Spinellus fusiger - A Parasitic Portrait

In the quiet decay of a woodland floor, where mushrooms rise and fall in usually unseen cycles, a second drama unfolds—one that’s rarely seen unless you look closely. Among the decomposing caps of delicate agarics, a ghostly fuzz begins to grow. At first glance, it might seem like mould. But this is no ordinary rot. This is Spinellus fusiger, a parasitic fungus that turns mushrooms into its own reproductive stage.

🧬 What Is Spinellus fusiger?

Spinellus fusiger belongs to the order Mucorales, a group of zygomycetes known for their fast-growing, spore-launching lifestyles. Unlike the mushrooms it colonizes, Spinellus doesn’t form a cap or gills. Instead, it sends up sporangiophores—long, hair-like stalks tipped with black sporangia that release spores into the air. These structures erupt from the surface of the host mushroom, often Mycena or Galerina species, creating a surreal halo of fungal filaments.

Mycoparasitism in Action

This interaction is a textbook case of mycoparasitism—where one fungus parasitizes another. Spinellus fusiger doesn’t just feed on dead tissue; it targets mushrooms that are still alive or just beginning to senesce. It invades the cap, hijacks its nutrients, and uses the mushroom’s structure as scaffolding for its own spore dispersal.  This parasitism is subtle and opportunistic. The host mushroom may still appear intact, but its tissues are being infiltrated by a network of hyphae. Eventually, the mushroom collapses, its reproductive mission aborted, while Spinellus completes its own.

Spinellus fusiger having fully parasitised a Mycena species mushroom - the hyphal stalks can be seen protruding from the cap and gills

Ecological Significance

In places like Slantry Wood or Oxford Island, where fungal diversity thrives in damp leaf litter and pine duff, Spinellus fusiger plays a quiet but crucial role. It accelerates decomposition, recycles nutrients, and adds another layer to the complex web of fungal succession. Its presence is a sign of a healthy, dynamic ecosystem—where even fungi have predators.

🔍 How to Spot It

•     Look for small mushrooms with a fuzzy coating, especially in late autumn.
•     The sporangia appear as tiny black dots atop translucent stalks.
•     A hand lens or macro lens reveals the intricate architecture of the sporangiophores.
•     Common hosts include Mycena galericulata, Mycena inclinata, and other woodland agarics.

Microscopy: 

Under the microscope, Spinellus fusiger reveals its true complexity. What looks like a soft fuzz to the naked eye becomes a forest of sporangiophores—slender, translucent stalks rising from the mushroom’s surface like fungal antennae. Each one terminates in a sporangium, a spherical capsule packed with spores, ready to be launched into the air.

Key Microscopy Features:

•     Sporangiophores: Hyaline (glass-like), often septate, emerging in dense clusters.
•     Sporangia: Black, globose, typically 50–100 µm in diameter.
•     Spores: Smooth, ellipsoid, often visible inside ruptured sporangia.
•     Hyphal Invasion: You may see hyphae penetrating the mushroom’s cap tissue—evidence of its parasitic strategy.

Spinellus fusiger spores showing the ellipsoid shape and large 40-50um size 

These structures are not just beautiful—they’re functional. The height of the sporangiophores helps spores catch air currents, while the sporangia rupture explosively, a hallmark of zygomycete dispersal.

Spinellus fusiger sporangia and spores with measurements

Distribution in Northern Ireland

Widespread Presence

Spinellus fusiger is the most common and widespread parasitic fungus on bonnet mushrooms in Britain and Ireland. This includes Northern Ireland, where it thrives in damp, shaded woodlands.  Habitat Preference - It’s typically found in areas with abundant leaf litter and decaying wood—ideal conditions for its preferred hosts like Mycena galericulata, M. inclinata, and other small agarics.

Likely Hotspots: Based on habitat suitability, you’re most likely to encounter it in places like:

•     Slantry Wood – rich in decaying conifer and broadleaf litter
•     Montiaghs Moss NNR – damp, shaded microhabitats with fungal diversity
•     Oxford Island – mixed woodland and lakeside margins
•     Brackagh Bog – mossy, decomposer-rich zones 

A close up of the sporangia through the macro lens

Spore Analysis

Spores showing the ellipsoidal shape