Wahlenbergiella mucosa

Wahlenbergiella mucosa is a species of saxicolous (rock-dwelling), crustose lichen in the family Verrucariaceae. It is a marine species that grows in the littoral zone, and therefore remains immersed in seawater for extended periods. The lichen forms a smooth, shiny thallus up to 1 mm thick that varies in colour from olive-green in shaded areas to dark green or black in full sunlight. First described by Erik Acharius in 1803 as a member of the genus Verrucaria, it was reclassified in 2009 to the genus Wahlenbergiella following molecular phylogenetics research. The species is very common throughout Britain and Ireland (except southeastern England), growing among barnacles and macroalgae, particularly on exposed shores where it forms extensive patches. It has been studied as a potential bioindicator of coastal water pollution....read more on Wikipedia.