Stellarangia namibensis Kärnefelt) Frödén, Arup & Søchting was originally described by (Kärnefelt (1988)Kärnefelt I. 1988. Two closely related species of Caloplaca (Teloschistaceae, Lichenes) from the Namib Desert. Bothalia 18: 51-56. as Caloplaca namibensis Karnef. based on his own material, collected in Namibia, and that of Welwitsch from southern Angola (Benguela). According to Kärnefelt (1988)Kärnefelt I. 1988. Two closely related species of Caloplaca (Teloschistaceae, Lichenes) from the Namib Desert. Bothalia 18: 51-56., C. namibiensis is close to Caloplaca elegantissima (Nyl.) Zahlbr., and differs from C. namibensis mainly by the presence of isidia, which are absent in C. elegantissima, as well as by the lobe width, which is narrower in C. namibensis (0.2-0.5 mm) than in C. elegantissima (0.5-1.8 mm). The author also pointed out to the difference in the separation between lobes and the thickness of the epinecral layer, observing a lower distance between lobes and a thinner epinecral layer in C. namibensis. The other species in the group, C. testudinea, is also isidiate, but in this case lobes are usually much shorter to sometimes absent (Wirth & al. 2005Wirth V., Kärnefelt I., Thell A. & Arup U. 2005. Caloplaca testudinea V. Wirth & Kärnefelt sp. nov. and C. rubelliana (Ach.) Lojka, new to southern Africa. Mycological Progress 4: 299-302. ).

In the most recent classification of the family Teloschistaceae, Arup & al. (2013)Arup U., Søchting U. & Frödén P. 2013. A new taxonomy of the family Teloschistaceae. Nordic Journal of Botany 31: 16-83. proposed the monophyletic genus Stellarangia to accommodate three species occurring in the deserts of south-western Africa and developing “beautiful long lobes”: Stellarangia elegantissima (Nyl.) Frödén, Arup & Søchting, S. namibensis and S. testudinea (V.Wirth & Kärnefelt) Frödén, Arup & Søchting.

The Atlantic Sahara specimens examined in this study are sterile, covered by isidia, mostly simple and scattered, but sometimes becoming clustered towards the centre of the thallus, and then appearing coralloid. Lobes are narrow, never exceeding 0.5 mm in width, mostly closely adpressed but occasionally more separated, and even disjunct. Such variation appears to be dependent on thallus developmental stage and/or determined by substrate microtopography. The surface is orange (K+ purple) to salmon pigmented due to a strongly developed white epinecral layer, which can leave the centre of the thallus virtually white (Fig. 1b). The epinecral layer is absent at the lobe tips but can be up to 50 µm thick towards the centre. Cortex is as described by Kärnefelt (1988)Kärnefelt I. 1988. Two closely related species of Caloplaca (Teloschistaceae, Lichenes) from the Namib Desert. Bothalia 18: 51-56., i.e. composed of strongly gelatinized hyphae that are mostly hyaline, with an orange-yellowish thin layer above, but is sometimes reduced to the pigmented layer directly contacting, and partially involving, the characteristic algal clusters. Algal clusters are frequently separated by hyphae stacks extending downwards from the cortex.

These specimens were found on January 5, 2016, about 6 km north of Sabkhat Fares (22.579995N, 16.35907W). They were growing on white quartz pebbles embedded in the sandy and gravelly coastal cliffs of the Hamada Plateau (Fig. 1a), which is in agreement with Kärnefelt’s description of the ecology of Caloplaca namibensis in the Namib Desert (Wirth 2010Wirth V. 2010. Lichens of the Namib Desert: a guide to their identification. Klaus Hess Publishers, Windhoek.; de los Ríos & al. 2022de los Ríos A., Garrido-Benavent I., Limóna A., Cason E.D., Maggs-Kölling G., Cowan D. & Valverde A. 2022. Novel lichen-dominated hypolithic communities in the Namib Desert. Microbial Ecology 83: 1036-1048. ). The climate is hot (average daily maximum temperature of 23°C) and arid, with scarce amounts of episodic rainfall (average less than 20 mm annual rainfall). Fogs blown in from the Atlantic are common, which condense and supply the majority of moisture needed for lichens to grow. The whole area is influenced by the cold Canary Current which flows southwards from the northern Atlantic Ocean along the north-western coast of Africa.

Our proposed explanation for the occurrence of Stellarangia namibensis in this particular locality of north-western Africa is a combination of the favourable environmental conditions of the Atlantic Coastal Desert, similar to the conditions found in the Namib Desert, and the suitable substrate for its growth provided by the stable quartz pebble field, which seems to be uncommon in this region, contrary to what happens in the Namib (Schieferstein & Loris 1992Schieferstein B., Loris K. 1992. Ecological investigations on lichen fields of the Central Namib. Vegetatio 98: 113-128. ; de los Ríos & al. 2022de los Ríos A., Garrido-Benavent I., Limóna A., Cason E.D., Maggs-Kölling G., Cowan D. & Valverde A. 2022. Novel lichen-dominated hypolithic communities in the Namib Desert. Microbial Ecology 83: 1036-1048. ).

The number of accompanying species in the locality was remarkably low. The most conspicuous was Elenkiniana gloriae (Llimona & Werner) S.Y.Kondr., Kärnefelt, Elix, A. Thell, Jung Kim, A.S.Kondr. & Hur., but Buellia vandenboomii Giralt & Brand, Diploicia canescens (Dicks.) A.Massal., Diplotomma alboatrum (Hoffm.) Flot., Niebla bourgaeana (Mont. ex Nyl.) Rundel & Bowler (salazinic acid chemotype) and Ramalina maciformis (Delise) Bory, were also present. Buellia vandenboomii is a new report for continental Africa. We searched for S. namibensis in other localities along the coast of the Atlantic Sahara, from Cape Bojador down to the border with Mauritania and no further occurrences were noted. Future survey efforts focused on finding suitable habitats for S. namibensis along the north-western African coast might well include this and other unexpected lichen taxa.