Heterosiphonia gunniana (Harv.) Reinbold

Scientific Description

Habit and structure. Thallus medium to dark red-brown, erect, with one to several main axes 10–20(–30) cm high and strongly developed lateral branches, denuded below, axes 1–1.5(–2) mm in diameter where heavily corticated, lateral branches alternately distichous, mostly 2–10(–50) mm apart, usually with further similar branching. Lesser branches terminating in flabellate tufts of rhodoplastic, ecorticate, basally polysiphonous but mostly monosiphonous, filaments to 4 mm long. Holdfast discoid, 2–15 mm across; epilithic. Structure. Apices of axes sympodial and distichous, developing 8–10(–12) pericentral cells, commencing 10–16 segments below the apical cell, produced in alternating sequence. Pseudolaterals (2–)3–5(–7) segments apart, each with several slender subdichotomous, tapering, upper monosiphonous filaments, lower cells 30–70 µm in diameter and L/D 1.3–3, tapering gently to upper cells 6–12 µm in diameter and L/D 8–15; the basal 5–7 segments becoming polysiphonous with 4–9 pericentral cells; segments 100–175 µm in diameter and L/D 0.8–2 (longer in deep water plants). Cortication commences well below the apices, by small cells cut off from the base of pericentral cells, developing into descending rhizoids which form a cortex 4–8 cells thick, outer cortical cells 12–25 µm in diameter, with contorted rhizoids. Lateral branches develop from lower cells of pseudolaterals. Rhodoplasts discoid, in chains in larger cells.

Reproduction. Gametophytes dioecious. Procarps develop on lower segments of the pseudolaterals subtending a subdichotomy, which develop 5 pericentral cells, the fifth formed bearing a carpogonial branch and 2 sterile groups which divide to form a number of pericarp initials. Post-fertilization, the auxiliary cell, residual supporting cell, axial cell and lower gonimoblast cells form a fusion cell below the much branched gonimoblast with terminal or short rows of ovoid carposporangia 45–65 µm in diameter, with later carposporangia developing from below. Cystocarps large, ovoid to subspherical, 1–1.5 mm in diameter, with little or no neck, on polysiphonous branches, often with remains of monosiphonous filaments protruding through the pericarp which is corticated with 3–4 layers of cells. Spermatangial branches occur as branches of the pseudolaterals, cylindrical with a short monosiphonous stalk and a terminal filament 6–12 cells long, 150–300 µm long and 30–50 µm in diameter, the axial cells with 5 pericentral cells which divide to form initials each producing 4–5 spermatangia. Tetrasporangial stichidia occur on basal polysiphonous segments of the pseudolaterals, cylindrical to lanceoid, 500–1500 µm long and 250–400 µm in diameter, terminated by a monosiphonous filament 4–8 cells long, with 14–25 fertile segments; each segment with 6 or 7 pericentral cells, all producing subspherical to slightly ovoid tetrasporangia 100–150 µm in diameter, decussately divided, each covered by the 4 cover cells.

Distribution. Fremantle, W. Aust., to Walkerville, Vic., and around Tas.

Habitat. H. gunniana is a common species, from shallow to deep water, often prolific in pools and on reef edges under rough-water conditions.

[After Parsons & Womersley in Womersley, Mar. Benthic Fl. Southern Australia IIIC: 496–499 (1998)]