Acarospora is a genus of mostly lichen-forming fungi in the family Acarosporaceae. Most species in the genus are crustose lichens that grow on rocks in open and arid places all over the world.[2] They may look like a cobblestone road or cracked up old paint, and are commonly called cobblestone lichens or cracked lichens.[3]: 216 [4] They usually grow on rock (are "saxicolous"), but some grow on soil (terricolous) or on other lichens.[2][3]: 216 Some species in the genus are fungi that live as parasites on other lichens (lichenicolous fungi).[2] Acarospora is a widely distributed genus, with about 128 species according to a 2008 estimate.[5]
Species in Acarospora may be shiny as if covered with a glossy varnish, or dull and powdery looking. They have a diverse range of colors, from the brilliant yellow bright cobblestone lichen, to the dark reddish-brown mountain cobblestone lichen, or they can appear tan, gray, or white, from a dusty-looking coating (pruina).[3]: 216 They may grow in crustose forms like a warty surface (verrucose), like cracking-up old crust of paint (rimose), like a bunch of "islands" in a dry lake bed (areolate), like the flakes of cracking up paint are peeling up at the edges (sub-squamulous), or like the flakes are growing over others like scales (squamulous).
They may grow as a warty crust (verrucose, a cracked crust rimose, or with the cracks separating island-like sections like in a dried lake (areolate – with the “islands” being called “areoles”).[2] The areolas may lift up at the edges (sub-squamulose), and these edges may overlap other areolas like scales (squamulose, with the areoles being called “squamules”).[2] The areoles may grow in lobes radiating from a center (placodioid. They may grow in irregular or indeterminate forms, sometimes with the areoles disconnected from each other and dispersed among other lichens.[2] Sometimes the squamules may be elevated with expansion of the mycelial base above the substrate ("gomphate"), or aside on “stems” called stipes, which are usually about usually half the diameter of areole.[2] The outer rim of the areola is usually down-turned.[2]
They may be shiny or dull, and in many shades from pale to blackish brown.[2] They may be smooth or rough (rugulose).[2] They may be different colors from brilliant yellow (from rhizocarpic acid) to brown to white.[2] They may or may not be covered with a powdery-looking surface (pruinose), which when present, may make them appear lighter in color, to almost white.[2]
Like other crustose lichens, their cross-section is generally divided into three layers, the cortex, photobiont layer, and medulla, and generally without a lower cortex as in foliose lichens.[2] The cortex itself is usually differentiated, with three layers including a syncortex (sometimes absent),[citation needed] epinecral layer (sometimes absent,[citation needed] and eucortex, which is where the pigment is located in the upper parts.[2] The photobiont of Acarospora are algae in the genus Trebouxia.[6]
Each wart, areola, or squamule may have 0 to many apothecia.[2] The apothecia are usually immersed in the thallus.[2] Sometimes the apothecia are raised on a wart[citation needed] and surrounded by a margin of thallus-like tissue, sometimes with the margin being a true exciple.[2] The apothecia are usually immersed, and round to very irregular in shape.[3]: 216 The apothecal disc is round to squished and irregular, and ranges in colors: black, brown, red, or yellow, or in-between. The disc may be smooth or it may be rough. The asci range from being narrow to being club-shaped (clavate). Spores are colorless, spherical to ellipsoid, and range from tens to hundreds per ascus.[3]: 216