Artillery Fungus
Sphaerobolus stellatus
Also called shotgun fungus, cannonball fungus, sphere thrower
A pinhead-sized fungus that splits into a tiny orange star and fires a single dark spore ball several metres into the air. It swarms over damp bark mulch and rotting wood.
Overview
Sphaerobolus stellatus is one of the smallest fungi anyone reliably notices, and only because of what it does. Each fruiting body is a whitish to cream sphere barely 2 mm across, embedded in mulch or rotting wood. At maturity the outer wall splits into a star of pointed rays, revealing a glistening orange inner cup that holds a single dark spore ball. The cup then turns itself inside out with sudden force and launches that ball as far as several metres.
The genus sits in the Geastrales, related to earthstars and other gasteroid fungi, but its structure is unlike anything else: rather than dispersing loose spores or multiple peridioles, it produces one packaged glebal mass per fruiting body and invests everything in throwing it. The ejection is among the fastest movements known in the fungal kingdom relative to size.
Colonies are typically enormous in number and easy to miss individually. The tiny sticky black balls that land and dry onto nearby surfaces are often the first evidence that the fungus is present.
How to identify it
Fruiting body
A minute globe 1.5–3 mm across, whitish, cream or pale ochre, sometimes flushed orange, growing partly sunk in the substrate. Individuals usually occur in vast scattered troops across a mulch surface or rotting log.
Opening
At maturity the outer wall splits from the top into 4–9 short, pointed rays that curve back, so the fruiting body looks like a tiny cup set in a pale star. The exposed inner cup is bright orange to orange-yellow and glistening.
Spore ball
Inside sits a single, roughly spherical glebal mass about 1 mm across, dark brown to blackish and slightly sticky. Unlike bird's nest fungi, which hold several peridioles connected by cords, Sphaerobolus has one ball and no funiculus. After ejection the empty star remains as a small pale cup.
Ejection and traces
The inner layer everts explosively, flinging the ball up to several metres horizontally and often high onto vertical surfaces. Landed balls adhere strongly and dry into hard, dark, tar-like specks a millimetre or two across — frequently seen dotted on light-coloured siding, fences, car paintwork and leaves near mulched beds.
Where and when it grows
Sphaerobolus stellatus fruits on damp, decaying plant matter: bark mulch, wood chips, sawdust, rotting logs and branches, old straw, compost and occasionally herbivore dung. Landscape mulch beds are its most familiar modern habitat, especially where chips are laid thickly, kept moist and partly shaded.
It is widely distributed across temperate regions of both hemispheres, including Europe, North America and Asia. Fruiting is concentrated in the cooler, wetter part of the year — late summer through autumn, sometimes into winter in mild areas — and colonies can persist in the same mulch bed for successive seasons as long as the substrate remains undecomposed and damp.
Ecological role
The fungus is a saprotroph and a vigorous decomposer of woody litter, breaking down cellulose and lignin in chips, bark and rotting wood. Its mycelium can permeate a mulch layer thoroughly, which is why fruiting is typically mass rather than scattered.
Dispersal is purely mechanical. Water pressure builds in the cells of the inner cup until that layer suddenly turns inside out, converting stored elastic energy into a launch that accelerates the spore ball extraordinarily quickly. The fruiting bodies orient towards light before firing, which tends to aim the ball at bright, open surfaces — an adaptation for landing on sunlit vegetation. The sticky ball glues itself where it lands and releases its spores as the wall gradually breaks down, or after being ingested along with the plant material it is stuck to.
Similar species
Bird's nest fungi are the nearest visual relatives. Crucibulum laeve, Cyathus striatus and Cyathus olla are all several times larger, form persistent open cups, and contain several flattened disc-shaped peridioles attached by cords; Sphaerobolus is far smaller, holds a single round ball, has no cords, and leaves only a tiny pale star behind.
Nidula species similarly contain many small peridioles in a sticky matrix within a taller cup. Very small immature puffballs or slime mould fruiting bodies can be mistaken for the unopened spheres, but neither splits into a symmetrical orange-cupped star. The scatter of hard, dark, strongly adhering specks on nearby surfaces is a supporting clue that no look-alike produces.