Synonyms: Dermatocarpon aquaticum (Weiss) Zahlbr.; Dermatocarpon fluviatile (Weber) Th. Fr.; Dermatocarpon weberi (Ach.) W. Mann; Endocarpon aquaticum (Weiss) P. Gaertn. G. Mey. & Scherb.; Endocarpon fluviatile (Weber) DC.
Distribution: N - Lomb (Rivellini 1994), Piem (Isocrono & al. 2004), VA (Piervittori & Isocrono 1999, Piervittori & al. 2004, Watson 2014), Emil (Fariselli & al. 2020), Lig (Brunialti & al. 2001). C - Tosc, Sar (Nöske 2000).
Description: Thallus foliose, umbilicate-polyphyllous, 3-10 cm wide, consisting of many interconnected, 5-15 mm wide, 0.15-0.4 mm thick (when wet) lobes with numerous holdfasts on the lower side, epruinose, pale to dark brown, turning bright green when wet, growing amphibious in cold freshwaters; lower surface pale to dark brown, smooth or slightly wrinkled, erhizinate. Upper cortex 60-130 µm thick, lower cortex: 30-90 µm thick, of paraplectenchymatous cells; medulla white, I+ red. Perithecia numerous, immersed in the thallus and scattered over almost the whole upper surface (appearing as black dots), in section pale, with only the tips brownish, c. 0.2-0.5 µm in diam. Hamathecium or periphyses and periphysoids, interascal filaments absent; hymenial gel hemiamyloid. Asci 8-spored, clavate, fissitunicate, the wall thickened above, not amyloid. Ascospores 1-celled, hyaline, elongate-elliptical 10-20 x 5-8.5 µm. Pycnidia black, immersed. Conidia bacilliform, 4-5 µm long. Photobiont chlorococcoid. Spot tests: all negative. Chemistry: without lichen substances.
Note: a cool-temperate to subarctic-subalpine, probably circumpolar species found on periodically inundated siliceous rocks near creeks and brooks, or on steeply inclined, shaded faces with frequent water seepage.