Verrucaria aquatilis Mudd

    Thallus crustose, very thin and continuous, 0.05 mm thick, smooth or seldom slightly cracked in some parts, sometimes minutely dotted (under magnification lens), dark grey-brown, dark brown, and in fine blackish when brought out of water (the only aquatic freshwater black Verrucaria?), prothallus black but most often absent. Perithecia usually numerous under favourable conditions, very small, forming low projections 0.1-0.2 mm, almost completely immersed at first then slightly protruding, but the young perithecia are covered with a layer of thallus at the beginning, ostiole sometimes bluish on a cut (so visible!). Ascospores very broadly ellipsoid, 7-9 x 5-7 µm. Chemical spot tests negative. Common on rocks and boulders totally submerged in winter and spring in high water quality rivers and streams (trout habitat).

N.B. This species probably is under estimated because the access is only possible in summer. Share the same aspect as some morph of smooth and thin thallus Hydropunctaria maura, seemingly with small perithecia growing on submerged rocks in freshwater rivers.

N.B. During the low water period in summer, this species is very difficult to spot on the rocks of streams because its dull colours make it easy to confuse with rotting algae.