Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, loosely adnate, 3-10 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 6-12 mm wide; apices: rotund, crenate, ascending, ciliate; cilia: up to 4 mm long upper surface: gray with some blackened areas, smooth, dull, sometimes white pruinose, usually white maculate isidia: simple to coralloid, common, laminal to submarginal; tips: sometimes ciliate, becoming eroded and subsorediate; soredia and pustulae: absent medulla: white with patches of yellow to orange brown, particularly in lower layers, continuous algal layer lower surface: black with brown to tan or mottled white naked zone peripherally, centrally rhizinate; rhizines: scattered, simple Apothecia: rare, substi ascospores: broadly ellipsoid, 16-22 x 10-14 µm Pycnidia: not seen Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC+ red or orange, P- Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with alectoronic acid (major), α-collatolic acid (minor) and skyrin (accessory). Substrate and ecology: usually on trees in open habitats, rarely on rocks World distribution: pantropical and southern USA Sonoran distribution: Sonora and Sinaloa, in the Sierra Madre Occidental.
Type: Saint Helena [exact locality unknown]. Melliss, J.C. 23 (K – holotype, US – isotype, fide Hale 1965, and Fleig 1997).
Description. Thallus corticolous; uppersurface whitish gray to ivory, occasionally with large, mottled-black patches, dull, epruinose, emaculate, but occasionally in parts irregularly cracked; sparsely to abundantly isidiate; isidia marginal, cylindrical, simple to sparsely or abundantly branched (coralloid) and then clustered (caespitose), brown to blackened at their tip and frequently ciliate (having a ‘tufted’ appearance); lobes broad, moderate-sized, 0.7–5(–7) mm wide, ± rotund, axils incised, margins abundantly ciliate; cilia long and slender, 0.5–5(–8) mm long, black, mostly simple, very rarely branched; lower surface black, with a distinctly erhizinate, 0.5–2.5 mm wide, dark brown to black marginal zone; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia absent or rare, 0.3–7 mm in diam., with a cupulate thallinemargin that is typically speckled by abundant isidia, disc dark brown, deeply concave, imperforate; asci clavate, Lecanora-type, ascospores 8/ascus, simple, broadly ellipsoid, (14.9–)16.0–21.3(–22.3) × (9.4–)11.0–14.1(–14.9) μm (n = 15), thick-walled (~ 1 μm thick). Pycnidia not observed.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with alectoronic and α-collatolic acid [P–, K–, KC+ rosé, quickly orange, C–, UV+ greenish blue].
Ecology and distribution. Oceania, Asia, Africa, North, Central and South America (Hale 1965; Awasthi 1976; Swinscow & Krog 1988; Malcolm & Galloway 1997; Louwhoff & Elix 1999; Kurokawa & Lai 2001; Kantvilas et al. 2002; Spielmann & Marcelli 2009; Marcelli & Benatti 2011). Previously reported from Galapagos only online (Bungartz et al. 2016); relatively rare, most collections from the humid zone, few from the upper transition zone; on sunny, wind- and rain-exposed rock or bark.
Notes. Well developed thalli of P. mellissii have repeatedly branched, coralloid isidia. These isidia are usually abundantly ciliate and thus ± ‘tufted’. Isidia of P. neosubcrinitum and P. ultralucens are typically not as elaborately branched, more densely clustered, but typically also abundantly ciliate. Of these similar species only P. mellissii contains alectoronic acid, with a medulla that reacts KC+ rosé, quickly turning orange, also displaying UV+greenish blue fluorescence. In Galapagos only P. rampoddense has a similar secondary chemistry. Its medulla also contains both α-collatolic and alectoronic acid, but P. rampoddense is sorediate and P. mellissii is isidiate.