Crab Brittlegill Identification Guide

First Impressions and the Setting

Russula xerampelina, the Crab Brittlegill, is a substantial, solid-looking brittlegill of woodland soil. You typically meet it in late summer and autumn under conifers, especially spruce and pine on sandy or acidic ground, though the wider group includes forms that partner with birch and oak. It grows from soil and litter, singly or in small scattered groups, and it has a heavy, well-fed look: a thick stipe, a broad firm cap and no trace of a ring or veil at any stage. From a metre away the impression is of a dark wine-red or purple-red dome, sometimes with a browner or olive-tinged centre, sitting on a stout white column that is often flushed with pink on one side. Even before you crouch down, two things separate brittlegills from most other agarics: the cap sits on the stipe like a nail head with no ring, and the whole fungus snaps rather than tears when you flex it.

Reading the Cap

The cap is 5 to 14 centimetres across, occasionally larger. It is convex at first, then flattens and finally becomes shallowly depressed in the centre, with a margin that stays even and smooth for a long time and only becomes faintly grooved in old, waterlogged specimens. Colour is the least reliable character, because this is a variable complex: expect dark vinaceous purple, wine-red, brownish-purple or purplish-red, frequently darker in the centre and often washed with olive, ochre or brownish tones as it ages or dries. The surface is dry to slightly greasy when damp, matt rather than shining, and the cuticle peels perhaps a quarter to a third of the way from the margin towards the centre when you lift it with a thumbnail. Underneath the peeled skin the flesh is white to faintly pinkish.

The Gills

Turn the cap over and study the gills carefully, because their colour is a major clue. They are moderately spaced, fairly thick, adnexed to almost free, and brittle in the characteristic Russula manner: press a finger sideways along them and they crumble instead of bending. They begin pale cream, and as the spores ripen they deepen through pale ochre to a definite deep ochre or yellowish-buff, a much warmer and more saturated colour than the white or cream gills of many red brittlegills. Any part of a gill that is bruised, insect-damaged or handled slowly stains dull brownish. The gill edges are even and concolorous. Look also at the interspaces: forking is limited, mostly near the stipe, and there are few or no short intermediate gills scattered between the full-length ones.

Stipe, Base and the Browning Reaction

The stipe is 4 to 10 centimetres tall and 1.5 to 3.5 centimetres thick, cylindrical or slightly club-shaped, solid at first and becoming softly stuffed or chambered inside with age. It is white with a fine longitudinal wrinkling, but on most collections one side or the base carries a distinct rose-pink or wine flush. Now perform the definitive test: rub the stipe firmly between finger and thumb, or scratch it with a fingernail, and set it aside for a few minutes. In this species the handled area discolours to a dingy honey-brown or ochre-brown, and by the time you get home the base is usually distinctly browned. This browning of the stipe surface, especially at the base, is one of the two anchor characters for the whole xerampelina group.

The second anchor is the smell. It is strong, penetrating and quite unlike the mild woodland scent of most brittlegills, best described as crustacean-like or reminiscent of a harbour at low tide. It becomes more obvious as the fungus ages and is strongest around the stipe base. That odour is exactly where the English name Crab Brittlegill comes from.

Flesh and Chemical Reaction

Cut the fungus in half lengthwise. The flesh is white, thick and firm in the cap, softer and more cottony inside the stipe, and it slowly discolours brownish rather than reddening or blackening. The classic laboratory-free chemical check is a crystal of iron sulphate, sold as ferrous sulphate, rubbed on the stipe flesh. In almost all brittlegills this produces a salmon-pink to orange-pink stain within a minute. In the Crab Brittlegill group it produces a dark green to blue-green stain instead. That single, easily carried reagent makes the group unmistakable and is worth keeping in a film canister in your bag.

Taking and Reading a Spore Print

Brittlegill identification depends heavily on the exact shade of the spore print, so make it a thick one. Cut a mature cap free of the stipe and place it gills-down on white paper, cover it with a bowl, and leave it for at least six hours, preferably overnight. Then scrape the deposit into a small heap with a knife blade, because a thin scatter always looks paler than the true colour. For this species expect a deep ochre to dark yellow deposit, clearly egg-yolk tinted rather than cream. Under the microscope the spores are broadly ellipsoid, 8 to 11 by 6.5 to 9 micrometres, ornamented with warts and short ridges that only partly join into a network, and the warts stain blue-black in Melzer reagent.

Substrate and Season

This is an ectomycorrhizal fungus that partners with forest trees, most typically spruce and pine in the strict sense of the name, with closely related forms under birch, oak and beech. It fruits from mid summer to late autumn, on acid to neutral soils, in both natural woodland and plantation. It never grows on wood. Because the group contains several segregate species distinguished largely by host tree and cap colour, note the trees within ten metres before you leave the spot.

Separating the Closest Look-alikes

Russula atropurpurea has a similarly dark purple cap but its gills stay white to pale cream, its spore print is whitish-cream, its stipe does not brown appreciably, and it lacks any strong odour.

Russula sanguinaria has a bright scarlet cap and a stipe strongly flushed pink, but its spore print is pale cream to light ochre, the flesh does not turn green with iron salts, and it has no crustacean smell.

Russula queletii and R. sardonia are slimmer, with purple-violet tints running down into the stipe, cream to pale ochre prints, and gills that never reach deep ochre.

Russula graveolens, R. clavipes and R. faginea sit inside the same broad group and share the browning stipe, ochre print, green iron reaction and characteristic smell. Separating those relies largely on cap colour, host tree and spore ornamentation rather than on any single field character, so a collection can legitimately be recorded at group level.

Russula lepida and R. rosea have hard, dry, matt caps with a bloom, white spore prints and firm flesh that stains orange with iron salts.

Field Checklist

• Stout brittlegill with wine-red to dark purple, often olive-tinted cap
• Thick brittle gills ripening deep ochre
• White stipe with a pink flush that browns where handled, especially at the base
• Strong crustacean-like odour, sharpest in age
• Deep ochre spore print
• Green stain with ferrous sulphate
• Soil under conifers or broadleaves, mid summer to late autumn