#027: Gilled Mushrooms (Agarics)

The gilled mushrooms, informally referred to as ‘agarics,’ are the type of mushroom with which we are most familiar. The most common edible mushrooms (white/button/portabella mushrooms, oyster mushrooms, and shiitake mushrooms), Amanita muscaria – the most recognizable mushroom in the world and the inspiration for almost all mushroom art – and the ‘magic mushrooms’ are all gilled mushrooms. All these mushrooms share one feature: vertical plates of spore-producing tissue stacked under a sterile cap.^1–5

Morphology

Gilled mushrooms are amazingly diverse. If you look at a field guide, chances are at least half of it will be devoted to agarics. What makes these organisms so successful? A large part of it is probably due to their gills. Gills (known to mycologists as ‘lamellae’) are plates of spore-producing tissue that form perpendicular to an upper sterile cap (called a ‘pileus’) and radiate out from a single point. The shape of these plates of tissue is reminiscent of fish gills, which resulted in the term ‘gills’. Gills are very efficient at increasing the surface area of the hymenium, which means the mushroom can produce more spores and give itself a higher chance of reproducing successfully.^>1

The above definition of “gills” is necessarily broad because gilled mushrooms have a highly variable morphology. Many agarics, like A. muscaria, have a central stalk called a ‘stipe’ that supports the pileus. The stipe serves to lift the spore-bearing surface above the layer of stagnant air near the ground (or, indeed, any surface). Gills on agarics with a stipe radiate out from the stipe and may be attached to the stipe in a number of different ways. The stipe itself is variable and may be positioned in the center of the pileus, on the edge of the pileus, or somewhere in between.

Some mushrooms, like the oyster mushroom, do not form a stipe or, if they do, it is small and lateral. These mushrooms have a ‘pleurotoid’ morphology (named for the oyster mushroom, Pleurotus ostreatus).^>6 In pleurotoid mushrooms, the gills radiate from the point at which the mushroom is attached to the wood. These mushrooms do not need a stipe because they are usually found growing on logs and are thus already elevated above the static layer of air.

Other variations on the agaric morphology include the color, shape, texture, decorations, and developmental strategy of the gills, pileus, and stipe. All of those differences as well as the ones mentioned above are useful in identifying agaric species. Microscopic characters can also be good diagnostics; the most important of these is the spore color, which can be assessed macroscopically by taking a spore print.

The chart below summarizes the key differences between 17 of the most commonly encountered groups of agarics. Blank cells mean that the feature is variable within the group and/or not important for identification.

If you want to speed up your identification in the field, you should memorize this chart. Actually, developing an intuitive sense for what kinds of mushrooms belong to which group is more helpful, but that takes years of identification practice. Until then, use this chart to help you decide on a starting point for identification. The list above is hardly exhaustive; many more groups of agarics exist, but those are generally less common.

Taxonomy

One reason that there is so much variability within this morphological group of mushrooms is because gills have evolved independently multiple times. Gilled mushrooms are all basidiomycetes (see FFF#013), but the morphological group is polyphyletic. At this point it is important to note that the terms “gilled mushrooms,” “agarics,” and “pleurotoid” do not really have any taxonomic meaning. Instead, they are informal terms that refer to a diverse group of mushrooms that happen to look similar. They are useful, however, in field guides and to help with mushroom identification.¹

Most agarics belong to the order Agaricales. Gills also evolved in the orders Russulales, (see FFF#028), and Polyporales (see FFF#030), although these have not diversified nearly as much as the Agaricales. All these orders contain fungi with gills indistinguishable from Agaricales gills. However, the Boletales and Polyporales also have some mushrooms with gills that are not quite so well formed. For example, Phylloporus rhodoxanthus (FFF#187) and Lenzites betulina (FFF#168) are known as the “Gilled Bolete” and the “Gilled Polypore,” respectively. Neither one looks quite enough like the rest of the agarics to hide their taxonomic affiliations. In both cases, the mushrooms are considered to have ‘false gills’. The order Cantharellales (see FFF#029) has also produced species with well-developed, gill-like ridges, although these are blunted and therefore also considered false.¹¹

All the orders with true gilled mushrooms also contain mushrooms with other, simpler morphologies.³⁵ This indicates that they did not all derive from a gilled common ancestor and must therefore have evolved gills have evolved independently of one another. This must have happened at least four times, but likely more than that. I suspect that the Agaricales were the first to evolve gills, since that order contains the greatest number and the most diverse group of agarics.

All agarics belong to the kingdom Fungi, division Basidiomycota, subdivision Agaricomycotina, and class Agaricomycetes. The table below lists some of the most common agaric genera by taxonomic divisions. This chart does not show you which families or genera are most closely related to one another, but it should give you an idea of the genetic diversity found in gilled mushrooms.

 

 

Tricholomatoid

These mushrooms used to be collected in an enormous family called Tricholomataceae. This family was the dumping ground for all the white-spored mushrooms that mycologists couldn’t decide where to put. As you might guess, DNA evidence is tearing this family to shreds. While the mushrooms will remain in the order Agaricales, mycologists have had to establish numerous new families to account for the extreme diversity of the tricholomatoid mushrooms. Many genera still remain in Tricholomataceae, but it seems likely that in a few decades the only genus left there will be Tricholoma.^11,19

Morphologically, the tricholomatoid mushrooms have only two things in common: 1) they have white or pale spore prints and 2) they do not belong to another group. Despite this lack of similarity, field guide authors continue to use the tricholomatoid group because it allows them to separate these fungi from all the other morphologically distinct groups of pale-spored agarics. The general strategy is this: make sure your mushroom doesn’t belong to Amanita, Russula, etc. before tackling the trichalomatoid mushrooms.^11,19

Unsurprisingly, tricholomatoid mushrooms have many different nutritional strategies and preferred habitats. Additionally, the group contains both edible and poisonous species.^11,19

Bolbitiaceae

Except for their brown to yellow spore colors, the Bolbitiaceae Singer³⁶ look for all the world like small coprinoid mushrooms. The stature and thin flesh are reminiscent of mushrooms in the genus Coprinopsis, but the small mushrooms in Bolbitiaceae do not have deliquescent gills and produce spores much lighter in color. Additionally, the surface of the pileus in Bolbitaceae mushrooms is composed of circular cells (‘cellular’) whereas the coprinoid mushrooms have a filamentous cap surface. The two main genera in the Bolbitiaceae are Bolbitius and Conocybe. Both contain small and relatively uninteresting species. Bolbitius mushrooms are nontoxic but inedible due to their size while Conocybe mushrooms contain hallucinogens and amatoxins (FFF#091) and so should be avoided.¹¹

Paxillaceae

Paxillaceae Lotsy³⁷ is related to the boletes and contains agarics with a yellow-brown to clay colored spore print, decurrent gills, an inrolled margin, and flesh that stains green in ferric sulphate. Some of the mushrooms in the group lack a stipe and therefore can be considered pleurotoid. The Paxillaceae most closely resemble either the Gomphidiaceae or species of Crepidotus. Both of those lineages have darker spores, so once you take a spore print it should be an easy matter of separating out the Paxillaceae. Paxillaceae contains both mycorrhizal and saprobic species, so the mushrooms appear on wood as well as on the ground. Mushrooms in Paxillus produce gastrointestinal irritants and are therefore considered poisonous.¹¹