Mychodea australis (Zanardini) Kraft

Scientific Description

Habit and structure. Thallus dark red-brown, (3–)10–20 cm high, complanately branched with more-or-less subdichotomous, flat, main branches 2–7 mm broad, traversed by a fairly distinct central nerve (axial filament), with numerous short, marginal, compressed, lateral branchlets of varying length, 0.5–1(–2) mm broad, together with acuminate marginal spines, especially near branch apices. Holdfast crustose, 1–3 mm across; epiphytic or epilithic. Structure uniaxial, with a distinct apical cell and a prominent, relatively broad, axial filament (containing yellow material and forming the nerve) throughout, each cell producing two periaxial cells and cross-connector cells; inner medullary filaments slight around the axial filament, outer medulla of large cells without slender filaments, and the cortex of short anticlinal rows, outer cells ovoid, 2–4 µm in diameter. Rhodoplasts discoid to elongate.

Reproduction. Sexual thalli monoecious; procarpic; polycarpogonial. Carpogonial branches 3-celled, 2–4 borne on enlarged inner cortical cells near apices, auxiliary cells developing gonimoblast filaments thallus inwardly and laterally, producing clusters of ovoid carposporangia 10–15 µm in diameter within a filamentous matrix. Cystocarps developing near apices of main and lateral branches, swollen, 300–500 mm across, with slight enveloping tissue and normal cortex which disintegrates for carpospore release. Spermatangial clusters scattered on the faces of branches, sunken in the outer cortex, with 2–6 initials each producing 2 ovoid spermatangia 2–3 µm in diameter. Tetrasporangia borne in slightly raised nemathecia on the surface of branches, laterally attached to mid-cortical cells, ovoid, 35–45 µm long and 20–25 µm in diameter, zonately divided.

Distribution.King George Sound, W. Aust., to Gabo I., Vic., and around Tas.

Habitat. M. australis is not a common species but occurs on rocks, wood and also tunicates, from low tide level to 60 m deep.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 468–470 (1994)]