Dichotomaria marginata (J.Ellis & Sol.) Lam.
Hist.Nat.Anim.Verteb. 2 (1816)
Conservation Code:
Not threatened
Naturalised Status: Native to Western Australia
Name Status:
Current
Scientific Description
John Huisman & Cheryl Parker,
Monday 20 June 2011
Habit and structure. Thallus dark red-brown, often grey-red above, 4–15(–20) cm high, much branched subdichotomously usually every 0.5–2 cm, branches flat, superficially smooth surfaced, (1–)1.5–2.5(–3) mm broad, 300–500 µm thick, not jointed but often with transverse banding and a thicker margin forming a ridge on drying; such branch systems arising from branched, terete, tomentose stipes 0.2–3 cm long and 1–3 mm in diameter, often with a tuft of longer filaments at the stipe apices and below the flat branches. Holdfast discoid, 0.5–2 cm across; epilithic. Structure of a pseudoparenchymatous cortex 2–3 cells thick, cells more or less isodiametric to ovoid, 30–45 µm across, with a central medulla of coarse filaments 10–15 µm in diameter with cells mostly L/D 4–8. Cortex in tetrasporophytes bearing a surface layer of 2 or 3-celled filaments, separated and with calcification between them on the pseudoparenchymatous surface, the outermost cells more or less pyriform, outwardly truncated, 20–30 µm in diameter and 30–40 µm long; in gametophytes, outer cortical layer compact, of isodiametric cells, polygonal in surface view and outwardly truncated, (12–)15–20 µm across, usually (especially in female plants) each bearing a clavate cell 20–25 µm long and 10–14 µm in diameter, with rounded to slightly apiculate ends; calcification in gametophytes in the outer medulla. Rhodoplasts campanulate with a central pyrenoid.
Reproduction. Sexual thalli dioecious. Carpogonial branches 3-celled, borne in the young cortex, with the hypogynous cell cutting off four large-celled branches and the basal cell four branches of smaller cells. Gonimoblast filaments from the fertilized carpogonium curve towards the surface to form the cystocarp wall. Cystocarps 300–600 µm in diameter, ostiolate, producing sessile or shortly pedicellate carposporangia, clavate when young, ovoid when mature, (25–)40–60 µm long and (12–)20–40 µm in diameter. Male conceptacles 200–400 µm in diameter, with tufts of branched filaments projecting inwards from the wall, with terminal and lateral ovoid spermatangia 4–6 µm in diameter and 6–8 µm long. The tetrasporophyte with cruciately divided tetrasporangia, subspherical, 30–40 µm in diameter, borne laterally below the terminal cell of the short filaments on the cortex.
Distribution. Widespread in tropical and subtropical seas. Extending from western and eastern Australian coasts around southern Australia and Tasmania.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 113–115 (1994) added as Galaxaura marginata]
