Starfish Fungus | Aseroe rubra Identification Guide

First impression: something red and radial on the mulch

You usually register this species with your nose before your eyes. Standing beside a mulched garden bed, a bark-chipped path or a compost-enriched border on a warm, wet morning, you notice a strong, cloying, putrid smell and a cloud of flies working over something at ground level. When you look down there is a red star lying flat on the wood chips, five to ten centimetres across, with a pale hollow column holding it a few centimetres above the surface and a black, glistening smear at the centre. Nearby, half-buried, are one or two whitish eggs the size of a large marble. That whole tableau, red arms plus white column plus buried eggs plus flies, is the identification in outline; the rest is confirming the details that separate it from its relatives.

Step 1: the egg stage

Always look for eggs, because they carry good characters and because they tell you the colony is still producing. An egg is 2 to 3 cm across, ovoid, whitish to pale pinkish-buff, soft and slightly yielding, and sits half-buried with a thick white cord running from its base into the mulch. Slice one vertically. You will see an outer skin, then a thick clear to whitish gelatinous layer, then a dark olive-brown band of immature spore slime, and inside that the compressed, folded arms and the accordion-pleated column waiting to expand. Expansion is fast: an egg can go from closed to fully open in a few hours.

Step 2: the column

The stalk that lifts the star is hollow, spongy and chambered in section, white to pale pink, 3 to 8 cm tall and 1 to 2 cm thick. It is fragile and watery, and the base is sheathed in a whitish to pinkish volva, the remains of the egg skin, with the white cord still attached beneath. There is no ring, no gills and no cap in the ordinary sense.

Step 3: the disc and the arms

At the top of the column the tissue expands into a flattened red to orange-red disc 3 to 4 cm across. From the rim of that disc, five to nine arms (most often six to eight) radiate outward, each 1.5 to 4 cm long, bright scarlet to orange, wrinkled or ridged on the upper face, tapering and often curling at the tips.

The single most important structural point is that each arm forks. Near its base or around its midpoint every arm divides into two tapering branches, so a fruit body with six arms presents twelve points. That bifurcation, combined with the flat central disc, is what separates this species from every other radial stinkhorn you are likely to meet.

Step 4: the spore slime

The gleba is a dark olive-brown to blackish, glutinous, strongly scented layer, and its position is diagnostic. It sits in the central depression of the disc and along the inner bases of the arms, not smeared along the whole upper length of each arm. Flies land within minutes and crawl through it, carrying spores away on their bodies and legs, which is the entire dispersal strategy of the group. After the slime has been removed by insects or rain, the disc surface beneath is revealed as pale and pitted, and the fruit body is far harder to name; try to work with fresh material.

Step 5: flesh and colour change

The tissue is spongy, watery and extremely fragile. It does not exude latex, does not bruise to a new colour and shows no chemical colour change on cutting. What it does do is collapse: within a day or two, sometimes within hours in hot weather, the whole structure slumps into a soft pinkish mass and then dissolves. Any identification therefore has to be made on the day.

Step 6: spores without a spore print

There is no spore print here, because spores are never dropped from an exposed fertile surface; they are carried away in slime. The equivalent step is to touch a clean glass slide or a scrap of card to the gleba, then examine the smear. The spores are minute, roughly 5 to 6 by 2 to 2.5 micrometres, cylindric-elliptical, smooth, and colourless to pale olive individually even though the mass looks black. Their small size and enormous numbers are what make the slime so dark and so effective at sticking to insect feet.

Substrate, ecology and season

This is a saprotroph of lignin-rich debris rather than a tree partner. It thrives in wood chips, bark mulch, sawdust, compost, and mulched garden beds, and in its native range in forest litter. It is native to Australia and the western Pacific and has been carried around the world repeatedly in mulch and nursery stock, so it now appears in gardens, botanic collections and municipal plantings well outside its original range. Fruiting is triggered by warmth and heavy rain: late summer and autumn in temperate areas, and effectively year-round in the subtropics. A colonised mulch bed will often produce successive flushes over several weeks, so it is worth returning to the same spot.

Separating it from look-alikes on structure alone

Clathrus archeri, the octopus stinkhorn. The commonest confusion. It has four to eight arms that are unforked, joined at their tips when young and splitting apart as they open, and the spore slime is spread along the whole inner surface of each arm. There is no flattened central disc.

Pseudocolus fusiformis. Three or four tapering arms that stay fused at the top, forming an arch or cage rather than a flat star, in orange to pinkish tones, with slime on the inner arm faces.

Lysurus mokusin. The stalk is distinctly fluted or angular in cross-section, and the short arms are gathered into a column at the apex rather than spread flat.

Lysurus cruciatus and relatives. Short, thick, unforked arms carrying the slime on their surfaces, and no expanded disc.

Phallus and Mutinus species. A single unbranched column topped by a slime-covered cap or a tapering coloured tip; no radiating arms at all.

Earthstars (Geastrum). Radial, but dry, papery, brown and odourless, with a distinct central spore sac. They open by weathering rather than expanding on a stalk.

Field sequence

1. Strong smell and flies over mulch or wood chips in warm wet weather.
2. White hollow column rising from a whitish volva, with buried eggs nearby.
3. Flat red disc at the top, 3 to 4 cm across.
4. Five to nine red arms, each forked into two tips.
5. Dark slime in the centre of the disc, not along the whole arms.
6. Tissue spongy, collapsing within a day.