Arctomia borbonica Magain & Sérus.

Thallus not exceeding 1 cm in diam., with distinct lobes when well-developed, lobes blue-grey to brown when dry, up to 0.2-0.3 mm wide and c. 200-400 µm thick, hardly distinguished in some specimens, with a surface typically wrinkled (even in young lobes), sometimes very much “crumpled”, always developing small goniocysts, mainly at the margins but also on the upper surface; cortex (Fig. 2C–E) developed on upper and lower sides, formed by a single layer of small rounded (in cross section) and jigsaw-like (in surface view) cells, less than 5 µm thick; goniocysts (Fig. 2F) 20-80 µm across, always containing compact chains of Nostoc cells and covered by a layer of isodiametric to rounded cells, 2–5 µm, best developed at the lobes margins where they eventually form a typical pale brownish soredioid edge, due to cortical disintegration. Photobiont belonging to the cyanobacteria genus Nostoc forming chains of small rounded cells 2–5 µm in diam. Ascomata and conidiomata unknown.

Arctomia borbonica has been collected at three different sites on the island of Reunion in the Mascarene archipepago, incl. in highly disturbed secondary tickets with Eucalyptus plantations; it grows on trunks (Eucalyptus, Acacia heterophylla) or on main stems of Erica tickets. It is probably widespread on the island. The two localities with natural vegetation belong to two different and typical habitats. The first one is the margin of the “Forêt de tamarins des hauts” with large boles of the endemic tree Acacia heterophylla (locality at the nature reserve “Roche Ecrite”, at 1500 m) and corresponds to the “Acacia mountain forest” in Strasberg et al. (2005). The other one is the wet upper montane ericoid tickets (type locality; locality in the Bébour forest at 1800–1850 m) and corresponds to the “Philippia mountain ticket” in Strasberg et al. (2005). Here the vegetation does not exceed 4–5 m in height and is formed by Erica arborescens, Erica montana, Eugenia buxifolia, Agauria buxifolia, Cordyline mauritiana (locally very abundant), Cyathea sp., Phylica nitida, Astelia hemichrysa, Blechnum attenuatum; ground is covered by very thick (up to 80 cm) layer of Sphagnum and other bryophytes. It is one of the most rewarding habitat for lichens on Reunion, with many interesting species, including representatives of the austral element (van den Boom et al. 2011), such as Gomphillus morchelloides, Gomphillus pedersenii and Sporopodiopsis mortimeriana.