Helminthora australis Levring

Scientific Description

Habit and structure. Thallus (gametophyte) red-brown, 3–20(–25) cm high, with irregular branching of several orders, varying from slender, open and sparse in calm water forms (forma epiphytica of Levring, on Posidonia) to profusely and fairly densely branched, more robust forms of broadly conical outline in medium water movement (on Posidonia or on rock) and to densely branched, robust, more narrowly pyramidal forms on rock on rough-water coasts (on forma pyramidalis of Levring). Axes 1–2 mm in diameter below, branches mostly 0.5–0.8 mm in diameter. Holdfast discoid, 1–5(–8) mm across; epiphytic or epilithic. Structure multiaxial, with a medulla 0.5–0.8 mm broad in larger branches, forming a compact strand of long, broad cells [(10–)20–40 µm in diameter], and a mature cortex (160–)200–400 µm broad, formed of tufts of anticlinal, straight or slightly curved filaments of ovoid cells mostly 8–12 µm in diameter, L/D 1.5–2.5, filaments dichotomously branched at each cell near their base, with the outer part unbranched and terminating in progressively smaller cells; slender rhizoidal filaments arise from the basal cell of cortical tufts, growing over the medulla; rhodoplasts irregularly stellate to lobed, each with a central pyrenoid; hairs usually present near apices of branches. Tetrasporophyte unknown.

Reproduction. Sexual thalli dioecious or occasionally monoecious. Carpogonial branches 4-celled, curved, laterally situated usually on the second cell from the base of a cortical branch below a dichotomy, with the carpogonium dumb-bell shaped. Zygote dividing transversely, the upper cell initiating the gonimoblast, carpogonial branch cells and especially the lower zygote cell becoming densely protoplasmic; gonimoblast compact, situated below the surface of the cortex, 150–200(–240) µm across in surface view, with terminal ovoid carposporangia mostly 10–18 µm long by 6–8 µm in diameter. Fusion cell not formed, and only occasionally do the pit-connections of the carpogonial branch enlarge. Sterile post-fertilization filaments arise from the cells of the dichotomy above the supporting cell and develop around the carpogonial branch as bridging filaments; from these, erect filaments develop to form a prominent involucre (of slenderer filaments than those of the cortex) around the carposporophyte, overarching it when mature; descending rhizoidal filaments penetrate towards the medulla, forming an indistinct and usually not strongly developed mass, 50–200(–320) µm long. Spermatangial clusters transformed from terminal 1–3 cells of cortical filaments, radially paniculate or sometimes digitate when young; spermatangial initials elongate, producing 1–3 sub-spherical spermatangia 2–3 µm in diameter.

Distribution.Cottesloe, W. Aust., to Walkerville, Vic., the N coast of Tas., and Kangaroo Point, Derwent R., Tas. New Zealand.

[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 83 (1994)]