Collemopsidium sublitorale

Collemopsidium sublitorale is a species of lichen in the family Xanthopyreniaceae. First described in 1871, it forms a "borderline" symbiotic relationship with cyanobacteria that is less structured than typical lichen partnerships. The species is characterised by an immersed thallus and black, roughly spherical fruiting bodies measuring 0.15–0.55 mm in diameter. It inhabits the intertidal and littoral fringe of marine environments, where it grows exclusively on calcareous surfaces such as limestone and marine shells. Its range includes north-west Europe and the Pacific coast of North America (United States and Canada)....read more on Wikipedia.