Crab Brittlegill

Crab Brittlegill (Russula xerampelina) is the best known member of a group of brittlegills unified by three characters that are rare in the genus: a distinct shellfish odour that strengthens with age, flesh and stipe that discolour ochre-brown when handled, and a deep ochre spore print. Together these make it one of the more confidently identifiable Russula species in the field.

It is better understood as a species complex than a single species. Members of the R. xerampelina group differ mainly in cap colour and tree partner — dark wine-red and purple forms under pine, olive and green-brown forms under spruce, and brighter red-brown forms with oak and birch — and various segregates such as R. graveolens, R. faginea and R. clavipes have been split off at different times.

The group also responds distinctively to iron salts: a crystal of ferrous sulphate rubbed on the stipe flesh gives a green reaction rather than the pink or salmon typical of other brittlegills, a spot test long used to place a collection in this group.

Cap

5–14 cm across, convex then flattening and often shallowly depressed. The classic form is dark wine-red, purple-red or blackish-crimson, usually darkest at the centre and paler, more olive or brown-tinted, toward the margin; other forms in the complex are olive-brown, coppery or brick. The surface is dry and matt to slightly velvety, not glossy or sticky, and the cuticle peels only a short distance from the margin. The margin becomes weakly grooved with age.

Gills

Pale cream when young, maturing to a warm ochre or pale buff-yellow — noticeably deeper than the white gills of most red brittlegills. Attachment is adnexed to nearly free. The gill edges and faces brown where bruised or where slugs have grazed, and old specimens have a rusty, spotted look.

Stipe

4–10 cm tall, 1.5–3 cm thick, stout, cylindrical or slightly club-shaped, white but usually flushed with rose-pink over part of its length. Handling turns it dull ochre-brown within minutes, and old specimens are brown throughout the lower half. It is firm, solid then softly stuffed, with no ring or volva, and snaps cleanly.

Flesh & spore print

The flesh is white, browning slowly on exposure. The odour is strong and characteristic — a fishy, shellfish or crab-like smell that becomes obvious as the fruiting body ages. Ferrous sulphate on the flesh produces a green stain. The spore print is deep cream to ochre; spores are ellipsoid with tall, isolated conical warts.

The complex occurs on the ground in coniferous, broadleaved and mixed woodland. The typical dark wine-red form is associated with pine on sandy, acid soils, while other members of the group partner spruce, oak, beech and birch. It often fruits along sunny woodland rides, on heathy ground and among needle litter, sometimes in loose troops.

Distribution spans Europe, northern Asia and North America. Fruiting is from late summer into autumn, peaking in September and October in temperate regions, and the fruiting bodies are relatively long-lasting for a brittlegill, though they discolour heavily as they age.

Crab Brittlegill is ectomycorrhizal. It forms mantled root tips on pine, spruce, oak and birch, threading hyphae out into the soil and litter to capture water, phosphorus and nitrogen that it exchanges with the host tree for sugars. Species in this complex are frequent members of the mycorrhizal community on sandy, nutrient-poor forest soils, where the fungal network greatly increases the effective root surface of the trees.

The strong shellfish odour is produced by volatile compounds released as the fruiting body matures, and it makes the mushrooms conspicuous to invertebrates: fungus gnats, beetles and slugs are frequent visitors, and the resulting damage speeds decay and returns nutrients to the litter. Spore dispersal is by air currents from the ochre gills, and the perennial soil mycelium can persist on the same root system for many years.

Russula atropurpurea shares the dark purple-red cap but has a white spore print, gills that stay cream, a stipe that greys rather than browning, and no shellfish odour. Russula queletii and R. sardonia are also wine-purple under conifers but have a distinctly violet-flushed stipe, a white to pale cream spore print, and no browning reaction.

Within the complex itself, Russula graveolens is a chiefly oak-associated segregate with a browner-red cap, and olive-capped forms under spruce are often listed as separate microspecies; all share the browning stipe, ochre spore print and shellfish smell, so separating them relies on tree partner and cap colour. In practice, the combination of browning handled stipe plus ochre spore print plus fishy odour places any collection in this group.