Diagnosis. Thallus erect-shrubby, branches not inflated, with abundant tubercles, fibercles and isidiofibrils, base blackish at the first millimeters, soralia irregular, becoming large and excavate at the branch apices, cortex mat, medulla very thin, axis very thick, with norstictic acid.
Type: Ecuador. Galápagos, Isla Isabela, Volcán Sierra Negra, around the parking place at the end of the dirt road to the crater of Sierra Negra, 0°49.0'45.1''S, 91°5.0'17.1''W, 913 m alt., humid zone, farming areas, with small trees of Psidium guajava, slope 25° SE, on living fencepost, 14-Aug-2008, Clerc 08-213 [holotype in CDS (40067); isotype in G]. %C/M/A: 9/7/68. Chemistry: usnic and norstictic acid.
Etymology. Named in honor of our good friend and colleague Helmut Mayrhofer, to whom this publication is dedicated on occasion of his 65th birthday.
Description. Thallus erect-shrubby, compact and stiff; ramifications ±isotomic-dichotomous; trunk blackish on the first millimeters (below first ramification), often with thin annular cracks extending on basal branches; branches cylindrical to irregular in diameter, not inflated; lateral branches not constricted at ramification; maculae absent; pseudocyphellae absent; papillae absent to sparse (see tubercles); tubercles typically abundant, hemispherical to verrucose, little eroded; fibrils slender, scattered to abundantly distributed on branches; fibercles absent; soralia developing on the cortex of terminal branches, plane to slightly stipitate, minute and aggregating in irregular clusters at young stage, at maturity becoming large and excavate (almost efflorescent) on the apices of branches; isidiomorphs usually abundant, developing into isidiofibrils within soralia and from eroded tubercles on basal and secondary branches; cortex opaque, moderately thick, (7.5 –)8.5 –12.5(–13 %); medulla compact to dense (hyphae visible individually), very thin, (5.5 –)8 –12(–12.5 %); axis white, very thick (52.5 –)53 – 65(–73.5 %), with an A/M-ratio > 4.5; apothecia and pycnidia not observed.
Chemistry. Medulla with norstictic acid only [P+ yellow, K+ yellow turning orange-red (crystals), C–, KC–].
Ecology and distribution. Usnea brattiae, U. cedrosiana, U. mayrhoferi, U. krogiana and U. patriciana all form a group of similar and possibly very closely related species, which apparently differ in their distribution. Usnea brattiae and U. cedrosiana are known from Southern California and the Isla Cedros, U. krogiana from the Azores, the West Indies and possibly from the Galapagos, and U. mayrhoferi and U. patriciana are currently known only from the Galapagos. Compared to most other species of Usnea in the Galapagos U. mayrhoferi is ecologically somewhat unusual: all specimens have been found in the humid zone, in exposed habitats, where they frequently grow close to the ground, often on small shrubs (esp. Pernettya howellii) and ferns (Polypodium), even on plant debris (dead fern fronds). Specimens growing as epiphytes of trees (or, like the holotype, on “living fenceposts”) typically grow among mosses or liverworts.
Notes. Usnea mayrhoferi is characterized by a stiff and compact thallus with non-inflated branches bearing abundant tubercles, fibrils and isidiofibrils. It has a distinctly blackened trunk (below the first ramification), bearing annulations all along its basal branches, and irregular, crowded soralia, becoming large and excavate at the branch apices. The species has an opaque cortex and a very broad axis (A/M-ratio > 4.5).
The new species much resembles U. krogiana P.Clerc, described from Macaronesia, also reported from the West Indies (Clerc 2006). The protologue of U. krogiana cites different chemotypes, and although the one with stictic and norstictic acid is most commonly found, a chemotype with norstictic acid only has also been reported. Usnea mayrhoferi can therefore not necessarily be distinguished by its chemistry, but the two species clearly differ in their soralia. Soralia of U. krogiana are punctiform and not delimited by a distinct rim. They are generally slightly stipitate, arise from tubercles, and although they are numerous and frequently closely grouped, they typically do not merge. Usnea mayrhoferi has soralia that are generally much larger, not distinctly punctiform, but irregular in outline. They are inconspicuously delimited and particularly towards the branch apices typically merge, thus obscuring the tip of the branches, occasionally becoming deeply excavate. Usnea brattiae P.Clerc from the Sonoran region (the Channel Islands of Southern California and Isla Cedros in Baja California) and U. cedrosiana P.Clerc (known only from Cedros Island) have a similar morphology: stiff branches, a blackened base and a similar CMA. Usnea mayrhoferi differs by its abundant tubercles and numerous fibrils, and by conspicuous annulations along it trunk and basal branches. In Galapagos specimens of U. mayrhoferi could be confused with Usnea patriciana, but this species contains salazinic, not norstictic acid and its soralia are distinctly delimited by a thin cortical rim. These soralia produce isidiomorphs that remain short, not developing into isidiofibrils. Usnea cedrosiana typically contains either norstictic or diffractaic acid, but chemotypes that contain usnic acid only are also known. In U. mayrhoferi only a single chemotype with norstictic acid has so far been observed. Thalli of U. cedrosiana are erect to subpendulous with very irregular branches, densely covered in abundant papillae and fibercles. Thalli of U. mayrhoferi are erect, stiff, and compact, the branches only have sparse if any papillae and they lack fibercles. Thalli of U. cedrosiana have an anisotomic-dichotomous branching pattern, and a brownish to jet-black base with no annulations; branches of U. mayrhoferi are isotomic-dichotomous and its base is blackish only at its first few millimeters, with distinct annulations up to the first branches. The two species differ further by the formation of their soralia. In U. mayrhoferi they originate on the cortex, but in U. cedrosiana are derived from tubercles. Both species differ in their CMA: Usnea mayrhoferi has a thick, opaque cortex; the cortex of U. cedrosiana is thinner and it appears shiny in section. The medulla of both species is compact, though in U. mayrhoferi perhaps not as dense; the axis of U. mayrhoferi is generally broader.