Taxonomic note: Lendemer & Hodkinson (2013) proposed the new combination as Lepraria tenellum (MycoBank no. 564057), but according to Art. 23.5 of the International Code of Nomenclature for algae, fungi, and plants (McNeill et al. 2012) the gender of the epithet, if used as an adjective, must follow the gender of the generic name, the epithet is thus corrected here to its female form tenella.
Thallus saxicolous; initially placodioid leprose, i.e., developing upon a common, shared hypothallus (structurally similar to L. vouauxii), but soon becoming leprose-fruticose by the formation of ill-defined pseudopodetia that lack a distinct axial strand, prothallus fine, inconspicuous, main thallus of distinctly delimited, granules soon budding into branched gnarled, pseudocorticate pseudopodetia; surface white to pale yellowish or greenish ivory white; hypothallus inconspicuous, exposed only as a fine prothallus along the thallus margin; rhizohyphae absent or at least indistinct; granules pseudocorticate, very compact, coarse [(157–)170–185(–200) µm in diam., rather uniform in size], aggregating into erect, coarse (0.8–)3–5(–11.5) mm tall pseudopodetia, without conspicuous extruding hyphae, immature or damaged, or eroded parts rarely with few protruding hyphae; photobiont green, coccoid, 7–10 μm in diam.
Spot tests and chemistry: P+ reddish orange, K+ yellow orange, C+ red, KC−; UV-(dark); atranorin, lecanoric acid, pannaric acid 6methyl ester [other secondary metabolites not confirmed for the Galapagos chemotype, Lamb (1974, table 1) also lists rangiformic and other fatty acids as well as grayanic, divaricatic, evernic, and sekikaic acid].
Distribution and ecology: Neotropical (Lamb 1974: Chile, Peru), first reported from Galapagos by Weber as Leprocaulon tenellum; in the archipelago it is rare in the dry and lower transition zone, is most common in the upper transition and humid zone, while a few specimens have even been collected in the high altitude dry zone; it typically grows on rock, rarely on soil and then often overgrowing mosses or detritus, rarely on wood and very rarely on bark. It occurs typically in ±sheltered, shaded habitats, below overhangs or on vertical rock faces and cliffs, rarely in ±exposed situations, but then usually not receiving direct sunlight.
Notes: Elix & McCarthy (1998) suggested that Lepraria tenella might be synonymous with Lepraria albicans (Th.Fr.) Lendemer & Hodkinson (2013: 1005) (º Stereocaulon albicans Th.Fr. (1857: 36). Pseudopodetia of L. tenella are, however, less well defined and consitently lack the central axis of L. albicans. Although both species share some secondary metabolites (Lamb 1974: table 1), L. tenella appears also well distinguished from L. albicans by the presence of lecanoric acid and dibenzofurans, in particular pannaric acid 6methyl ester (which, according to Leuckert & Kümmerling 1989 corresponds to the unidentified dibenzofuran LP-1 mentioned by Lamb 1974).
In the Galapagos the only other species with dibenzofurans is L. vouauxii (see below). Both L. tenella and L. vouauxii lack a well defined hypothallus and the immature thalli of L. tenella which have not developed pseudopodetia, can be strikingly similar to the coarsely granular thalli of L. vouauxii. Both species have compact, pseudocorticate granules that are largely devoid of extruding hyphae (Fig. 6), although individual granules of L. tenella are generally much larger than those of L. vouauxii. Like many, but not all species of Lepraria s.str. that form pseudopodetia (i.e., species that were previously included in 'Leprocaulon'), the pseudopodetia of L. tenella lack a distinct central, axial strand. This central strand is typical for the type species of Leprocaulon s.str., L. quisquiliare (Leers) M. Choisy (1950: 166) and is also present in the recently described Leprocaulon americanum Lendemer & Hodkinson (2013: 1007).
Although the species containing dibenzofurans do not form a monophyletic group separate from Lepraria s.str. (Lendemer & Hodkinson 2013, Lendemer 2013), it nevertheless appears worth investigating whether at least some of these species with structurally similar granules, indistinct hypothalli and no axial strand, are more closely related than others.