Rhodocallis elegans Kütz.

Scientific Description

Habit and structure. Thallus erect, medium to dark red and fading to carmine, 10–20(–28) cm high, much branched complanately and pinnately with subdichotomous to irregular, terete to slightly compressed, main branches 400–800 µm broad, bearing regularly and closely arranged, distichous, compressed, simple determinate branchlets (1–)2–4 mm long, with slight, spinous or blunt, marginal serrations on their upper half (especially on the abaxial edge). Holdfast rhizoidal, 2–10 mm across; epilithic. Structure. Apical cells 10–12 µm in diameter and L/D 0.8–1, dividing by oblique alternate walls, each axial cell cutting off an initial periaxial cell, followed 1–2 axial cells lower by 2 more periaxial cells, then a fourth periaxial cell several axial cells below, formed in alternating sequence. The second and third formed periaxial filaments become transverse and are caducous (leaving only basal tissue) within a few mm of the apices, leaving the first formed filaments to form alternate, distichous, determinate laterals which become compressed, curved towards the branch apices, and have an apical spine and spines on the abaxial edges. Axial cells increasing to 120–180 µm in diameter and L/D 2–4 in the indeterminate axes; all branches becoming corticated by compacted filaments of short cells, arising from basal cells of the determinate laterals, with indeterminate branches producing rhizoidal filaments around the axial cells; these rhizoidal filaments extend out between the cortical cells close to the holdfast to form a hirsute surface coating. Lateral branches originate by continued growth of the apical cells of the distichous branchlets, with transition to spiral development. Cells uninucleate; rhodoplasts discoid in small cells, ribbon like in larger cells.

Reproduction. Gametophytes dioecious. Carpogonial branches occur in short series on successive axial cells of dwarf indeterminate axes, on the second and third formed periaxial cells which become the supporting cells, bearing 4-celled carpogonial branches, without sterile cells. Post-fertilization the supporting cell cuts off laterally an auxiliary cell, which, after nucleus transfer via a connecting tube, divides to form a foot cell and a gonimoblast initial, developing 3–6 gonimolobes each 400–600 µm across of ovoid carposporangia 20–30 µm in diameter. Rhizoidal involucral filaments arise from the basal cell of the sterile branchlet on the fertile axial cell and form a dense envelope around the developing carposporophyte, with the gonimolobes extending beyond the envelope at maturity; the carposporophyte is also surrounded by incurved determinate branchlets. Spermatangia occur on the distichous branchlets and near the apices of indeterminate branches, with initials arising directly from outer cortical cells. Tetrasporangia occur on distichous branchlets near the tips of indeterminate axes, around the branchlets or in linear sori along the adaxial edges, developed directly from surface cortical cells, subspherical, 40–50 µm in diameter, tetrahedrally divided.

Distribution.Isles of St Francis, S. Aust., to Port Phillip Heads, Vic., and E Tas.

Habitat. R. elegans is found in deep water on rough-water coasts.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIC: 364–366 (1998)]