Caloplaca ceracea J. R. Laudon
Caloplaca caesiorufa auct. brit.
Thallus crustose, areolate, surface roughened, widespreading, pale to dark grey, grey-greenish when moist, to 5 cm diam., prothallus inconspicuous or absent. Apothecia numerous, usually crowded, to 1 mm diam., rounded to flexuose; proper margin thick, swollen, matt, persistent for a long while and looking like small caruncles when old, paler than disc, yellow-orange to yellowish; disc flat or very slightly convex matt brown-orange, olive brown, bright brown when dry, and greenish to olive-green when moist, contrasting with exciple. This green colour is due to algae, forming a dense continuous green layer of closely packed cells running beneath a colourless hypothecium (visible in sections) and is characteristic of this species. Ascospores polarilocular, ellipsoid, septum 3-5 µm wide, about 1/3 of the length of the ascospore, 13-15 x 8 µm according to literature, 14-17 x 6-8 µm, septum de 5-7 µm wide according to our measures. Photobiont : trebouxioid. Thallus K-, apothecia K+ purple. Common, on slightly acid siliceous nutrient-rich rocks, in sheltered and little lit habitats from the xeric-supralittoral zone to the terrestrial-halophilic zone subjected to spray. Frequently confused with species belonging to the Caloplaca crenularia cluster that can extend to marine environments but distinct by its apothecia, bright brown-rusty slightly convex with a concolourous exciple, disappearing later and by the lack of greenish colour on the whole surface of apothecia (but many intermediate specimen exist, difficult to determine). Confused synonymy with closely related species, notably Caloplaca subpallida.
Caloplaca ceracea is a highly variable species, strictly dependent on its habitat, and althought several taxa may be described under the name of Caloplaca ceracea, we suggest here to restrict its use to a well characterized taxon of the xeric supralittoral zone. N.B. A different taxon, growing on slightly acid or calcareous rocks, in little-lit areas near the foreshore may be found: Caloplaca ceracea forme saxicole calcicole, its thallus is thicker, green-yellow and apothecia more brown-orange tinged olivaceous with bright yellow exciple. There is also a particular form with a slightly scaly greenish thallus on the old walls: Caloplaca ceracea forme saxicole des murs.
N.B. In the strictly xeric supralittoral zone there exists on the acid rocks a very similar species but with a gray-blackish thallus with protruding black pycnidia and more red apothecia with a clean-cut margin that does not disappear: Caloplaca fuscoatroides