Diagnosis. A chalky, bright white, densely pruinose, continuous crust, that, when well developed, forms conspicuously cerebriform, saxicolous thalli, with large lecanorine apothecia, with a thick thalline margin and pallid, pruinose disc, lacking soredia; containing atranorin and protocetraric acid.
Type: Ecuador. Galápagos: San Cristóbal, Cerro Partido along trail from entrance to Cerro Pelado to El Ripioso, 0˚51’23’’S,89˚27’37’’W, 376 m alt., transition zone, rocky SW-exposed slope of hill with Jasminocereus thouarsii, Clerodendrum molle var. glabrescens, Psidium galapageium, Bromeliaceae and ferns growing in rock crevices, on rock, 28-Apr-2007, Bungartz, F. 6633 (CDS 34853–holotype).
Description.Thallus saxicolous, continuous, not distinctly fissured, uneven to ±verrucose (‘lumpy’), typically significantly thickened, soon distinctly convoluted, gnarled, soon becoming conspicuously cerebriform particularly in the center, i.e., with conspicuously inflated, swollen and convoluted protuberances, isolated or merging into a very thick crust; surface white, dull, densely whitish pruinose, lacking soredia; prothallus absent. Apothecia sparse to numerous, occasionally crowded and deformed by mutual pressure, circular or slightly undulate in outline, 0.5–1.5(–1.9) mm in diam., soon sessile and typically basally ±constricted, distinctly lecanorine with a markedly thickened, entire or barely crenate, whitish pruinose margin, concolorous with the thallus, disc concave to plane, pale beige to faintly carneous, densely whitish pruinose; hymenium hyaline, not inspersed, epihymenium with a diffuse, dull brown pigment (elachista-brown: dissolving in K, HCl± dull greenish, N−) and few to abundant minute crystals, mostly dissolving in K (residue of fine, rounded, reddish granules persistent, 0.5–1 μm); proper exciple thin, indistinct, with few minute crystals; thalline exciple very thick, of thin-walled, radiating hyphae with numerous, but inconspicuous photobiont cells obscured by abundant small crystals soluble in K, ecorticate or indistinctly corticate (outer cells more deeply brown); hypothecium hyaline; ascospores 8/ascus, simple, broadly ellipsoid to subglobose, (5.9–)8.1–10.3(–10.8) × (3.9–)4.7–6.3(–6.9) μm (n = 45). Pycnidia not seen.
Etymology. Named for its unusual, brain-like growth form.
Ecology and distribution. Known only from the Galapagos, typically growing in the transition zone, occasionally also in the upper dry zone, on exposed to semi-sheltered, sunny to semi-shaded steep rock surfaces (large boulders, steep cliffs).
Notes. When well developed, L. cerebriformis has a conspicuous, unusual thallus morphology that may best be compared to inflated, puffed up “kernels of popcorn”. These strongly swollen, bullate, often irregularly ramified protuberances are often dispersed so that the substrate appears to be sprinkled with popcorn. Thalli can also merge into a thick, irregular crust with an extremely convoluted surface. The apothecia of the species have a thick margin, superficially similar in appearance to those of Ochrolechia, but with typical Lecanora-type asci. This typical growth form of the species is termed ‘cerebriform’ since the thalli often appear to be folded, convoluted and swollen like the lobules of a brain, i.e., a cerebrum. The thalli can be surprisingly similar to those of Mobergia calculiformis, a species endemic to Baja California. Lecanora cerebriformis, however, has almost white, rarely very faintly pink (‘carneous’) apothecial discs and it has simple, hyaline spores. Apothecial discs of Mobergia are black to bluish black, and the species forms deeply olive brown, thick-walled ascospores characteristic of the Physciaceae. Despite its name L. cerebriformis does not always form the distinctly cerebriform thalli; very rarely contiguous, indistinctly fissured thalli can be observed. Even these thalli, however, become thicker and ‘verrucose’ in the center and chemically and anatomically clearly belong to this species. In the Galapagos a second species of Lecanora is also characterized by a cerebriform morphology but L. cerebrosorediata contains usnic acid rather than atranorin. It has rarely been found fertile (ascospores have not been observed) and is typically densely sorediate, the soredia brighter, more greenish than the otherwise pale yellowish green, pruinose surface. Despite their similarity, the species are probably not closely related and may represent ecological adaptations to the same habitat, as they often co-occur.