Mushroom Monday: The Mossy Maze Polypore (Cerrena unicolor)

Mushroom Monday: The Mossy Maze Polypore (Cerrena unicolor)

In the complex world of forest ecology, some of the most fascinating stories involve multiple species working together – or against each other. Today’s featured fungus, Cerrena unicolor, commonly known as the mossy maze polypore, plays a central role in one such story, involving two different wasps and the trees they inhabit.

Identification

Cerrena unicolor forms semicircular, fan-shaped fruiting bodies that grow in overlapping clusters on hardwood trees. These brackets range from 0.5 to 10 cm wide (approximately 0.2 to 4 inches). While young specimens display a fuzzy, whitish to grayish upper surface, older specimens become bare and darker, often hosting a coating of green algae that can be mistaken for moss. The underside begins with a maze-like pattern of pores that develops into tooth-like structures as the fungus ages, changing from whitish to grayish over time.

Habitat and Role

Cerrena unicolor acts primarily as a saprophyte, breaking down dead wood, but can also act as a weak parasite on stressed or wounded trees. It typically appears on various hardwood species, particularly maple (Acer species) and birch (Betula species), especially in moist areas and along watercourses. It produces powerful enzymes that break down both cellulose and lignin, the main structural components of wood, contributing to nutrient recycling in forest ecosystems.

A Complex Relationship

What makes Cerrena unicolor particularly interesting is its relationship with two different wasp species. The pigeon horntail wasp (Tremex columba) carries spores of this fungus. When female horntail wasps emerge from their pupal stage, they pick up fungal spores in their body. Later, when laying eggs in trees, they introduce these spores along with their eggs – a mutualistic relationship where the wasp transports the fungal spores to new hosts while the fungus helps break down the wood, creating better conditions for the developing wasp larvae.

The presence of Cerrena unicolor also produces chemical signals that attract a second species – the giant ichneumon wasp (Megarhyssa species). These large wasps use their long ovipositors to detect horntail larvae within the wood and lay their eggs directly into the larvae, which will eventually be consumed by the ichneumon larvae. The ichneumon wasp is a parasitoid, meaning its larvae develop by feeding on and eventually killing a single host – in this case, the horntail wasp larvae. The chemical signals that attract these parasitoid wasps may be a byproduct of the fungus’s wood-decomposing activities, rather than being produced intentionally to attract the wasps. Further research into this complex relationship may reveal additional insights about the ecological role of these chemical signals.

Ecological Role

The horntail wasps typically target stressed or weakened trees rather than completely dead ones, as their larvae require living tissue for initial development. This suggests that Cerrena unicolor may begin its relationship with trees while they are still alive but stressed, transitioning to purely saprophytic activity as the affected tissues die. This complex interaction between fungus, wasps, and trees demonstrates the intricate connections that exist within forest ecosystems.

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