The thing in the photo below is called a lobster mushroom. But it is neither a lobster, nor a mushroom.
It is actually a fungus in the phylum Ascomycota — Hypomyces lactifluorum. It parasitizes mushrooms in the genera Lactarius (milk caps), Lactifluus, and Russula (russulas), gradually covering the entire fruiting body until the host is unrecognizable and slowly dies. Infected specimens turn bright orange-red, like the shell of a cooked lobster.
Hypomyces lactifluorum is mainly found in North America. It tastes like seafood, with a dense, meaty texture, and is sold commercially there as food. Parasitism even rewrites the host's flavor: Lactifluus piperatus, the peppery milk cap, is naturally hot — once parasitized, it is no longer spicy.
A Genus of Mushroom Parasites
Hypomyces belongs to the Ascomycota: class Sordariomycetes, order Hypocreales, family Hypocreaceae, genus Hypomyces. Every species in this genus parasitizes mushrooms.
Hypomyces cervinigenus parasitizes saddle-shaped mushrooms of the genus Helvella.
This is what the host normally looks like — which is why the parasite is called H. cervinigenus ("deer-colored"):
Hypomyces chrysospermus parasitizes boletes. At first it forms a thin layer over the mushroom, which gradually turns golden yellow as the parasite's spores develop.
Later, the golden spore layer covers the bolete completely.
Hypomyces hyalinus parasitizes Amanita mushrooms. The host's fruiting body is gradually wrapped until it is unrecognizable and finally dies. Unless an uninfected mushroom of the same species grows nearby, you cannot tell what the host was at all.
Beyond Hypomyces, there are many other fungi that parasitize fungi. Hypomyces species are ascomycetes; mushrooms are mostly basidiomycetes. Given that most ascomycetes parasitize plants and animals, an ascomycete parasitizing a basidiomycete seems almost reasonable.
When a Mushroom Parasitizes a Mushroom
Basidiomycetes can also parasitize other basidiomycetes.
The best-known example is Asterophora lycoperdoides — a mushroom that grows on mushrooms, mainly parasitizing species of Russula.
Pseudoboletus parasiticus parasitizes the hard earthball Scleroderma citrinum.
When infected, the earthball sprouts these bolete-like fruiting bodies of its parasite.
And ascomycetes can parasitize ascomycetes: Tolypocladium ophioglossoides (formerly in the genus Elaphocordyceps, the "deer truffle club") parasitizes the false truffle Elaphomyces.
Infection turns the false truffle into a base for the parasite's club-like fruiting bodies.
The Silver Ear Mushroom: A Fungus That Lives on Fungus
Among fungi that parasitize fungi, the most famous is the silver ear mushroom, Tremella fuciformis. In fact, the sexual state of every species in the entire class Tremellomycetes — including the silver ear mushroom you see in daily life — is parasitic on other fungi, while their asexual states need no fungal host.
The sexual-stage silver ear cannot get nutrients by simply degrading rotting wood. It mainly parasitizes ascomycetes in the family Hypoxylaceae (carbon fungi), with a strong preference for Annulohypoxylon archeri. That is why commercial cultivation must sow carbon fungus spores together with the silver ear — and why this is a textbook example of a basidiomycete parasitizing an ascomycete.
Tremella mesenterica, the golden ear or witch's butter, parasitizes basidiomycetes instead — for example, the crust fungus Peniophora.
When the Parasite Itself Gets Parasitized
Everything described so far is a fungus parasitizing another fungus, but the hosts were mostly saprophytes or plant-symbionts. What if the host fungus is itself a parasite?
