Lichen it a lot

Quick blip on the way home from work. Crustose lichen I am told is what these are.

Thanks to wikipedia i know know that lichen are 'composite organisms consisting of a symbiotic association of a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc)'. Crikey.

Strange inter-species relationships put me in mind of an episode from the excellent This American Life I listened too recently about parasites (is better than it sounds - you should listen to it).