Asterophora parasitica, the silky piggyback mushroom (pictured here on Russula nigricans)

A mushroom growing on another mushroom (hyperparasitic mushroom)! This is a highly unusual fungus, with an exceptionally adapted reproductive strategy - for good reason: due to its niche substrate - growing on only a select few species of host mushrooms. Whereas most agarics (cap and stem mushrooms) reproduce by releasing sexual spores from basidia (club-shaped cells in their gills), the piggyback mushroom will have very few of these. This species is very unlikely to be able to find a suitable mate in its very limited substrate - hence cannot rely on sexual reproduction. Instead it has evolved to release asexual spores from the top of its cap, via a powdery secretion.