Polysiphonia subtilissima Mont.

Scientific Description

Habit and structure. Thallus dark red-brown, (1–)4–12 cm high, very slender, lax, much branched, arising from prostrate and entangled basal filaments. Attachment by rhizoids; epilithic (or on wood) or epiphytic. Structure. Prostrate filaments 100–180 µm in diameter with segments L/D 1–1.5(–2), attached by scattered rhizoids in open connection with the pericentral cells, and with occasional endogenous branches; erect main filaments usually 70–180 µm in diameter with segments L/D 1.5–2(–6) below, tapering gradually to 35–60 µm in diameter above with segments mostly L/D 1–2(–3), irregularly branched but usually with terminal flat-topped branch systems; branches arising exogenously usually every 3–7 segments near apices, independent of trichoblasts, spirally arranged on a phyllotaxis of one third or one quarter usually continuing as indeterminate laterals; branches 15–25 µm in diameter several segments from the apices, segments L/D 0.5–1, apical cells relatively prominent. Pericentral cells 4, ecorticate; trichoblasts and scar cells rare to occasional in vegetative and tetrasporangial thalli, more frequent in sexual thalli; trichoblasts slender, lower cells 8–16 µm in diameter, simple to twice furcate. Rhodoplasts discoid, scattered or in chains.

Reproduction. Gametophytes dioecious. Carposporophytes with a small basal fusion cell and short branched gonimoblast with clavate terminal carposporangia 20–30 µm in diameter. Cystocarps short-stalked, ovoid to slightly urceolate, 120–450 µm in diameter; pericarp ostiolate, 2 cells thick, outer cells isodiametric, angular. Spermatangial branches 150–350 µm long and 30–65 µm in diameter, replacing the whole trichoblast and with a sterile tip 4–6 cells long. Tetrasporangia forming a relatively uniform straight series in ultimate or subultimate slender branchlets 60–80 µm broad, swelling each segment slightly, 35–50 µm in diameter.

Distribution.Tropical and subtropical eastern America; Korea; Hawaiian I, Indian Ocean. In southern Australia, from Swan R. estuary, W. Aust., to Port Phillip, Vic. and around Tas, and to Towra Point, Botany Bay, N.S.W.

Habitat. P. subtilissima usually occurs in sheltered bays in shallow water or just intertidal.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 178–180 (2003)]