Polysiphonia atricapilla J.Agardh

Scientific Description

Habit and structure. Thallus dark brown-red, 4–12 cm high, much branched on all sides and densely tufted above with frequent short, patent, branchlets, with a single erect, basal axis. Holdfast discoid, rhizoidal, commonly epiphytic on Amphibolis. Structure. Basal axis suberect to very shortly prostrate, with rhizoids cut off from the lower pericentral cells. Lower main axis 0.7–1 mm in diameter with segments L/D 0.3–0.5, decreasing gradually to 400–600 µm in diameter with segments L/D 0.3–1 in mid parts and to 200–300 µm in diameter with segments L/D 0.2–0.5 in branchlets, then tapering fairly evenly to straight apices, usually with profuse trichoblasts often extending well below apices; lateral branches arising from basal cells of trichoblasts, with some cicatrigenous branches below. Pericentral cells 10–12, elongate throughout most of thallus, ecorticate; trichoblasts relatively persistent, brown, formed on every segment with a divergence of one quarter or one fifth, commonly 1–2 mm long with (2–)3(–4) furcations, basal cell 45–60 µm in diameter. Rhodoplasts ribbon-shaped.

Reproduction. Gametophytes dioecious. Carposporophytes with a small, erect, basal fusion cell and branched gonimoblast bearing ovoid to clavate terminal carposporangia 30–45 µm in diameter. Cystocarps subsessile, globular to slightly ovoid, not or slightly urceolate and with small ostiolar cells, 400–500 µm in diameter; pericarp ostiolate, 2 cells thick, outer cells isodiametric, angular. Spermatangial branches developing as one basal branch of a trichoblast, elongate-conical, tapering from near the base, 200–450 µm long and 100–150(–200) µm in diameter, without a sterile apical cell when mature. Tetrasporangia forming spiral series with a divergence of about one fifthin upper, simple, not or slightly swollen branchlets, not distorting the segments, occupying about one third of the branch diameter when mature, subspherical, 70–100 µm in diameter.

Distribution.King George Sound, W. Aust., to Guichen Bay, S. Aust.

Habitat. Most specimens of P. atricapilla are epiphytic on stems of Amphibolis or Posidonia, and the species may be largely confined to these hosts.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 209 (2003)]