Ceramium isogonum Harv.

Scientific Description

Habit and structure. Thallus grey-red to red-brown, 0.5–3(–6) cm high in rough-water forms, to 12 cm high under slight to moderate water movement, with one to several axes arising from prostrate basal parts. Branching regularly pseudodichotomous, rarely with a few proliferous laterals, branches in rough-water forms 400–700 µm in diameter below, tapering gradually to about 200 µm near the apices, in slight water movement 200–400 µm in diameter below, tapering to about 100 µm near the apices; apices usually involute. Attachment by numerous rhizoids which arise from the periaxial and inner cortical cells and consist of one to a few uniseriate cells and a digitate pad which becomes multicellular; epilithic or epiphytic on various algae or Posidonia. Structure. Axial cells L/D about 1(–1.5) in rough-water forms, extending in lower parts of plants in slight water movement to L/D 1.2–3(–4), with distinct internodal spaces throughout and regular, well defined, nodal cortical bands. Periaxial cells 7–8(–9), each cutting off two cells acropetally and basipetally, these cutting off (1–)2 further cells, which divide further in older parts to form nodal bands 5–7 cells long and usually broader than long; ultimate acropetal cortical cells generally smaller than basipetal ones, developed more-or-less synchronously, leaving a very short (especially in rough-water forms) and well defined internodal space in the younger branches, this space in older parts becoming as long as the nodal band in rough-water forms and up to 4 times as long in forms in slight water movement. Outer cortex developed from the larger inner cortical cells and periaxial cells, with small cells which may appear as rosettes. Darkly staining gland cells occur occasionally to profusely, sometimes absent, derived from the smaller outer cortical cells, scattered over the nodes. Rhodoplasts discoid to elongate in cortical cells, ribbon like in axial cells.

Reproduction. Gametophytes dioecious. Carposporophytes 150–350 µm across, with 2–4 short involucral branchlets; carposporangia ovoid, 20–30 µm in diameter. Spermatangia cut off from outer cortical cells, covering the node on all sides. Tetrasporangia prominent, external and completely naked, derived initially from the first acropetal cortical derivatives of the periaxial cells and thus forming an irregular ring around the upper edge of the nodal cortication, later from other cortical cells, decussately or tetrahedrally divided, subspherical, 50–80 µm in diameter.

Distribution. Houtman Abrolhos, W. Aust., to Bondi N.S.W. (and Lord Howe I.) and the Great Barrier Reef, Qld; Tas (?).

Habitat. On rough-water rock platforms just below low tide level (stouter forms) and also 2–4(–8) m deep in conditions of slight to moderate water movement.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 402–404 (1998)]