Dictyomenia harveyana Sond.

Scientific Description

Habit and structure. Thallus medium to dark brown-red, erect, 10–40 cm high, complanately and usually regularly pinnately branched for 3 or 4 orders, with flat main axes (1–)2–5 mm broad bearing lateral branches 2–15 mm apart, the lesser branches with alternate determinate laterals (bearing trichoblasts), 2–3 mm long and 2–3 mm apart, with branches more-or-less terete. Midrib of main branches present but not conspicuous, lateral veins also inconspicuous. Main branches corticated, heavily so near thallus base where blades are lost, giving a subterete stalk 1–3 mm thick. Holdfast branched or fibrous, 5–15 mm across; epilithic or on shells. Structure. Apical cell hemispherical to dome-shaped, 12–16 µm in diameter, with the axial cells branching alternately distichously 2–4 cells apart, cutting off 6 pericentral cells with 2 larger lateral ones and 2 smaller ones on each side, but of equal size in the terete branches of the ecorticate determinate laterals (which can become indeterminate). Lateral pericentral cells of the indeterminate branches divide laterally to form rows of 4–6 cells in the 2–5 mm broad branches (fewer in slender thalli) which become lightly corticated from close to the apices. Determinate branches 60–270 µm in diameter, with segments L/D (0.6–)0.8–1.5, with trichoblasts from each segment near their apices, branched, 1–2 mm long, lower cells 20–30 µm in diameter and L/D 3–4 (basal cells isodiametric). Cells uninucleate; rhodoplasts discoid.

Reproduction. Propagules often formed abundantly and terminally on determinate branches, flat, ovate, 1–1.5 mm long, 400–600 µm broad, with 3–4 short spinous laterals and with a dormant apex, borne on a slender terete stalk. All reproductive organs occur on the marginal determinate laterals. Gametophytes dioecious. Procarps borne on the second cell of trichoblasts, which rapidly cuts off 5 pericentral cells, with the supporting cell cutting off a 4-celled carpogonial branch and a group of sterile cells. Carposporophytes with a basal fusion cell and branched gonimoblast with elongate-ovoid to clavate terminal carposporangia 40–80 µm in diameter. Cystocarps on branches of determinate laterals, short stalked, subspherical to ovoid, 500–1200 µm in diameter; pericarp ostiolate, 3 cells thick, corticated. Spermatangial organs replacing trichoblasts, elongate and tapering apically with 1(–2) sterile basal cells and a sterile apical cell, 40–80 µm in diameter and 180–360 µm long. Tetrasporangia in terete branches of the determinate laterals, single per segment, in slightly spiral rows with 1–4 mature, 120–180 µm in diameter, with 2 cover cells.

Distribution.Port Denison, W. Aust., to Green Cape, N.S.W., and around Tas.

Habitat. D. harveyana is mostly a deep-water species, though in shallow water on Tasmanian coasts.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 349–351 (2003)]