Polysiphonia scopulorum Harv.

Scientific Description

Habit and structure. Thallus dark red-brown, 5–15(–40) mm high, usually forming spreading mats on rock platforms just emergent at low tide, sometimes as dense tufts, normally with an extensive and relatively prominent prostrate basal system bearing erect branches, simple or usually with few laterals (occasionally moderately branched). Attachment by rhizoids from prostrate filaments; epilithic, occasionally epiphytic (on Scaberia and Amphibolis). Structure. Prostrate filaments 80–130 µm in diameter with segments L/D 0.5–1.5, attached by scattered rhizoids in open connection with pericentral cells, with actively developing apices and producing adventitious erect branches usually arising several segments from the apices; erect branches 80–120(–140) µm in diameter with segments L/D 0.5–1.5, often slightly narrower towards their base, bearing a few lateral branches of similar diameter and arising independently of trichoblasts, probably mainly cicatrigenously. Pericentral cells 4, ecorticate; trichoblasts and scar cells occasional only on lower parts of erect branches, more frequent and sometimes on every segment with a phyllotaxis of one quarter near well-developed apices; trichoblasts slender, several times furcate. Rhodoplasts discoid, scattered.

Reproduction. Gametophytes dioecious. Carposporophytes with a small basal fusion cell and branched gonimoblast with clavate terminal carposporangia 20–30 µm in diameter. Cystocarps short-stalked, globular to slightly urceolate, 300–400 µm in diameter; pericarp 2 cells thick, outer cells isodiametric, angular. Spermatangial branches developed commonly from both basal branches of a trichoblast and thus appearing paired, sometimes replacing whole trichoblast, 120–200 µm long and 40–50 µm in diameter, without sterile tip cells. Tetrasporangia forming slightly spiral series in upper laterals, slightly swelling and distorting the lateral (100–200 µm in diameter), sporangia 50–60(–70) µm in diameter.

Distribution. Dampier Archipelago, W. Aust., to Barwon Heads, Vic, N.S.W. and southern Qld.

Habitat. P. scopulorum occurs mainly on rough-water rock platforms or in relatively calm localities, usually forming spreading mats on rock platforms just emergent at low tide, occasionally sublittoral on older axes of Scaberia.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 175–178 (2003)]