Diagnosis. Superficially similar to Lecanora subcrenulata, characterized by a similar chemistry, but very different apothecial anatomy [epihymenium yellowish brown, not soluble in K, lacking crystals (glabrata-type), the exciple abundantly filled with fine crystals soluble in K, lacking larger crystals (allophana-type)].
Type: Ecuador. Galapagos: Isabela, Volcán Sierra Negra, Volcán Chico on the northeastern slope of Sierra Negra, 0˚46’56.77’’S, 91˚6’4.298’’W, 938 m alt., transition zone, young lava flow with very scattered vegetation (Macraea laricifolia, Darwiniothamnus tenuifolia), on SW-exposed trunk of Jasminocereus thouarsii, 14-Aug-2008, Bungartz, F. 8178 (CDS 40824–holotype).
Description. Thallus corticolous (on cacti, Jasminocereus), thin and in parts ±discontinuous, rimose-areolate to verrucose areolate; surface white to pale gray, strongly roughened, matt, epruinose or with faint, whitish pruina, lacking soredia; prothallus white, weakly developed, often indistinct. Apothecia sparse to numerous, typically ±isolated or a few aggregating, circular to very irregular in outline, often quite large, 0.5–1.7(–2.2) mm in diam., soon sessile and conspicuously constricted at the base, distinctly lecanorine, margin persistent, conspicuously fissured, often broadly undulating and at least in parts ±crenate, epruinose, concolorous with the thallus, disc deeply concave to ±plane, pale beige to brown, darkening with age (due to parasitic fungi), occasionally almost blackened, epruinose; hymenium hyaline, not inspersed, epihymenium lacking crystals, with a diffuse yellowish brown pigment (cf. leptocline-brown: turning orange in K, forming a yellowish solution, HCL+ intensely brown, N−), pigment not soluble in K, (cf. glabrata-type), proper exciple thin, indistinct, with abundant small crystals, thalline exciple thick, ±corticate, filled with abundant small crystals that are completely soluble in K (allophana-type); subhymenium and hypothecium hyaline; ascospores 8/ascus, simple, broadly ellipsoid to subglobose, (5.9–)6.2–8.0(–8.8) × (4.9–)5.1–6.0(–6.4) μm (n = 20). Pycnidia not seen.
Chemistry. Thallus cortex including apothecial margin P+ yellow, C−, KC−, K+ yellow, UV−; with atranorin [major], isonotatic acid [major], 4-O-methylhypoprotocetraric acid [minor], norisonotatic acid [minor], and 2’-O-methylperlatolic acid [major]; [all specimens cited were analyzed with TLC].
Etymology. Growing on cacti (Jasminocereus).
Ecology and Distribution. Known only from Galapagos, where relatively few specimens have been found on the columnar, endemic cactus, Jasminocereus thouarsii.
Notes.Lecanora cactacea is morphologically very similar to L. subcrenulata and the two species share the same chemistry. However, they are anatomically distinct, as the thalline exciple of L. cactacea is entirely filled with small crystals. In L. subcrenulata the exciple may appear densely filled by small crystals, but when K is applied the small crystals rapidly dissolve to reveal a few larger crystals. The thalli of L. cactacea and Pertusaria oahuensis H. Magn. are superficially extremely similar. A specimen of P. oahuensis collected growing side by side on Jasminocereus (Bungartz 8176, CDS 40822), was initially believed to be the sorediate form of L. cactacea. However, chemistry clearly distinguishes the two species as P. oahuensis contains lichexanthone with stictic, cryptostictic and constictic acid (Bungartz et al. 2015). Lecanora cactacea belongs to an unusual series of Galapagos lichens that commonly inhabit endemic cacti, either the pads of Opuntia species or the columnar trunks of Jasminocereusthouarsii (e.g. Opegrapha cactacearum, Parmotrema dominicanum, Lecanographa lyncea, Pyxine cocoës, Pyrenula ochraceoflava, P. cerina, Roccella gracilis, etc.).