Sphacelaria biradiata Askenasy

Scientific Description

Habit and structure. Thallus medium brown, 0.2–1.0(–1.5) cm long, forming loose to (usually) dense, much branched, tufts or mats on larger algae (especially Fucales) or seagrasses, with many axes arising from a compact, cellular-filamentous base on the host surface. Branching of main axes frequent, irregularly radial, alternate or occasionally opposite, with 2–3 orders of largely determinate laterals each slenderer than the parent branch and arising at moderate to broad angles; phaeophycean hairs abundant, 12–18 µm in diameter. Axes (40–)50–80(–90) µmin diameter with segments L/B(0.5–)0.6–0.8(–1.0) and showing (3–)4–7(–11) longitudinal walls; laterals 25–40(–45) µmin diameter with segments L/B(0.4–)0.6–1.0(–1.2) and showing 2–4(–5) longitudinal walls; secondary transverse walls absent except where lateral branches or reproductive structures arise; pericysts frequent.

Reproduction. Propagula with two arms, each basally constricted and tapering above, straight to curved, 80–160(–200) µmlong and (20–)25–40 µmin maximum diameter, borne on a pedicel 80–160 µmlong, and with a long apical hair 8–12 µmin diameter. Unilocular sporangia scattered, with a short, unicellular pedicel, subspherical, (50–)60–80 µmin diameter. Plurilocular organs scattered, borne on pedicels 1–3 cells long, ovoid to obovoid, 40–60(–75) µmlong and 30–40(–60) µmin diameter, with small loculi.

Distribution. From Rottnest I., W. Aust., around southern Australia and Tasmania to Walkerville, Vic.

Habitat. S. biradiata is the commonest epiphytic Sphacelaria on southern Australian coasts, occurring on many larger Fucales and cartilaginous red algae, as well as on seagrasses.

[After Womersley, Mar. Benthic Fl. Southern Australia II: 162–164 (1987)]