Scytonema Agardh ex Bornet & Flahault (1886: 85)

Mcgregor, Glenn B., 2018, Freshwater Cyanobacteria of North-Eastern Australia: 3. Nostocales, Phytotaxa 359 (1), pp. 448-450 : 448-450

Scytonema Agardh ex Bornet & Flahault (1886: 85)

Type: S. hofmannii Agardh ex Bornet & Flahault (1886: 99)

Filamentous, thallose; solitary branched filaments or forming mats on the substrate. Filaments free or in fascicles, sometimes densely coiled, prostrate or with erect branches, solitary or geminate falsely-branched. Branching initiates after trichome dissociation following the formation of necridic cells, typically between two adjacent heterocytes, usually not at heterocytes, loop-like lateral formations may occur after which the tops of the trichomes later divide. Trichomes isopolar, cylindrical, uniseriate, constricted at the cross walls; terminal parts of branches cylindrical or slightly widened, with rounded apical cell; middle parts of trichomes sometimes with elongated, cylindrical cells. Sheaths firm, limited, parallel or diverging, lamellated, usually yellow-brown, coloured by scytonemin, particularly in mature filaments. Cells pale or olive-green, usually with solitary, irregularly disposed granules or with granular content, rarely yellowish or pinkish coloured; apical cells sometimes vacuolate. Heterocytes intercalary, solitary, rarely in pairs, cylindrical or barrel-shaped. Akinetes not produced. Cells divide crosswise to the trichome axis, mainly in meristematic zones near the ends of branches. Reproduction by hormogonia, which develop at the ends of branches and liberate from the sheaths.

A worldwide genus with 126 species currently taxonomically accepted; most are aerophytic, subaerophytic or metaphytic. Six species are described here from north-eastern Australia, four other species are known from elsewhere in Australia. Bibliography: Skinner & Entwisle (2001), Komárek (2013a, 2013b), Komárek et al. (2014), Sendall & McGregor (2018).

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Trichomes of the same width along the entire length.........................................................................................................................2 Trichomes narrowed in the central parts, distinctly widened towards the ends.................................................................................5 Vegetative cells isodiametric or shorter or longer than broad............................................................................................................3 Vegetative cells always shorter or longer than broad.........................................................................................................................4 Vegetative cells 3.5–7.5 (–13.5) μm long × 8.0–11.0 (–13.5) μm wide.............................................................................. S. coactile Vegetative cells 3.5–8.0 μm long × 4.0–6.5 μm wide.......................................................................................... S. tolypothrichoides Vegetative cells shorter than broad, 3.3–8.7 μm long × 7.1–9.3 μm wide.............................................................. S. cf. sanpaulense Vegetative cells longer than broad, 6.8–14.5 μm long × 7.9–10.5 μm wide............................................................................ S. sp. A Vegetative cells isodiametric to cylindrical, 6.5–16.0 μm long × 6.3–10.5 μm broad....................................................... S. mirabile Vegetative cells isodiametric to shortly barrel-shaped, 3.5 –5.5 μm long × (5.0–) 7.0–9.5 (–11.5) μm wide...................... S. subtile