Type: Ecuador. Galápagos, Isla Santa Cruz, above Mina Granillo Rojo on the N-side of the island, 0°37.0'7.5''S, 90°21.0'55.5''W, 607 m alt., transition zone with Burseragraveolens, Psidiumgalapageium, Scalesiapedunculata and Opuntiaechios, slope 10°N, on Scalesiabranches, 7-Aug-2008, Truong 1127 [holotype in CDS (39438); isotype in G]. %C/M/A: 6.5/36.0/15.0. Chemistry: usnic, salazinic, traces of protocetraric acid.
Short Description. A detailed description is provided by Truong & Clerc (2016). The species is characterized by a stiff, erect, shrubby thallus with ±inflated branches, ±constricted at their ramifications, irregularly to densely covered by ±elongated, apically eroded tubercles with a distinct cortical rim. Molecular studies confirm that both sorediate and non-sorediate specimens belong to the same lineage and are thus considered part of the same species (Truong et al. 2013a). If present, mature soralia are large and circular to elongated, plane to strongly excavate, often crowded at the terminal branches, eroding the branch apices. On the secondary branches the isidiomorphs sometimes develop into isidiofibrils. When soralia are absent, apothecia are usually numerous. The specimens are further characterized by a thick medulla composed of loose hyphae [axis/medulla-ratio (A/M-ratio) < 0.75]; the thalli contain salazinic acid and thus react K+ yellow turning red.
Sorediate specimens are similar to Usnea fragilescens Lynge and U. cornuta. Unlike U. clerciana these two species typically lack tubercles; they have branches that are regularly terete, not deformed. Usnea fragilescens has not been found in Galapagos; the species can further be distinguished from U. clerciana by soralia that remain isolated and do not become crowded at the terminal branches, thus not eroding the branch apices. Usnea cornuta is reported from the Galapagos (see below). It forms only minute soralia that may be sparse and then difficult to detect, although these punctiform soralia may secondarily aggregate into irregular clusters not resembling the much larger, regularly delimited and circular to elongate ones of U. clerciana. Like U. clerciana some specimens of U. cornuta contain salazinic acid and thus react K+ yellow turning red, while others contain stictic acid, reacting K+ yellow.
Specimens of Usnea clerciana that lack soredia and are abundantly fertile could be confused with Usnea cirrosa Motyka, a species that may also be present in the Galapagos. Usnea clerciana is genetically distinct (Truong & Clerc 2016), but according to Gerlach et al. (2017) U. cirrosa may be paraphyletic. On the continent, we have so far only found specimens that agree well with U. cirrosa. Usnea clerciana is therefore probably endemic to the Galapagos and all material from these islands, which has so far been examined with molecular tools, clearly belongs to this species (Truong & Clerc 2016). So far we have found only two Galapagos specimens that morphologically more closely resemble U. cirrosa [Aptroot 63221 B (G), and Bungartz 6745 (CDS 34996, G), details cited below]. Both species are characterized by numerous apothecia and inflated, basally constricted branches. Thalli of Usnea cirrosa, however, appear to be more flaccid, with regularly terete, not irregular or deformed branch segments. Unlike U. clerciana, its cortex surface can be papillate, but in our opinion the species lacks true, well-developed tubercles. Although specimens of U. cirrosa may thus appear tuberculate, we believe these “false” tubercles are in fact fibercles, i.e., fibrils which have broken off, leaving circular scars that could be confused with the often ±elongated and apically eroded true tubercles of U. clerciana. Usnea clerciana apparently can further be distinguished also by a lax medulla composed of coarse hyphae (almost arachnoid); in U. cirrosa the medulla does not appear distinctly arachnoid, it is denser and composed of thin hyphae.
Although these characters might be sufficiently distinct to reliably distinguish the two species, we nevertheless hesitate to identify the two only Galapagos specimens, which morphologically more closely resemble Usnea cirrosa, as this species. Although we included U. cirrosa in the key, we nevertheless hesitate to accept this record until the specimens have been examined with molecular tools.
Chemistry. Medulla with salazinic, ±norstictic and traces of protocetraric acid [P+ reddish yellow, K+yellow turning deep red, C–, KC–].
Ecology and distribution. Endemic to the Galapagos, where it is among the most common Usnea species, particularly in the upper dry and lower transition zone, though it extends through the humid zone even into the high altitude dry zone; primarily corticolous on various trees and shrubs (incl. cacti), rarely saxicolous or lignicolous. Only U. rubicunda is still more common and more widely distributed throughout the archipelago.