Stirtonia rhizophorae Kalb & Mongkolsuk sp. nov. (Fig. 2K–L) Mycobank MB564186

Sicut Stirtonia macrocarpa Makhija & Patw. et S. schummii Aptroot, sed acidum confluenticum continet.

Type:— THAILAND. Trat Province: Muang District, near Ban Nam Chieo, in a ± disturbed mangrove forest on Rhizophora apiculata, ± 3 m, 12°10’25’’ N, 102°28’37’’ E, 25 February 2011, K. Kalb & P. Mongkolsuk (holotype RAMK, isotype hb. Kalb 38845) .

Etymology:—The new species is named after the phorophyte on which the type species was collected.

Thallus spreading, covering an area of up to 7 cm diam., contiguous, smooth to slightly warty, whitish grey, with calcium oxalate crystals, I+ blue. Ascigerous zones delimited, round to slightly elongated in outline, 0.5– 1.5 mm diam., raised, always higher than the thallus, white, with calcium oxalate crystals, I+ blue. Asci globose to ovoid, 8-spored, ascospores 12–15-septate, ellipsoid, 60–85 × 25–33 µm, cells of ± equal size (Fig. 2 L). Chemistry: confluentic acid (TLC). Thallus and ascigerous zones C-, Pd-, KOH-, UV-.

Notes:—At present, no other species of Stirtonia is known to have this chemistry. It should be noted that a chemical reinvestigation of the type of Stirtonia schummii did not confirm the presence of perlatolic acid as mentioned in the protologue, but rather psoromic acid. This finding was confirmed by co-chromatography with the pure acid. Furthermore, Stirtonia macrocarpa probably does not contain 2’- O -methylperlatolic acid as mentioned by Aptroot (2009), but three unknown substances (Makhija & Patwardan 1987). Confluentic acid is a common substance in the related genus Cryptothecia which differs from Stirtonia in having muriform ascospores. However, ascospore septation is variable in many genera and both transversely septate and muriform ascospores can be found in closely related species of the same genus (Lücking 2009). The genera of Arthoniales are currently being revised by G. Thor and A. Frisch (pers. comm.) so that no taxonomic changes are made here.