Aquatic gilled mushrooms: Psathyrella fruiting in the Rogue River in southern Oregon.

A species of Psathyrella (Basidiomycota) with true gills has been observed fruiting underwater in the clear, cold, flowing waters of the upper Rogue River in Oregon. Fruiting bodies develop and mature in the main channel, where they are constantly submerged, and were observed fruiting over 11 wk. These mushrooms develop underwater, not on wood recently washed into the river. Substrates include water-logged wood, gravel and the silty riverbed. DNA sequences of the ITS region and a portion of the ribosomal large subunit gene place this fungus in Psathyrella sensu stricto near P. atomata, P. fontinalis and P. superiorensis. Morphological characters distinguish the underwater mushroom from previously described species. Fruiting bodies have long fibrillose stipes with small diameter caps. Immature stages have a thin veil that is soon lost. Gills lack reddish edges. Cystidia are ventricose with subacute apices. Spores were observed as wedge-shape rafts released into gas pockets below the caps. Underwater gills and ballistospores indicate a recent adaptation to the stream environment. This particular river habitat combines the characteristics of spring-fed flows and cold, aerated water with woody debris in shallow depths on a fine volcanic substrate. Based on molecular and morphological evidence we conclude that the underwater mushrooms are a new species, Psathyrella aquatica. This report adds to the biodiversity of stream fungi that degrade woody substrates. The underwater environment is a new habitat for gilled mushrooms.