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DescriptionThallus: areolate to rimose, up to 10(-15) cm in diam., (0.1-)0.2-0.4(-0.8) mm thick; areoles: angular to sometimes round, flat to slightly convex or uneven, (0.2-)0.4-1(-2.5) mm in diam., contiguous, rarely dispersed; prothallus: sometimes present as a narrow dark zone at the thallus edge, rarely fimbriate, black to blue-black or gray-black, 0.1-0.5(-1) mm wide; surface: gray, sometimes blue-gray, often with a yellow or olive tinge, rarely olive-brown, dull, often velvety; upper cortex: (20-)30-40 µm thick, uppermost part usually ±brown, 5-10(-20) µm thick, cortical cells (3-)4-6(-8) µm in diam.; cortex covered with an epinecral layer (0-)2-5(-10) µm thick; photobiont: chlorococcoid, cells ±round, 5-16(-19) µm; Apothecia: aspicilioid, usually rather common, (0.1-)0.2-0.5(-1) mm in diam., 1-2(-4) per areole, round, rarely angular; disc: black, usually concave, sometimes plane, without pruina, often surrounded by a distinct, dark exciple; thalline margin: flat to slightly elevated, rarely prominent, concolorous with thallus or darker, or rarely forming a thin, white rim; exciple: (20-)40-60(-100) µm wide , usually I- but sometimes partly I + blue medially; uppermost cells brown, ±globose, 5-6(-8) µm in diam.; epihymenium: green to olive, without or with a few crystals, N+ green to blue-green, K+ brown; hymenium: hyaline, I+ persistently blue, (120-)150-200(-280) µm tall; paraphyses: submoniliform to scarcely moniliform, with 1-3(-5) upper cells ±globose to subglobose or broadly ellipsoid, 3-5(-6) µm wide, in lower part 1.5-2(-2.5) µm wide, slightly branched and anastomosing; subhymenium and hypothecium: pale, I+ persistently blue, together (30-)40-60(-70) µm thick; asci: clavate, (75-)85-140 x (20-)25-35 µm, 8-spored; ascospores: ellipsoid to broadly ellipsoid, (17-)20-28(-35) x (10-)13-19(-20) µm; Pycnidia: rare, 1(-2) per areole, 120-200 µm in diam., with a black to brown, punctiform ostiole, 60-100(-130) µm in diam.; conidia: filiform, straight or slightly curved, variable in size, (10-)13-22(-33) x 0.8-1(-1.2) µm; Spot tests: cortex and medulla I-, K-, P-, C-; Secondary metabolites: none detected by TLC.; Substrate and ecology: on siliceous rocks and boulders in moist habitats by streams and lake shores; World distribution: Eurasia and North America, cirumpolar; temperate to mainly boreal-alpine; Sonoran distribution: southern California and Arizona, at 2020-3290 m.; Notes: Aspicilia aquatica is characterized by an areolate to rimose thallus, a tall hymenium, submoniliform paraphyses, large to very large, broadly ellipsoid spores, conidia of medium length, the absence of secondary metabolites and its habitat. Aspicilia aquatica may sometimes resemble A. caesiocinerea but the latter has a blue-gray thallus color and shorter conidia, and it contains aspicilin and grows in less moist habitats. The conidia length is known to vary in A. aquatica, but normally the conidia in European material do not exceed 20 µm. In most Sonoran specimens they are 16-22µm long but in three specimens from California (El Dorado, Inyo and Ventura Counties) they are up to 33 µm. Further studies are needed to evaluate these differences. Aspicilia americana, A. confusa, A. cyanescens and A. fumosa all differ from A. aquatica by a different thallus color, moniliform paraphyses, and by the habitat. Aspicilia cyanescens sometimes grows close to brooks, but not submerged, and in addition to the mentioned differences also exhibits a blue-green epithecium and usually a prominent prothallus.