Rhizocarpon oxydatum Fryday
Biblioth. Lichenol. 88: 140. 2004.
Thallus orange-red, thin, < 0.05 mm thick, cracked-areolate; areoles flat, 0.4-0.6 mm across, usually contiguous. Photobiont chlorococcoid, cells globose, 10-15 µm diam.
Apothecia black, lecideine, orbicular, 0.4-0.6 mm diam., sessile; disc concave; margin thick, raised and persistent, 0.1 mm wide, often radially cracked. Excipulum thick and carbonized, composed of radiating hypahe 5-7 µm wide, dark brown throughout. Hypothecium thick, dark brown, carbonaceous, >50 µm, mainly composed of vertically orientated hyphae. Hymenium hyaline, 90-100 µm thick, I+ blue; epihymenial zone very pale blue, 15-20 µm thick. Paraphyses relatively thick 2.0-2.5 µm thick, sparingly branched and anastomosing, lax in water, slightly swollen at the apices to 3.5 µm with a brown cap. Asci cylindrical, 70-85 x 17-25 µm, Rhizocarpon-type. Ascospores hyaline, becoming brown when over-mature, 1-septate, 18-21(-24) x 9-12 µm, halonate.
Conidiomata not observed.
Chemistry: no substances detected by TLC.
Distribution & Ecology: Known only from siliceous rocks on Campbell isalnd and the Auckland Islands.
Notes: The orange-red thallus and black apothecia give this species a superficial resemblance to the cosmopolitan species Tremolecia atrata (Ach.) Hertel, from which it is most readily distinguished, macroscopically, by its sessile apothecia. Microscopically R. oxydatum has a strong resemblance to R. hochstetteri (Körb.) Vain., in having medium-sized, 1-septate ascospores and similar paraphyses. It differs from this species mainly in the colour of its thallus and its carbonized exciple, although this latter may not be a good character for distinguishing species in the Subantarctic (see below under Rimularia maculata).
Associated species include Fuscidea subasbolodes, Lecidea lygomma, Placopsis sp. Stereocaulon sp., and Rimularia maculata (see below).
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