Hypomyces lactifluorum or "Lobster Mushroom", contrary to its common name, is not a mushroom, but rather a parasitic ascomycete fungus that grows on certain species of mushrooms, turning them a reddish orange color that resembles the outer shell of a cooked lobster. H. lactifluorum specifically attacks members of the genera Lactarius and Lactifluus (milk-caps), and Russula (brittlegills), such as Russula brevipes and Lactifluus piperatus in North America. At maturity, H. lactifluorum thoroughly covers its host, rendering it unidentifiable. Lobster mushrooms are widely eaten and enjoyed fresh. They are commercially marketed and sometimes found in grocery stores; they have been made available at markets in Oregon. They have a seafood-like flavor and a firm, dense texture.

Source: Wikipedia