Scoresbyella profunda Womersley

Scientific Description

Habit and structure. Thallus medium to dark brown below, light brown near apices, 6–19 cm long, complanate and moderately branched, with a single axis arising from a rhizoidal holdfast 2–5 mm across and 2–5 mm long; epilithic. Fronds branched dichotomously at apices, becoming laterally branched at intervals of (0.2–)0.5–2(–3) cm, branches entire, 3–5 mm broad near the truncate to slightly concave apices, mostly 5–10 mm broad throughout the main thallus, decreasing to 1–3 mm broad near their bases. Growth from a single apical cell in an apical depression, lenticular in surface view and rectangular in transverse section, segmenting on both lateral sides; daughter cells dividing transversely and then with two walls parallel to the thallus surface, giving three layers, each row of daughter cells with an apical cell which segments in similar manner and lies on one side of the apical groove, separated from similar cells on the other side by a gelatinous plug. Structure of mature thallus at first with division of the cortical layer giving two layers of fairly densely phaeoplastic cells, the inner layer of which enlarges and loses most of its phaeoplasts, while the central layer becomes distinctly larger. Further divisions of the cortical and outer medullary cells produce a 6–8 cell thick medulla, with the inner cells progressively larger and arranged more or less in layers but not in regular transverse rows. Thallus near apices 60–80 µmand 3 cells thick, slightly thicker near margins, mid thallus 150–300 µm and 5–8 cells thick, and near the holdfast biconvex in section, 400–500 µmand 8–12 cells thick. Cortical cells in vague lines, becoming irregular, isodiametric to L/B 2, 10–25(–40) µm across; phaeoplasts discoid, numerous per cell, without pyrenoids. Hair tufts scattered, phaeophycean, external to the cortex; hairs densely clustered, (10–)12–16 µm in diameter.

Reproduction. Sporangial and oogonial plants unknown. Male plants with scattered, often confluent, sori, irregular in shape and mostly 0.5–1.5 mm across, with elongate, sterile paraphyses surrounding the sori. Antheridia densely packed, formed from each cortical cell, elongate, (60–)80–120 µm long and 15–25 µm in diameter.

Distribution. Known from Rottnest I., W. Aust., to York Pen and Investigator Straight, S. Aust.

Habitat. S. profunda appears to be a rare alga, confined to deep water.

[After Womersley, Mar. Benthic Fl. Southern Australia II: 257–259 (1987)]