Dipterosiphonia prorepens Falkenb.

Scientific Description

Habit and structure. Thallus dark red-brown, densely clothing the host for up to several cm, 2–6 mm high (thick) with prostrate axes producing alternate pairs of determinate laterals which soon become more-or-less erect and closely adjacent on the axes, with the branches of the anterior laterals developing as well as the posterior laterals; the anterior laterals are flexed forward on the axes and the posterior one emitted more directly. Attachment by rhizoids; epiphytic, especially on the stems of Amphibolis. Structure. Axial apical cells hemispherical, 12–20 µm in diameter, segmenting transversely and the subapical cells obliquely in the pattern of the genus, with the anterior determinate lateral branched and the posterior simple, all soon flexed upwards and free from the substrate. Axes and laterals with (7–)8 pericentral cells, axes 130–200 µm in diameter with segments L/D 0.3–0.5, determinate laterals terete, 50–120 µm in diameter with segments L/D 0.2–0.5(–0.8), tapering slightly throughout. Trichoblasts arising from subapical cells of determinate laterals, 300–1000 µm long, much branched, basal cells 16–30 µm in diameter, L/D 1–2, longer and slenderer above. Rhizoids cut off from axial pericentral cells, unicellular with multicellular haptera. Cells uninucleate; rhodoplasts discoid to elongate, chained in larger cells.

Reproduction. Gametophytes dioecious. Procarps on subrabasal cell of trichoblasts, soon polysiphonous. Carposporophytes with a small basal fusion cell and a much branched gonimoblast with clavate terminal carposporangia 20–30 µm in diameter. Cystocarps lateral on determinate laterals, short-stalked, ovoid, 450–650 µm in diameter; pericarp ostiolate, 2 cells thick, ecorticate, outer cells angular, isodiametric. Spermatangial organs replacing trichoblasts, tapering, 80–180 µm long and 35–55 µm in diameter, with a sterile basal cell and usually 2–3 sterile apical cells. Tetrasporangia in determinate laterals, usually in straight series, not or slightly swelling the segments, 40–80 µm in diameter, with 2–3 cover cells.

Distribution.Kalbarri, W. Aust., to Kingston, S. Aust.

Habitat. D. prorepens is largely restricted to the stems of the seagrass Amphibolis, occasionally on other algae.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 306–308 (2003)]