Lichen is a fungus, usually of the class Ascomycetes, that grow symbiotically with algae, resulting in a composite organism. Lichen characteristically form a crust like or branching growth on shingle granules. Lichens put out root-like tentacles that penetrate deep into the shingle’s then an oil base for nourishment. Eventually lichen will ruin your shingles by creating dark spots, or pock marks where they grow. Click here to read more.