Verrucaria halizoa Leight

    Thallus crustose, very thin, less than 0.1 mm thick, 1-3 cm wide, potentially forming mosaics with conspecific colonies, translucent when wet, smooth, without ribs or wrinkles and only sometimes dotted with pycnidia, olive-green, olive-grey, pale olive-brown, prothallus sometimes though rarely present, well delimited, thin, narrow and blackish. Perithecia homogeneously scattered over the thallus, more or less hemispherical, occasionally deformed, not angular (unless 2 or 3 are clustered), prominent, projections 0.15-0.3 mm diam., ostioles regular, apex crater-shaped. Ascii-8spored, clavate, 20-30 x 8-15 µmaccording to our measures. Ascospores simple, oblong-ellipsoid to narrowly oblong-ellipsoid, 8-11 x 3,5-5 µm according to literature, 8-10 x 3-5 µm according to our measures. Pycnidia usually rare, visible as minute black dots c. 0.05 mm diam. (though sometimes abundant in some forms and not to be mistaken with ribs or dots of Verrucaria (Wahlenbergiella) striatula). On rocks on the foreshore, in the mid-littoral zone out of the main force of the sea and away from rapid drying-out, continuously wet and shaded during low tides, at the barnacles level with Verrucaria (Wahlenbergiella) striatula and Verrucaria (Wahlenbergiella) mucosa and the red algae Hildenbrandia rubra, seldom immediatly above. Uncommon and understated species when thalli are isolated. N.B. This species is in some cases hardly distinguishable from Verrucaria (Wahlenbergiella) striatula of which the ascospores are slightly bigger, thalli blacker, not smooth, minutely ribbed  and with very irregular, angular and much larger perithecia (see pictures of the species intertwined). In some circumstances, microscope analyses are required to evidence the larger ascospores of Verrucaria (Wahlenbergiella) striatula  but this implies the presence of mature ascospores, which is not always the case. There is a form (under study) of areas with very little sun and shade: Verrucaria halizoa forme de l'ombre.