Psathyrella aquatica

Psathyrella aquatica is a slender, fragile, dark-spored agaric in the family Psathyrellaceae, the group that contains the brittlestems and their inkcap relatives. What sets it apart from every other member of the genus is not its shape but its address: it fruits underwater, with cap, gills and stipe developing while completely submerged in a flowing stream.

The species was noticed in the upper Rogue River drainage of southern Oregon, where clusters of small brown mushrooms were seen standing upright on submerged wood in cold, clear, spring-fed current. Because gilled fungi normally rely on air to release spores from their gills, an aquatic fruiting habit was long assumed to be impossible, and the discovery drew wide attention among mycologists.

In all other respects it is a typical brittlestem: thin-fleshed, hygrophanous, easily torn, with a hollow whitish stipe and a dark spore deposit. Its distinctiveness lies almost entirely in the combination of ordinary Psathyrella structure with an extraordinary habitat.

Cap

The cap is small, roughly 1 to 4 cm across, beginning bluntly conical or bell-shaped and expanding to broadly convex. The surface is smooth, slightly slimy in water, and tan to olive-brown or dull yellow-brown, fading paler as it dries. Fine translucent striations may show at the margin, and shreds of thin whitish veil tissue can cling to the edge in young specimens.

Gills

Gills are attached to narrowly adnate, moderately crowded, and pale grey-brown when young, darkening with age to deep brown or nearly blackish as the spores mature. The gill edges are often slightly paler than the faces. Unlike true inkcaps, the gills do not dissolve into an inky fluid.

Stipe

The stipe is slender, whitish to pale cream, hollow, and brittle; it is often more or less straight and holds the cap clear of the substrate against the current. A thin fibrillose veil may leave a faint ring zone but no persistent membranous ring, and there is no volva at the base.

Flesh and spore print

The flesh is thin, watery and fragile, breaking rather than tearing cleanly, with no marked colour change on cutting. The spore deposit is dark brown to blackish brown. Under the microscope the spores are smooth and ellipsoid with a germ pore, typical of the genus.

This fungus is known from cold, clear, spring-fed streams where the water temperature stays low and relatively constant year-round. Fruit bodies grow from waterlogged wood, submerged twigs and organic debris lodged on the stream bed, generally in gently flowing rather than turbulent water, and are usually found at depths of a few tens of centimetres.

The documented range is very restricted, centred on the upper Rogue River area of southern Oregon in the Pacific Northwest of North America. Fruiting has been recorded chiefly through the warmer months, from around mid-summer into early autumn, when the submerged wood is well colonised. Its apparent rarity may partly reflect how seldom anyone looks for mushrooms below the waterline.

Psathyrella aquatica is saprotrophic, digesting waterlogged wood and plant debris on the stream bed rather than forming any partnership with living plants. Its mycelium permeates submerged sticks and coarse organic litter, contributing to the slow breakdown of woody material that enters streams from the surrounding riparian forest.

The most intriguing part of its life cycle is spore release. Spores are shed from the gills into water rather than air, and are thought to be carried away by the current or to rise as they are released, which represents a substantial departure from the ballistospore-into-air mechanism used by nearly all gilled fungi. The cap's upright posture on a firm stipe within slow current appears well suited to letting spores drift free of the fruit body.

Numerous small brown Psathyrella species grow on wet streamside wood and debris just above the waterline, and any of them can be mistaken for this fungus if the specimen has been dislodged and washed up. Habitat is the single most useful separator: only P. aquatica is found with the entire fruit body developed while submerged, and a mushroom collected from damp bankside soil is far more likely to be a common terrestrial brittlestem such as Psathyrella candolleana, which is larger, paler cream to buff, and carries conspicuous white veil fragments on the cap margin.

Psathyrella piluliformis forms dense clusters on stumps and buried hardwood, with reddish-brown hygrophanous caps and a fibrillose veil, but always in air. Inkcaps such as Coprinellus micaceus share a dark spore print and streamside wood habitat but are separated at once by their glistening mica-like cap granules and by gills that deliquesce into black fluid, which Psathyrella gills never do. Species of Galerina growing on wet mossy wood have a rusty-brown spore print rather than a blackish one, and a tougher, less brittle stipe.