Thuretia australasica (Sond.) M.J.Parsons

Scientific Description

Habit and structure. Thallus erect, tufted, with terete axes 3–7 cm high and 2–4 mm in diameter bearing alternate and more or less distichous laterals 3–10 mm long, with obtuse to truncate apices. Holdfast discoid, 1–4 mm across, usually with several axes; epilithic or epiphytic. Structure. Apices of axes and laterals sympodial, forming alternately distichous pseudolaterals which become semicircular with laterals developing to similar length and thus producing the terete network of monosiphonous filaments, with outer, free, short filaments 200–500 µm long. Cells of network 20–50 µm in diameter and L/D 1–4, those of outer free filaments 40–60 µm in diameter and L/D (0.2–) 0.6–1. Pericentral cells 4, formed in alternating sequence, becoming corticated from close to the apex, the thick mature cortex enveloping the basal cells of the pseudolaterals. Lateral axes arise from basal cells of pseudolaterals. Rhodoplasts discoid, becoming elongate or chained in larger cells.

Reproduction. Gametophytes dioecious. Procarps occur on lower cells of the pseudolaterals, with 5 pericentral cells the fourth of which (occasionally also the fifth) is the supporting cell and cuts off the carpogonial branch and 2 groups of sterile cells. Post-fertilization the fusion cell develops from the auxiliary cell, central cell of the fertile segment, sterile pericentral cells and lower gonimoblast cells, and the sterile groups divide. The much branched gonimoblast bears terminal clavate to ovoid carposporangia 20–25 µm in diameter. Cystocarps lie within the monosiphonous network, urceolate and 0.6–1.2 mm in diameter with a relatively long protruding neck, opening straight to flared; the pericarp arises pre-fertilization from the sterile pericentral cells and becomes 4–5 cells thick. Spermatangial branches ovoid, 60–100 µm in diameter, borne terminally on short monosiphonous stalks within the network, 3–6 cells long with each cell forming 4–5 pericentral cells which cut off initials each of which produces 3–4 spermatangia. Tetrasporangial stichidia develop from lower branched parts of the pseudolaterals, becoming 300–500 µm and 10–15 segments long, with each central cell cutting off 6–10 pericentral cells most of which form tetrasporangia 25–50 µm in diameter and 3 pre-sporangial cover cells.

Distribution. Hopetoun, W. Aust., to Wilsons Promontory, Vic.

Habitat. T. australasica occurs just below low tide level and deeper on rough-water coasts, both on larger algae or Amphibolis or directly on rock.

[After Parsons & Womersley in Womersley, Mar. Benthic Fl. Southern Australia IIIC: 506–508 (1998)]