Diagnosis. A saxicolous species with orange apothecia that, like its sister taxon O. isidiosa, contains the anthraquinone isidiosin, the coarsely granular to irregularly subsquamulose thallus lacking secondary metabolites, its surface olive to pale gray, occasionally with isidia.

Type. Ecuador, Galápagos: San Cristóbal, Pan de Azúcar, inland from Bahía Sardinas at the NW-coast of the island, 0°43′6″S, 89°21′19″W, 88 m alt., dry zone, NE-facing slope near the top of the first small hill with low shrubs of Alternanthera filifolia and few Bursera graveolens trees, on rock, 24-Apr-2007, Bungartz, F. 6417 (CDS 34632‒holotype; GenBank Accession number nrITS: MT967415).

Description. Thallus coarsely granular to irregularly subsquamulose, of discrete, strongly convex (‘bullate’), up to 0.5 mm broad areoles, effuse, lacking a prothallus or hypothallus; surface olive to pale gray, smooth, but not shiny, dull, with sparse to abundant white pruina (on well-developed specimens entirely masking the gray cortex), occasionally becoming granular isidiate, with scattered, 0.1 mm thick, ± coralloid isidia. Apothecia numerous, dispersed, sessile to somewhat stipitate, up to 1.2 mm in diam., lecanorine; thalline margin prominent, often curved inwards and strongly crenate, flexuose, 0.05–0.06 mm broad, pale orange, color often fading and increasingly whitish to pale orange pruinose towards the outside; disc concave to mostly flat, deep orange, epruinose, C–, K+ purple; epihymenium with orange pigment contiguous with the outer exciple; hymenium hyaline, not inspersed; proper exciple indistinct, almost entirely reduced to a few hyaline hyphae, thalline exciple differentiated into an inner part abundantly filled with small colorless crystals, dissolving in K, obscuring the photobiont cells in between; subhymenium and hypothecium not differentiated, hyaline, not inspersed; asci clavate, Teloschistes-type; ascospores 8/ascus, polaribilocular, oblong to ellipsoid, (9.7–)11.7–15.1(–16.7) × (4.5–)5.3–7.6(–9.5) μm, with a thick, (2.7–)3.5–5.4(–6.4) μm wide septum (n = 43). Pycnidia unknown.

Chemistry. Thallus P–, K–, C–, KC–, UV– (dull); apothecia P–, K+ purple, C–, KC+ purplish (fading), UV– (dull); no secondary metabolites detected in the thallus, apothecia with isidiosin.

Etymology. Named for the presence of idiosin, an unusual chemistry that the species shares with Oceanoplaca isidiosa.

Ecology and distribution. Currently known only from Galapagos (Santa Cruz, San Cristóbal, Pinzón), where it is found in the dry zone, on sunny, wind- and rain-exposed rock surfaces.

Note. According to the molecular data, Oceanoplaca chemoisidiosa is a sister species of O. isidiosa, with which it also shares an identical chemistry. In O. chemoisidiosa, the characteristic isidiosin can, however, only be found in the apothecia; the thallus lacks secondary metabolites. Both species are morphologically distinctly different: Oceanoplaca isidiosa forms placodioid, deep orange to orange-red thalli with well developed, radiating marginal lobes. It is typically abundantly isidiate and often lacks apothecia. Oceanoplaca chemoisidiosa is not placodioid, its thalli are effuse, of irregularly dispersed granular to bullate or even subsquamulose, gray to whitish pruinose areoles. Specimens rarely form isidia, but are typically fertile. The Sonoran Desert species ‘Caloplaca’ lagunensis Wetmore is morphologically somewhat similar, but the spores are larger (18–22 × 10–12.5 μm) and consistently have three locules. The species also contains lichexanthone instead of isidiosin.