Crystal Brain Identification Guide

The First Impression

Cold, wet weather, ideally after a run of mild rainy days between autumn and early spring. You are looking at a dead deciduous branch, either lying on the ground or still hanging in the shrub layer, and along a section of it there is something that reads at first as spilt water or a smear of frogspawn. It is barely coloured and almost transparent, and if the light is flat you can walk past it repeatedly without registering it at all.

The habit of scanning branches at an angle, so light glances off wet surfaces, is what makes this species findable. Once you catch the glint, get close, because the confirming character is invisible from more than a hand's breadth away.

Reading the Jelly Body

There is no cap, no stipe and no separate underside. The whole structure is one gelatinous mass with the fertile layer covering its exposed surface.

Individual lobes are cushion-shaped to shallowly brain-like, generally 0.5 to 2 cm across, and they coalesce along the branch into irregular masses that may run to 10 cm or more. The surface is smooth to gently folded, never spiny, never fringed, never leaf-like. Colour is the point: watery white to greyish, essentially translucent, so that the bark shows through the jelly. It is not opaque white, not cream, and not tinted yellow or pink.

The Crystals

This is the character the species is named for and the one that closes the identification. Detach a lobe and hold it up against the sky or a torch beam. Scattered through the jelly, and usually denser near the point of attachment, are opaque chalk-white granules from about half a millimetre to a millimetre and a half across. Backlit, they show as clear white nuclei suspended in clear jelly.

You can also feel them. Crush a lobe between finger and thumb and it grinds slightly, as if there were fine sand in it. These inclusions are mineral, and no other white gelatinous fungus on hardwood branches carries them. If a specimen looks right but shows no crystals under a strong backlight, treat the identification as unproven.

Texture and Behaviour

Handle a lobe. It is soft and slack, more watery than rubbery, and it tears or smears rather than snapping cleanly. Firmer, springier jellies belong elsewhere.

Watch what dry weather does to it. The mass shrinks to a thin, colourless, varnish-like film pressed onto the wood, almost unrecognisable, and then revives to full size within hours of rain. If you note the locations of colonies you can return after a wet spell and find them restored, which is often the easiest way to get good material.

Attachment and Substrate

The fruitbody is broadly attached along its whole underside, without a stalk or a distinct sterile base. It grows on dead hardwood branches, both decorticated wood and bark, with beech, ash, hazel, sycamore, oak, elder and willow all frequent. It commonly shares a branch with resupinate crust fungi, and it favours small to medium diameter wood in the earlier and middle stages of decay.

Its role is as a wood-rotting saprotroph, contributing to the white rot of branch wood, and colonies typically follow the same branch for a season or two before it becomes too far decayed.

Spores and Microscopy

A spore print is difficult but not impossible: rest a lobe on dark glass inside a closed box with a damp tissue and leave it overnight. The deposit is white and very fine.

Microscopy is where the confirmation becomes airtight. The spores are sausage-shaped, smooth and colourless, roughly 10 to 14 by 4 to 5 micrometres, and germinate by budding off secondary spores. More importantly, the basidia are divided lengthways into four cells by longitudinal septa. That cruciate arrangement places the fungus among the true jelly fungi and rules out the gelatinous cup fungi, whose spores are packed into cylindrical asci.

Season and Weather

Fruiting is essentially a wet-weather phenomenon spanning autumn, winter and early spring, with mild damp winter spells often the most productive time of all. Summer records are scarce, and summer specimens are usually the dried varnish stage rather than expanded jelly.

Separating the Look-alikes on Morphology

Exidia thuretiana, the white brain, is the closest match. It is opaque milky white rather than watery translucent, noticeably firmer and springier, more strongly folded into brain-like convolutions, and it contains no crystalline granules. Backlighting a lobe of each side by side settles it in seconds.

Ductifera pululahuana, in North America, forms white cushions on hardwood but is again opaque and crystal-free.

Tremella fuciformis and its relatives build frilly, leaf-like or petal-like lobes with thin translucent blades rather than solid cushions.

Exidia glandulosa and Exidia nigricans are blackish-brown and often warted; Tremella mesenterica and Tremella aurantia are bright yellow to orange; Dacrymyces stillatus is orange and typically on conifer.

Neobulgaria pura, a cup fungus, forms firm pinkish-buff rubbery discs on beech, with a definite disc shape and a distinct margin, and no crystals.

Slime moulds such as Enteridium and Fuligo can look like colourless jelly for a few hours, but they change dramatically within a day, becoming crusted and then powdery, and they flow over leaves and stems as well as bark.

Working Checklist

• Watery, translucent, almost colourless jelly on dead deciduous branches
• Lobed cushions 0.5 to 2 cm, coalescing into masses to 10 cm
• Opaque chalk-white granules visible inside the jelly when backlit, gritty when crushed
• Soft and slack, tearing rather than snapping; whole surface fertile
• Shrinks to a colourless film in dry spells and revives after rain
• Mild wet weather from autumn to spring