Acarospora castaneocarpa M. Westb. & Wedin, in Ariyawansa et al., Fungal Diversity 75: 131 (2015)
Index Fungorum number: IF 813044; MycoBank number: MB 813044; Facesoffungi number: FoF 00928

Etymology – In reference to the chest-nut coloured ascocarps.
Holotype – FINLAND, Varsinais-Suomi (Regio Aboensis), Lohja, Paavola, NE of Rautaniemi, stony SE-slope, young Pinus sylvestris plantation, on calcareous stone, 50 m alt., 21 May 2005, Juha Pykälä 26880 (H).

Lichenized, photobiont a chlorococcoid alga. Thallus only present as a thalline margin in the ascocarps. Apothecia  lecanoroid, rounded or more often irregular in outline, 0.6–1.1 mm diam., margin distinct, raised above the disc, up to c. 0.12 mm thick, reddish brown, shiny, in the paratype with a bluish white pruina along the inner rim (adjacent to the disc); disc concolourous with the margin, flat, smooth, shiny, epruinose; true exciple indistinct from the outside, I–, prosoplectenchymatous, below the hymenium of thickwalled cells with narrow lumina, towards the surface expanding and becoming fan-shaped, composed of branching, narrow, elongated cells, becoming shorter and rounded towards the surface, uppermost cells rounded, up to c. 5 μm diam., with a reddish-brown colour; epithecium reddish brown; hymenium colourless, I+ blue, 65–80 μm tall; paraphyses, easily distinguished in water, 1–1.5 μm in mid hymenium, tips clavate, up to 4.5 μm wide, upper part of the top cell often reddish brown; subhymenial layers, colourless, flat (i.e., not forming a wedge-shaped structure), up to c. 40 μm thick, prosoplectenchymatous, compact, of small, indistinct cells, oil-drops colourless, small, very few seen; Asci clavate, multi-spored (no well-developed asci found). Ascospores 3.5–5×1–1.5 μm (only few spores found), colourless, simple, narrowly ellipsoid–cylindrical. Pycnidia not seen. All spot tests negative.

Material examined – FINLAND, Varsinais-Suomi (Regio Aboensis), Lohja, Torhola, 300 m N-NE of Kallioranta, young pine plantation, S-slope, small rock wall of calcareous rock, 60 m alt., 22 July 2004, Juha Pykälä 25280 (H).

Ecology and distribution: Acarospora castaneocarpa is so far only known from two localities near the lake Lohjanjärvi in southernmost Finland where it was collected on calcareous rocks in young Pinus sylvestris forests.

Notes – Surprisingly this characteristic species is closely related to Sarcogyne algoviae, a very different species with black, lecideine apothecia with a strongly carbonized margin. In A. castaneocarpa the margin of the apothecium has a cortical layer that is densely filled with small, colourless crystals (calcium oxalate?). We have not seen this character in any other species in Acarosporaceae and as far as we know this is a very unusual character in the family.

Fig. 1  f–i Acarospora castaneocarpa f–i Finland, Pykälä 26880 (H holotype) g, h Section through apothecium showing dense layer of crystals in the cortex and anatomy of the true exciple i Spores. Scale bars: a–c, f=1 mm, c= 50 μm, e=10 μm, g=50 μm, h–i=10 μm. Photographs by M. Westberg