Most aquatic mushroom

The most aquatic species of mushroom is Psathyrella aquatica, native to the Rogue River in Oregon, USA. It is the only species of basidiomycete whose basidiocarp (the mushroom) exists underwater, and is anchored up to 0.5 m (1 ft 7 in) deep in sediment in order to withstand the river's strong, fast-moving currents. All other basidiomycete mushrooms presently known to science are terrestrial.

This remarkable, unique species was formally named and described as recently as 2010 (initially appearing online in 2009), after having been discovered five years earlier in the Rogue River by Professor Robert Coffan from Southern Oregon University. As Coffan and his colleagues noted when describing this species: "Fruiting bodies develop and mature in the main channel, where they are constantly submerged, and were observed fruiting over 11 weeks. These mushrooms develop underwater, not on wood recently washed into the river. Substrates include water-logged wood, gravel and the silty riverbed". It is the only fungus to form a macroscopic structure underwater.