Thallus fruticosus, albidus vel cinereo-albidus; rami numerosi, teretes, 2-10 cm longi. Cortex hyphis verticalibus. Ascocarpia numerosa. Soralia rara. Medulla albida vel palida brunneola. Thallus C+ rubescens, discus C+ rubescens, K non reagens.
Holotype: Ecuador. Galapagos Islands, Santa Fé NE part at Bahia Paraiso, small ridge approx. 300 m from the sea. (Barrington) S 0° 42’11” W 90° 2’00”, elevation 50-100 m. 15 Feb. 2005, Tehler 8653 (CDS); isotypes: (B, BM, E, H, LD, NY, S-L68229 TFMC, UPS, US)
Etymology. The epithet albida refers to the whitish colour of the thallus as compared to the more brownish-grey thalli of the closely related species Roccella galapagoensis, R. margaritifera and R. nigerrima.
Thallus branches terete, smooth or slightly foveate; usually 5-20 cm long; white to white-greyish; soredia rare, C-; medulla byssoid or nearly byssoid, white or sometimes pale yellowish-brown in lower parts. Ascomata common often sessile with strongly undulating margin giving it a knotty appearance (Fig. 2b); ascospores 22-26 x 5-6 µm (average=23.8 x 5.5 µm). Spot tests: medulla K-, C-; cortex K-, C+ red; disc C+ red. Secondary metabolites erythrin, lecanoric acid and protocetraric acid (by HPTLC; present study).
Distribution and habitat. Roccella albida is endemic to the Galapagos Islands and has been found on Santa Fé, Floreana, Pinzón, Rábida and Española. It grows in coastal regions on the vertical rocks and cliffs.
Notes. Roccella albida is habitually similar to the sympatric species R. galapagoensis. It is distinguished from the latter species by its smaller size, the smaller growth and the whitish thallus colour and whitish medulla. Roccella albida is more rarely collected than the common R. galapagoensis. Sorediate thalli of R. albida may be confusingly similar to R. margaritifera but differ from the latter species by their smaller, more delicate thallus in combination with a whiter medulla composed of more loosely interwoven hyphae.
from: Tehler, A., Irestedt, M., Bungartz, F. & Wedin, M. (2009) Evolution and reproduction modes in the Roccellagalapagoensis aggregate (Roccellaceae, Arthoniales). Taxon58: 438–456.