Melanothamnus nudus (Mamoozadeh & Freshwater) Freshwater

Scientific Description

Habit and structure. Thallus mostly upright, to 12 mm tall, pale red, from limited prostrate axes. Prostrate axes 300–340 µm diam. [segment L:B 0.5–0.6], attached by slender rhizoids arising 1–5 from pericentral cells. Rhizoids unicellular with digitate ends, 35–60 µm diam., pit-connected to the bearing cell, mostly from proximal ends but also from lower lateral face of cell. Upright axes subdichotomously branched every 8–25 segments. Lower axes 250–300 µm diam. [segment L:B c. 1]; mid-thallus segments 145–170 µm diam. [L:B 1–2], tapering to filiform upper branches 40–50 µm diam. [segment L:B 1–3]. Axes with 4 pericentral cells, except in fertile segments of tetrasporophytes where 3 cover cells are formed from the reduced fertile pericentral cell that bears the tetrasporangium, resulting in segments with 6 cells (3 pericentral cells and 3 cover cells). Trichoblasts absent on tetrasporophytes, simple and rudimentary on female gametophytes, forming spermatangial branches on male gametophytes.

Reproduction. Spermatangia formed in heads replacing trichoblasts, mostly in unilateral series on 3 or 4 consecutive upper segments, 40–50 µm diam., 15–18 cells and 290–400 µm long, with a naked basal cell and a sterile tip 1 or 2 cells long. Procarps with an associated trichoblastic filament composed of 3 or 4 subspherical cells, each bearing a single-celled lateral branch, these loosely enveloping the trichogyne. Cystocarps ellipsoidal when young, becoming spherical when mature, stalked, to 300 µm diam., with lachrymiform carpospores 70–95 µm long and 25–35 µm diam. Pericarp with enlarged ostiolar cells. Tetrasporangia formed in straight series of up to 25 sequentially maturing sporangia, the proximal 6–8 mature and slightly to distinctly distending the bearing segment, ellipsoidal, 80–90 µm long, 50–70 µm diam.

Distribution. Known from the Houtman Abrolhos Islands and Montebello Islands, Western Australia, Coffs Harbour, New South Wales, and probably south-eastern Queensland.

Habitat. epiphytic on Dichotomaria marginata and Titanophora pikeana.

[After J.M. Huisman in Algae of Australia: Marine Benthic Algae of North-western Australia, 2. Red Algae: 545–547 (2018)]