Halicnide similans (J.Agardh) J.Agardh

Scientific Description

Habit and structure. Thallus light red to dark red-brown, 3–14 cm high, complanately branched usually with a single main blade (0.5–)1–3(–4) cm broad, bearing from the margins flat lateral blades (0.5–)1–4 cm long and 3–10 mm broad, all blades basally constricted to a short stipe (0.2–)0.5–1 mm in diameter; midrib becoming heavily corticated, bearing opposite lateral veins, blade margin and the older surface bearing short, spinous, multicellular filaments. Holdfast discoid, becoming stoloniferous and fibrous; epilithic. Structure. Growth by means of a dome-shaped to obconical apical cell, segmenting within 1 or 2 segments to lateral and transverse pericentral cells, the lateral cells forming second-order rows with some cells dividing irregularly to cut off third-order cells both abaxially and adaxially, forming a compact but irregular cell pattern. Original second-order rows become 2–4 axial cells apart due to intercalary divisions and form the opposite lateral veins which become heavily corticated, with the blade between veins remaining monostromatic or becoming 3 cells thick with cortical cells on each side; thallus sectional view regular. Second-order cell rows project beyond the blade margin as prominent, branched, tapering spines 200–300 µm long, base 80–200 µm and 3–5(–10) cells across with corticating cells, the upper 4–8 cells uniseriate and 15–20 µm in diameter, cells isodiametric. Smaller but similar spines are formed from any marginal cells, and the veins and surface cells of older blades form, sparsely to densely, simple or occasionally basally branched, uniseriate spines mostly 90–200(–500) µm and 6–25 cells long, 15–30 µm in diameter. Mature cells multinucleate; rhodoplasts discoid.

Reproduction. Gametophytes dioecious. Procarps scattered on blades. Carposporophytes with a basal fusion cell and branched gonimoblast filaments with terminal chains of 3–5 ovoid carposporangia 15–20 µm in diameter. Cystocarps few per blade, 1–1.5 mm across, markedly swollen on both sides of the blade; pericarp 250–350 µm thick, with compact anticlinal rows of isodiametric cells 15–20 µm in diameter, the surface cells often projecting as a short uniseriate spine. Spermatangial sori extensive, covering most of the blades on both sides, with cortical cells dividing further and each cell producing several spermatangia. Tetrasporangial sori in small, compressed and ovate, marginal leaflets which develop profuse surface spines; tetrasporangia cut off from inner cortical cells on both sides, covered by smaller outer cortical cells, subspherical, 30–40 µm in diameter, with varying ages in the sorus.

Distribution.Penguin I., Safety Bay, W. Aust., and Pearson I.,S. Aust., to Point Hicks, E Vic., and E and S Tas.

Habitat. H. similans is a deep-water species, most common in Tas., where it also occurs in shallower but shaded situations.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 75–77 (2003)]