Basionym: Lecanora diplacia Ach., Syn. meth. lich. (Lund): 154. 1814; MB#388093.
Caloplaca diplacia (Ach.) Riddle, Memoirs of the Brooklyn botanical Garden 1: 113. 1918; MB#381493.

Description. Thallus rimose to rimose-areolate, thin to strongly thickened, ± effuse, but typically delimited by a compact blackish prothallus, although compatible thalli are confluent, not distinctly separated, merging with one another; surface dark to pale bluish gray, sometimes tinged with a pinkish hue, smooth, but not shiny, epruinose, with laminal, strongly pustulate and frequently confluent soralia, 0.2–0.5(–0.7) mm in diam. (larger if confluent), with farinose pale green soredia [(20–)25–35(–40) μm in diam.], which are, if not eroded, typically strongly tinged bluish black. Apothecia mostly absent, if present, dispersed to loosely aggregated, rarely very closely grouped and strongly deformed (‘gall-forming’), sessile, up to 1 mm in diam., but mostly smaller, lecanorine (although appearing ‘biatorine’ or even ‘lecideine’, initially pale, but soon discolored, first becoming reddish-black and increasingly darkening, with maturity the entire surface deep reddish-black, the discoloration either starting with the disc, eventually extending to the margin, or first the margin becoming pigmented, later the disc); thalline margin regular to deformed, moderately to strongly thickened, up to 160 μm, ± waxy and smooth, initially pale, ‘fleshy’, almost transparent, soon irregularly tinged reddish-black, eventually deeply blackened throughout, epruinose, C-, K-; disc plane, smooth, initially pale, whitish to almost transparent (‘glassy’), soon irregularly tinged reddish-black, eventually deeply blackened throughout, epruinose, C-, K-; epihymenium reddish orange to strongly violaceous, pigmentation diffuse, C+ yellowish brown, K-, pigment granules absent, ± contiguous with the outer exciple; hymenium hyaline, not inspersed; proper exciple indistinct, almost entirely reduced to a few hyaline hyphae; thalline exciple differentiated into a central, lower part with abundant trebouxioid photobionts, a transitional part abundantly filled with small crystals that dissolve in K, and an outer part, which is initially pale, but with age becomes increasingly pigmented by a reddish orange to strongly violaceous pigment, C+ yellowish brown, K-; subhymenium grayish, hypothecium hyaline, both not inspersed; asci clavate, Teloschistes-type; ascospores 8/ascus, polaribilocular, broadly ellipsoid to almost globose, (7.1–)8.9–11.0(–11.5) × (3.9–)4.9–7.0(–8.3) μm, with a thick, (2.7–)3.4–5.3(–6.4) μm wide septum (n = 35). Pycnidia not observed.

Chemistry. Thallus P+ yellow, K+ yellow, C–, KC–, UV– (dull), apothecia P–, K–, C–, KC–, UV– (dull); thallus with atranorin, isofulgidin, vicanicin and caloploicin.

Ecology and distribution. Originally described from St. Thomas (Virgin Islands), but on coastal rock probably very widely distributed throughout the Caribbean or even Central America. In Galapagos, this is probably the most abundant saxicolous species of Caloplaca s.l. Specimens have been collected especially along the coast throughout the dry zone into the lower transition zone, but the species can occasionally still be found even in the lower humid zone. It typically grows in ± sheltered, semi-shaded, ± nitrophytic, dust-rich habitats, often close to the ground, but one specimen (Aptroot, A. 64559, CDS 31131) has been collected on bark.

Note. In the field, specimens of Sucioplaca diplacia can easily be confused with several superficially similar species of Lecanora that often grow together in the same habitat, even side by side on the same rock (see Bungartz et al. 2020). Lecanora austrosorediosa is perhaps the species most similar to S. diplacia. Both form sorediate thalli with apothecia transitioning from pale to dark. Soralia of S. diplacia are strongly pustulate, frequently confluent, their outer surface, when not eroded, typically tinged a dark bluish gray. Soralia of L. austrosorediosa are rarely confluent, pale creamy beige, not discolored by a bluish or blackish tinge. The species consistently forms thin, pale, whitish gray, rimose-areolate thalli, sometimes with a whitish prothallus, rarely delimited by a thin, black line. Thalli of S. diplacia are instead deeply lead to olive gray, generally much thicker, more irregular, its areoles in part almost subsquamulose; the thallus is typically delimited by a thick, conspicuous, black prothallus. The two species have apothecia that are confusingly similar, both initially pallid. In L. austrosorediosa, apothecia are a pale creamy white, of a ‘waxy’ appearance. In S. diplacia, they are initially colorless, ‘± glassy’, almost transparent. The apothecia of both species become at least partially discolored with age. In both species, they turn distinctly dark, ultimately blackening with age. The pigments causing this transition are, however, different: apothecia of S. diplacia become disfigured by a deep violet to purple. These blackened parts of the apothecia thus have a violaceous hue. Eventually, apothecia of S. diplacia appear lecideine, carbonized throughout. In L. austrosorediosa, the pigment is instead a  grayish black, apothecia become irregularly carbonized, but they are rarely completely blackened throughout. In section, the different color of both pigments is immediately evident, but then of course the two species can also be distinguished by their spores and asci: Sucioplaca has polardiblastic spores produced in Teloschistes-type asci; Lecanora has simple spores in Lecanora-type asci. Chemically, the two species are also distinct: though both contain atranorin, L. austrosorediosa is further characterized  by ± 2’-O-methylhyperlatolic acid (or closely related secondary metabolites) and zeorin, whereas S. diplacia contains isofulgidin, vicanicin and caloploicin. Sterile specimens of S. diplacia (i.e., those with soralia only) could further be confused with sorediate specimens of L. avium or L. prosecha. Both Lecanora species, when well developed, form areoles with distinctly interlocking, serrate margins. These thalli, therefore, do not resemble those of Sucioplaca, even though sterile specimens of L. avium may have bluish gray soralia. Soralia of L. prosecha are pale, creamy white, similar to the ones of L. austrosorediosa, but the species has cryptolecanorine apothecia. Schumm & Aptroot (2019b) refer to the sorediate morphotype as Caloplaca diplacioides (Vain.) Zahlbr. The protologue of this taxon, however, doesn’t suggest that apothecia are always absent. Instead, Vainio (1896, p. 67) describes apothecia of Placodium diplacioides Vain. as ‘... only rarely forming and often partially developed ...’ (‘apothecia increbra et saepe parcius evoluta’). Clearly, there is no molecular evidence that the sorediate morphotype of Sucioplaca diplacia warrants taxonomic recognition and specimens are regularly both sorediate and apotheciate. We have not examined type material of C. diplacioides, but it is possible that the taxon must be considered a synonym.