Verrucaria species

Schiavo, Lo, 2015, Revision of the Verrucaria elaeomelaena species complex and morphologically similar freshwater lichens (Verrucariaceae, Ascomycota), Phytotaxa 197 (3), pp. 161-185 : 173-174

Key to freshwater Verrucaria species with spreading involucrellum, median ascospores lengths> 20 μm

and fully epilithic, subgelatinous thallus.

Specimens with poorly developed thallus can easily be confused with the sub-aquatic V. margacea agg. and the little known V. filarszkyana with slender ascospores, V. muralis s. lat. or similar but mostly terrestrial species with broad ascospores. These species lack subgelatinous thalli and they are usually found in habitats which become less frequently inundated. Thallus sections show at least locally empty spaces between fungal cells.

1 Ascospores mostly slender (length/width ratio> 2.2).......................................................................................................................2

- Ascospores mostly broader (length/width ratio typically 1.8–2.1, occasionally up to 2.3) [ Verrucaria elaeomelaena agg.]......................7

2. Mean ascospore length ≤23 μm, if ascospores longer then perithecia often in close proximity with neighbouring involucrella merging, thallus thick (>60 μm).................................................................................................................................................................3

- Mean ascospore length>23 μm, perithecia widely spaced and involucrella not merging, thallus always thin (<60 μm)............................6 3 Thallus with olive-blackish cortex pigment (in thin section)...................................................................................... V. pachyderma

- Thallus cortex pigment absent or yellowish to brownish or (brown-)black.......................................................................................4

4 Thallus thin (15–60 μm) and always with transparent area between exciple base and pigmented part of involucrellum (in thin sections).................................................................................................................... V. hydrophila ( V. hydrela auct. p.p.)

- Thallus thick (usually> 60 μm), if thinner usually without transparent area between exciple base and pigmented part of involucrellum or involucrellum variable with apical and dimidiate forms sometimes in the same thallus.......................................................5

5 Thallus blackish-green when dry, involucrellum variable from apical to reaching the thallus base and laterally spreading and forming a black basal layer,Ascospores short and slender, absent from Central European lowland areas.......................................... V. pachyderma

- Thallus with green, brownish or yellowish colours, involucrellum never apical, always reaching the base and often fused with neighbouring perithecia forming a black basal layer. Ascospores long and slender to elongate, from sea level to alpine areas across Europe................................................................................................................................ V. funckii

6 Mean ascospore length>30 μm (26–31.7 μm in the British Isles), perithecial mounds 400–800 μm............................... V. alpicola

- Mean ascospore length 22–27 μm, perithecial mounds 400–600 μ. .................................................................................... V. placida

7 Exciple base transparent, rarely pale brown. Median ascospore length usually 20–25 μm. Thallus grey to yellowish or light brown, with yellowish pigment or without cortex pigment. Mostly at low elevations (<600 m)..................................................................................8

- Exciple base brown to transparent, median ascospore length usually>25 μm. Thallus brownblack, grey to yellowish, without cortex pigment or with yellowish brown to blackish cortex pigment. In Central Europe mostly at high elevations, also at lower elevations in N- and NW Europe.......................................................................................................................................................................................10

8 Ascospores with length/width ratio of 1.0–1.5, (12–) 16–20 μm wide......................................................................... V. pachyspora

- Ascospores with length/width ratio of 1.6–2.4, up to 16 μm wide.....................................................................................................9

9 Perithecia 220–360 μm in diameter, thalline mantle absent or restricted to the perithecia base, median of ascospores 20.7–24.6 μm, median of spore length/width ratio 2.0–2.3......................................................................................................................... V. humida

- Perithecia (200–) 400–1250 μm in diameter, usually covered in thalline mantle, median of ascospores (20–) 23–28.3, median of spore length/width ratio 1.6–2.4........................................................................................................................ V. elaeomelaena agg.

10 Algal layer continuous, not interrupted by carbonised columns protruding from the basal layer. Involucrellum usually covered by thalline mantle..................................................................................................................................................................................11

- Algal layer interrupted by carbonised vertical columns,which reach the thallus surface.Upper half of involucrellum exposed. ........................... V. maas-geesterani

11 Thallus brownblack but cortex transparent (not pigmented), surface with irregular cracks...................................................... V. basaltica

- Thallus brownblack to yellowish or grey, dark specimens with brown to black pigment in cortex, pigment pale or absent in yellowish to grey (shade)forms, surface continuous to cracked. ................................................................................................................ V. alpicola