Heterosiphonia muelleri (Sond.) G.De Toni

Scientific Description

Habit and structure. Thallus medium to dark red, erect, with one to several main axes 5–25 cm high, heavily corticated below and 3–4 mm in diameter, bearing alternately distichous lateral branches at intervals of 1–2 mm, in turn bearing alternately distichous, tufted, pseudolaterals; lower axes (probably perennial) clothed with monosiphonous, mostly unbranched, adventitious filaments. Holdfast flat to conical, rhizoidal, 2–7 cm across; epilithic or rarely epiphytic. Structure. Apices of axes and branches sympodial and distichous, developing 7–8 pericentral cells in alternating sequence, commencing 16–18 segments from the apex. Pseudolaterals 2–3(–5) segments apart, (1–)1.5–2.5 mm long, each with 3–5 lower subdichotomies (1–)2–3 cells apart and upper unbranched ends 10–15 cells long; the basal 3–4 segments usually become polysiphonous, with 4–8 pericentral cells; lower cells of pseudolaterals 70–110 µm in diameter and L/D 1–1.5, tapering gradually, upper cells 20–35 µm in diameter and L/D 1.5–2.5. Cortication commences early by small cells cut off the lower end of pericentral cells, developing rhizoidal filaments which rapidly develop a thick cortex, the cells becoming of similar size to the pericentral cells which become indistinct. Adventitious filaments 1–2 mm and 12–25 cells long, curved upwards, tapering apically and basally, some cells slightly swollen, 70–110 µm in diameter and L/D 1–1.4. Lateral branches develop from lower cells of the pseudolaterals.

Reproduction. Gametophytes dioecious. Procarps develop usually on the third subdichotomy of pseudolaterals, with the fifth pericentral cell bearing a carpogonial branch and 2 groups of sterile cells, with up to 10 and 8 cells in each group. Post-fertilization, a fusion cell occurs with an upper much branched gonimoblast bearing terminal lachrimiform to clavate carposporangia 18–25 µm in diameter. Cystocarps prominent, ovoid to subspherical, 1–2 mm in diameter, with little or no neck, borne on short polysiphonous laterals; pericarp with the erect filament cells cutting off 1–3 outer pericentral cells which form a 2–4 layered cortex of small cells. Spermatangial branches developed as branches of pseudolaterals, with a monosiphonous stalk 4–6 cells long and terminal filament of up to 10 cells, 100–300 µm long and 35–50 µm in diameter, with 10–15 fertile segments each with 4 pericentral cells producing a layer of initials each forming 3–4 outer spermatangia. Tetrasporangial stichidia occur as pseudolateral branches, usually with a polysiphonous stalk and a short terminal filament 1–6 cells long, lanceoid, 150–1500 µm long and 80–120(–230) µm in diameter, with 20–35 fertile segments each with 6 pericentral cells and ovoid tetrasporangia 40–55 µm in diameter, and 2 (subdivided) cover cells.

Distribution. Lancelin, W. Aust., to Walkerville, Vic., and N Tas. N Indian Ocean.

Habitat. H. muelleri is one of the commonest species of Heterosiphonia on southern and southwestern Australian coasts, from shallow water on rough-water coasts to deeper water on calmer coasts where current flow occurs.

[After Parsons & Womersley in Womersley, Mar. Benthic Fl. Southern Australia IIIC: 489–492 (1998)]