Habit and structure. Thallus rose-red to dark red-brown, 5–16 cm high, erect and much branched, spreading, variable in robustness, mucilaginous, with a main axis 2–3 mm in diameter bearing irregularly radially arranged laterals to 3 or 4 orders, terete and tapering and 0.5–1 mm in diameter in lesser branchlets which are alternate to opposite. Holdfast discoid, 1–3 mm across; epiphytic on Amphibolis, Heterozostera or larger algae, or possibly epilithic. Structure uniaxial, with an apical cell dividing transversely and producing a row of discoid cells which become cylindrical axial cells 300–400 µm long and 50–250 µm in diameter when mature, with 4(–6) periaxial cells each bearing tufted fascicles basally divided at each or most cells below, then unbranched and when mature gently tapering, usually 12–16 cells long with cells (5–)8–14 µm in diameter and L/D 2–3(–5). The outer medulla grades to the cortex and the outer cortical cells near branch apices bear long, slender, floridean hairs. Lateral branches arising from periaxial or inner cortical cells, first developing as unbranched primordia of 15–20(–40) discoid cells. Young thallus branches usually show slight annulations corresponding to the periaxial whorls. Each periaxial cell also produces a descending rhizoid of elongate cells, each cell of which may produce a lateral branch system similar to and lying between the periaxial whorls, and resulting in a more continuous medulla and cortex. The descending rhizoids pass down over two or three axial cells and thus each axial cell becomes surrounded by 12–18 rhizoids, which ultimately enlarge to similar diameter as the axial cells and tend to obscure the latter. In the mature thallus, numerous secondary descending rhizoids develop from other cells of the periaxial laterals, forming an interwoven filamentous medulla.
Reproduction. Sexual thalli usually dioecious. Carpogonial branches develop usually on periaxial cells of lateral thallus branches, 6–10 cells long with the upper 3 cells reflexed and cells 2 and 3 smaller; the basal or sub-basal cell frequently bears a single lateral cell. Following fertilization, an extension from the carpogonium fuses with cell 4 and frequently extends to cell 5, and the resultant large fusion cell produces several connecting filaments each with a basal pit-connection. Auxiliary cell branches develop from periaxial cells and also from basal cells of lateral branch systems developed from the descending rhizoids, and thus considerably outnumber the carpogonial branches. Auxiliary cell branches 11–20 cells long, with 3–5 mid cells larger and the central of these (often fifth from the base) is the smaller, compressed, auxiliary cell; the terminal 6–8 cells often taper and a sterile lateral cell may occur on the basal cell as in carpogonial branches. Nearly all auxiliary cells are contacted by connecting filaments and two gonimoblast initials develop opposite the fusion side of the auxiliary cell. Carposporophytes compact (initially two lobed), of different ages, scattered profusely in the outer medulla but scarcely swelling the thallus, 100–175 µm in diameter with most cells forming rounded to angular carposporangia 10–27 µm in diameter. Male gametophytes with spermatangia formed in dense clusters from 1–4 terminal and subterminal cells of the outer cortex. Tetrasporophytes have denser rhizoidal development than gametophytes, and this obscures the basic whorled branch pattern to a greater extent. Tetrasporangia large, zonately divided, 25–45(–50) µm long and 12–20 µm in diameter, terminal or lateral on outer medullary cells of both the periaxial branch systems and those arising from the descending rhizoids. Further tetrasporangia may arise from short branches originating below the earlier tetrasporangia.
Distribution. King George Sound, W. Aust., to Western Port, Vic., and the N coast of Tas.
Habitat. D. australis is a sublittoral species known from 5 to 33 m deep and is commonly epiphytic on the seagrass Amphibolis, but also on the brown alga Caulocystis.
[After Womersley, Mar. Benthic Fl. Southern Australia IIIA: 225–226 (1994)]
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