad, all basally constricted; actively growing branch systems pyramidal in form. Holdfast discoid, 2–7 mm across; epilithic or epiphytic on Amphibolis. Structure multiaxial, developing a laxly filamentous medulla (becoming dense in older branches) and a pseudoparenchymatous cortex with a single inner cortical layer of large, compact, isodiametric cells 60–80 µm across and an outer cortex 1–2 cells thick; outer cells ovoid, 8–10 µm in diameter, evenly spread in surface view and not forming rosettes. Rhodoplasts laminate, 1–3 per outer cell.
Reproduction: Sexual thalli monoecious; probably procarpic. Carpogonial branches borne on inner cortical cells, 3-celled, directed inwards with reflexed trichogynes. Auxiliary cell becoming partly surrounded by dense nutritive tissue, with several gonimoblast initials developing towards the thallus surface and forming filaments which line the inner surface of the dense pericarp of vegetative filaments; sporogenous filaments developing centripetally, forming branched tufts with rows of 1–2 obovoid carposporangia 10–15 µm in diameter. Cystocarps (Fig. 122E) protuberant, slightly basally constricted, 0.8–2.2 mm in diameter, with enveloping tissue and a thick outer cortex, ostiolate. Spermatangial initials cut off from outer cortical cells, each forming 3–4 spermatangia 2–3 µm in diameter.
Tetrasporangia developed from mid cortical cells, intercalary (usually bearing two outer cortical cells), basally pit-connected, ovoid, 20–30 µm long and 15–25 µm in diameter, zonately divided.
Type from Israelite Bay, W. Aust. (Brooke); holotype in Herb. Agardh, LD, 34452.
Selected specimens: Eucla, W. Aust., drift (Womersley, 2.ii.1954; AD, A19363). Elliston Bay, S. Aust., 10–11 m deep (Shepherd, 20.x.1969; AD, A35034 and 24.x.1969; AD, A34965 and 18.iv.1970; AD, A35852) and on Amphibolis antarctica, 7 m deep (Shepherd, 25.x.1971; AD, A42611).
Distribution: Israelite Bay, W. Aust., to Elliston, S. Aust.
Taxonomic notes: A. racemosa is apparently restricted in its distribution and differs from A. hymenocladioides in form and the complete cover of outer cortical cells.
AGARDH, J.G. (1892). Analecta Algologica. Acta Univ. lund. 28, 1–182, Plates 1–3.
KRAFT, G.T. (1977a). Studies of marine algae in the lesser-known families of the Gigartinales (Rhodophyta). I. The Acrotylaceae. Aust. J. Bot. 25, 97–140.
KYLIN, H. (1932). Die Florideenordnung Gigartinales. Lunds Univ. Årsskr. N.F. Avd. 2, 28 (8), 1–88, Plates 1–28.
The Marine Benthic Flora of Southern Australia Part IIIA complete list of references.
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.
Illustration in Womersley Part IIIA, 1994: FIG. 122D, E.
Figure 122 enlarge
Fig. 122. A–C. Amphiplexia hymenocladioides (AD, A44750). A. Habit. B. Longitudinal section of a branch apex. C. Cross section of mature cystocarp. D, E. Amphiplexia racemosa (AD, A35852). D. Habit. E. Cross section of cystocarp, with ostiole. F. Antrocentrum nigrescens (AD, A48318). Habit. [B–E as in Kraft 1977a.]
State Herbarium of South Australia