Buellia stellulata

Buellia stellulata, commonly known as the disc lichen ) is a species of crustose lichen that is widely distributed throughout the Northern Hemisphere.

Thallus
The thallus is crustose, areolate, and thin to moderately thickened, often forming a continuous layer. The prothallus is conspicuous and black, strongly developed in most specimens, growing between the areoles to form a hypothallus. The thallus surface is usually white to whitish gray, rarely dark gray, and can be dull or slightly shiny. The thallus is epruinose and phenocorticate, and lacks soredia.

Medulla
The medulla is white and lacks calcium oxalate.

Apothecia
The apothecia exhibit a lecideine structure, with a diameter ranging from (0.2-)0.3-0.4(-0.5) mm. They are typically immersed to adnate, occasionally found in a sessile form. The margin may appear black or masked by grayish remnants of necrotic thalline material, forming a thin and usually persistent boundary, occasionally excluded as they mature. The disc is characterized by a black, epruinose surface, typically flat but occasionally developing a slight convexity over time. Within the apothecia, the proper exciple appears narrow and lacks differentiation, resembling the aethalea-type. The inner excipular hyphae are narrow, hyaline, and often prosoplectenchymatous, exhibiting structural and orientational similarities to the paraphyses. The hypothecium is characterized by a deep reddish-brown color, described as leptoclinos-brown with a textura intricata. Outer excipular hyphae are arranged parallelly, moderately swollen (textura oblita), and heavily carbonized, displaying a range of brown shades and aeruginose pigments.

Epihymenium
The epihymenium is brown, with pigmentation continuous with the outer exciple.

Hymenium
The hymenium is hyaline and not inspersed with oil droplets. The paraphyses are simple to moderately branched, with apically swollen tips containing a brown pigment cap.

Asci and ascospores
The asci are clavate, Bacidia-type, and 8-spored. The ascospores soon become brown, 1-septate, oblong to ellipsoid, usually not constricted, with obtuse ends, and not curved, measuring (8-)8.7-[9.9]-11.1(-13) x (4.5-)4.8-[5.5]-6.1(-7) μm. The proper septum is narrow and does not thicken during spore ontogeny (Buellia-type). The spore ornamentation is microrugulate.

Chemistry
The lichen contains the secondary metabolites atranorin, 2'-O-methylperlatolic acid, and confluentic acid. Spot tests are K+ yellow (sometimes weak), P- or + faintly yellow, C-, KC-, and CK-. UV fluorescence is -(dark).

Substrate and ecology
Buellia stellulata, an epilithic lichen, thrives on siliceous rock surfaces (typically HCl-) and is commonly found in coastal regions. However, its habitat is not restricted solely to seashore environments.

Distribution
Buellia stellulata exhibits a broad distribution across the Northern Hemisphere. Within the Sonoran region, this species is notably present in coastal areas spanning from southern California to Baja California and Baja California Sur.