Psathyrella aquatica Identification Guide

The Setting You Would Be Standing In

Everything about finding this species is unusual, beginning with the fact that you are looking down into moving water rather than at the ground. The setting is a cold, clear, spring-fed river in southern Oregon during the low-water months of summer, from roughly late June into September. The bed is cobble and gravel, with waterlogged branches, bark fragments and packed organic debris wedged among the stones. The water is shallow enough to see through and cold enough year-round that a wetsuit or at least waders and a polarising lens are part of the kit.

Psathyrella aquatica fruits completely submerged, sometimes tens of centimetres below the surface, standing upright in the current on stalks that arise from that submerged woody debris. The first impression is of small pale honey-brown pinheads on the riverbed, holding their position while everything else drifts. Two details give them away: they stand vertically rather than lying flat like waterborne litter, and they frequently trap a bright silvery air bubble beneath the cap margin or among the gills, which glints when the light shifts.

A polarising filter or simply looking into a shaded stretch removes surface glare and turns a confusing riverbed into a readable one. Once the eye adjusts, colonies of a dozen or more become obvious.

Step One: Read the Cap

The cap is small, roughly 5 to 25 mm across, hemispherical to convex, occasionally with a low broad umbo, and it never becomes fully flat. Colour is hygrophanous, meaning it tracks water content: saturated caps are tan, dull orange-brown or fairly dark brown, and as the tissue drains the margin pales toward buff and cream, producing a two-toned look in specimens lifted from the water.

The margin is translucent-striate — hold a fresh cap to the light and the grooves show the gill positions through the thin flesh. Young caps carry scattered whitish veil fibrils or small cottony patches along the edge, remnants of a delicate partial veil that vanishes quickly. The surface is smooth, not scaly and not obviously slimy, and the flesh is thin enough that the cap feels almost membranous between the fingers.

Step Two: The Gills

Turn the cap over while it is still wet. The gills are adnate to narrowly adnexed, moderately close, and progress from pale grey through cinnamon-brown to a dark blackish-brown as the spores ripen. Mature gills often look mottled or unevenly blotched rather than uniformly coloured, which is a hallmark of this whole group and reflects spores maturing in patches.

Examine the gill edge with a lens. It is usually noticeably paler than the faces, appearing whitish and finely fringed. Under a microscope that pale edge resolves into abundant sterile cells (cheilocystidia), the standard confirmation of the genus.

Step Three: Stipe, Attachment and Flesh

The stipe is 20 to 60 mm long and only 1 to 4 mm thick, white to whitish, hollow, brittle and finely fibrillose, sometimes with a few scattered veil fibres left on the upper half. There is no true ring, though a faint fibrillose zone may persist briefly on young specimens.

The attachment is the character that carries the identification, so handle the collection carefully. Trace the stipe down to its base and establish exactly what it is fixed to: submerged twigs, bark, waterlogged wood fragments or packed organic sediment among the cobbles. Lift the whole substrate piece out of the water rather than snapping the stalk. Photograph the attachment in situ if you can, because a specimen without a demonstrated submerged anchor is not a record.

The flesh is thin, watery brown in the cap, whitish in the stipe, and extremely fragile — the genus name refers to that friable texture. It does not change colour when cut, and there is no latex and no distinctive odour.

Step Four: Taking a Spore Print From Waterlogged Material

A saturated cap will not drop spores onto paper, so the usual method needs adapting. Lift the specimen, stand it on absorbent paper in still air and let it drain for ten to twenty minutes, until the surface is merely damp rather than streaming. Then cut the stipe flush, place the cap gill-side down on white paper or a glass slide, cover it with a cup or a small box to hold humidity steady, and leave it four to eight hours. Water-loaded tissue is slow, and it is worth checking rather than abandoning the attempt early.

The print is dark purple-brown to blackish-brown, which is the family signature and immediately separates the collection from the rust-brown and white-spored small agarics that share wet habitats. Under the microscope the spores are smooth, ellipsoid, dark brown in potassium hydroxide, and carry a distinct germ pore at one end; the gill edge is crowded with cheilocystidia.

Substrate, Season and Ecology

The fungus is saprotrophic, breaking down submerged wood and organic debris rather than partnering with a tree. What makes it remarkable is that it completes fruiting underwater: the cap expands, the gills mature and spores are shed while the whole fruitbody is submerged in flowing water, so dispersal happens downstream in the current rather than on air. The trapped air bubbles so often seen under the caps are part of that unusual picture.

The habitat is narrow — cold, clear, well-oxygenated, spring-fed flowing water with stable summer temperatures and abundant submerged woody debris. The species is documented from a short reach of the upper Rogue River in Jackson County, Oregon, fruiting during the low-flow summer months, which makes any claim from elsewhere something to document exhaustively rather than assume.

Separating It From Look-Alikes on Structure Alone

The overwhelming majority of small brown mushrooms found in or beside a river are not this species, and the checks are straightforward.

Washed-in specimens are the biggest source of error. A fruitbody dislodged upstream and lodged among cobbles will usually show abraded tissue, a torn or missing stipe base, waterlogged collapse and no living attachment. Confirm a firm, intact, growing connection to submerged substrate, and confirm that the substrate itself sits below normal flow rather than on an exposed gravel bar.

Other members of the genus fruit abundantly on damp streamside wood, stumps and buried debris, but above the waterline. Species such as P. piluliformis grow in dense tufts on rotting stumps, run considerably larger, and have date-brown caps; the shared purple-brown print means you must decide the question on habitat and attachment rather than on spore colour.

Galerina and Conocybe species can look similar in size and cap colour on wet ground, but their spore prints are rust-brown to cinnamon rather than purple-brown, and their gills mature to a warm rusty tone rather than blackish and mottled.

Coprinellus and Coprinopsis species also produce small striate caps with dark spores, but their gills deliquesce into ink or dissolve as they age, and their prints are black.

If a collection matches the structure but comes from outside the known range, the honest approach is to record it as an unresolved member of the genus, backed by in-situ photographs of the submerged attachment, notes on water temperature and flow, and preserved material for spore and cystidia examination.