Rhabdonia clavigera J.Agardh

Scientific Description

Habit and structure. Thallus medium to dark red-brown, fading to yellow-grey 5–15 cm high, moderately to much branched with 2–4(–6) branches from the apex of each segment; mature segments elongate-ovoid to clavate, 0.5–2 cm long, 1–2(–3) mm in diameter; lower segments not or only slightly thickened, 0.5–1 mm in diameter, without adventitious laterals. Holdfast stoloniferous and hapteroid, 0.5–2 cm across, with several axes; epilithic. Structure uniaxial, with rhizoidal filaments enlarging to form a core of 4–6 similar filaments 8–20 µm in diameter together with slenderer filaments, surrounded by a broad, lax medulla of periaxial filaments with divergent end cells lying just inside the cortex, some cells becoming pseudoparenchymatous, 2(–3) cells thick, inner cells ovoid and 12–20(–30) µm in diameter, outer cells (8–)10–16(–20) µm in diameter. Rhodoplasts ovoid to elongate, becoming ribbon shaped, several per cell.

Reproduction. Sexual thalli monoecious; non-procarpic. Carpogonial branches 3-celled, 1(–2) borne on inner cortical cells, directed inwards with reflexed trichogynes. Connecting filaments not observed. Auxiliary cell an inner cortical cell which becomes darkly staining, with attached outer cortical cells dividing into short chains. First gonimoblast cut off inwardly, later radially from the developing fusion cell, the carposporophyte producing short chains of ovoid carposporangia (terminally mature) 20–30 µm in diameter, sometimes germinating before release. Cystocarps lying within the medulla, enveloping tissue absent, cortex thickened around the narrow ostiole. Spermatangia scattered or in small patches, with surface cortical cells cutting off 3–4 initials which form 2–3 ovoid spermatangia each 1.5–2 µm in diameter. Tetrasporangia scattered in the cortex, laterally pit-connected to an inner cortical cell, protruding into the medulla, ovoid, 40–70 µm long and 30–60 µm in diameter, zonately divided.

Distribution. Point Sinclair, S. Aust., to Western Port, Vic.

Habitat. R. clavigera is a deep-water species.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 349–350 (1994)]