Hydropunctaria maura (Wahl.) Keller & Gueidan  s.l.

   Thallus crustose, potentially covering large surfaces through merging of contiguous conspecific colonies, tightly appressed to rocks, highly variable but usually thin (< 1 mm thick), typically cracked (dried mud-like), areolate to fissured, areoles 0.3-0.6 mm, plane to gently convex, matt black, dark brown-black, surface tinged greyish in old well-lit specimen, scattered with warts and little prominent wrinkles, 20-30 µm, dispersed or aligned. Prothallus thin, narrow, sometimes absent, blackish, and fading. Perithecia hemispherical to conical-hemispheric, 0.2-0.5 mm, partially immersed, flattened and inconspicuous, ostiole regular, crater-like. Ascospores simple, ellipsoid, 15-18 x 8-10 µm. Very common, from rocks immersed at high tides of greater ranges or beaten by the waves, i.e. the higher part of littoral zone up to the aerohaline zone. The most common among the black belt Verrucaria species, so that blackened rocks seem to be polluted with oil residues as during oil spills (but it is sometimes the case: see photograph!).N.B. If the common use is to calling the black belt 'Verrucaria maura belt', this use is improper since several black species are involved in its composition. At the same level as Hydropunctaria maura s. str. two cryptic species may coexist, undistinguishable if isolated, but potentially distinguisable by a comparative approach when growing coterminous to Hydropunctaria maura, namely Hydropunctaria oceanica and Hydropunctaria orae. Hydropunctaria oceanica has a thinner thallus, more greenish with an incomplete network, a brownish prothallus and small conical perithecia, ascospores slightly smaller and narrower (although this is hardly decisive). Hydropunctaria orae is common in estuaries, has a thinner thallus, more greenish and a pale whitish prothallus. At the lower level of Hydropunctaria maura thrives Hydropunctaria amphibia of which the thallus is ribbed, numerous perithecia are eroded and ascospores narrower. Below the Hydropunctaria maura level, Verrucaria ditmarsica / erichsenii may be found. Its perithecia are rounded and prominent, and this not to mention other undescribed "black" species.