Verrucaria ditmarsica Erichsen ss. auct. brit.

     Thallus crustose, 1-3 cm diam. but may be larger by joining to contiguous conspecific colonies to form large dark brown areas, very thin when young, gelatinous and translucent, bright and glossy when moist, brownish, dark brown, brown greenish, grey-olive-greenish, smooth and uncracked, getting thicker and covered by minute blackish dots 0.01-0.1 x 0.01-0.06 mm, scattered all over the surface and then merging into small wrinkles or small ridges that, if numerous, can blacken the surface. Perithecia small, broadly conical to hemispherical, 0.1-0.3 mm, bright black (pearly). Ascospores simple, broadly ellipsoid, 8-11 x 5-7.5 µm. Typically on the upper part of rocks or boulders in sheltered areas, notably estuaries at the level of high-waters, since this species tolerates brackish water and a light siltation. May be superficially confused with Collemopsidium halodytes which grows theoretically lower, bearing flatter apothecia, with 1-septate ascospores, and has a cyanobacterial photobiont. N.B. This is a sensitive taxon subject to various interpretations: to some authors it is an intertidal species, or at least a species growing at the high-water tidal mark, to others it extends to the basis of the Hydropunctaria (verrucaria) maura black belt but out of range of extreme high tides. Today the tendancy is to interpret this species as a morph of Verrucaria erichsenii (that would be a synonymous) but the latter would be distinguished by growing in more exposed zones and of which perithecia are more irregular ; however all the possible intermediary between these two taxa exist. To our current knowledge, molecular analysis show that several taxa are designated under these two names, but the sampling procedures and heterogeneity within herbaria often impair the quality of results and discussions, so all the present conclusions have to be cautiously considered until additional data allow to decide whether it is a Hydropunctaria or a Verrucaria (our interpretation being the latter). N.B. Specimens presented here are now studied by sequencing both fungi and algae.