Oyster Mushroom – Pleurotus ostreatus

The laterally attached stipe (stem) and rounded white to brown semicircular pileus (cap) of the Oyster mushroom are readily recognizable. They look like a “side-ear.”

Common Name: Oyster Mushroom, Tree Oyster Mushroom, Grey Oyster Mushroom, Hunter’s Mushroom, Hiratake (Japanese for ‘flat mushroom), Píng gū (Chinese for “flat mushroom”) – The semicircular white fruiting body of the mushroom is reminiscent in shape and color to the shell of an oyster.

Scientific Name: Pleurotus ostreatus – The genus name is descriptive, deriving from the Greek words pleura meaning ‘side’ and ot meaning ‘ear.’ The oyster shaped mushroom grows out of the side of a tree bole like a side ear. Ostreatus is Latin for “like an oyster shell.”

Potpourri:  The “side-ear” or pleurotoid morphology of the oyster mushroom was recognized as distinctive by the earliest pioneers of fungal taxonomy. The renowned German botanist Nees von Esenbeck, who postulated the origins and development of fungi in 1816, called one section of his numerous divisions Pleurotus. While originally assigned to the catch-all generic assignation Agaricus in 1774 by Nicolas von Jacquin, the director of the Vienna Botanical Gardens , the oyster mushroom was permanently assigned to the new genus Pleurotus by Paul Kummer in the seminal work Der Führer in die Pilzkunde (Guide to Mycology) in 1871. And that is the way things stood until about 10 years ago, when the biological revolution engendered by DNA sequencing supplanted classical taxonomy based on similarities in appearance. Phylogenic studies have revealed a complex web of geographic and genetic diversity in the oyster mushroom clade that is only now emerging in the new taxonomy of evolutionary heredity.

In spite of their taxonomic heterogeneity, the oyster mushroom archetype is easy to identify in the field, and, whatever species it may be, it is edible and considered choice by most. The laterally attached stipe (stem) and rounded white to brown semicircular pileus (cap) are readily recognizable. They look like a “side-ear,” according to the genus, or perhaps like an oyster, according to the common name. It is sometimes mistakenly avowed that the name oyster mushroom is due to the smell that emanates from the fungus; presumably this is the smell of oysters which would imply a fishy, seashore aroma. There is no consensus among the fungus experts as to the nature of the smell: Tom Volk, the noted University of Wisconsin mycologist describes it as “from very mild to very strong, sometimes sweet with the smell of anise (licorice)” while the self-proclaimed mushroom expert Michael Kuo attests that they have a peculiar smell “but I would be hard pressed to describe it, an ‘oyster mushroom’ smell.” While there are no deadly doppelgangers of the oyster mushroom, there are a few species that are similar in appearance to the extent that they are whitish and grow in brackets on wood. The most notable is the Lentinellus ursinous, or Bear Lentinus, which a casual inspection would reveal as having toothed gills and not the uniform gills of the oyster. However, even if one were to mistakenly harvested, one taste would provide adequate deterrence. The bitter and acrid taste is so acute that, according to Tom Volk, it is “the only mushroom that raccoons will spit out.”

The Lentinellus ursinous, or Bear Lentinus looks a bit like the oyster

The gustatory excellence of the oyster mushroom varies according to the preferences of the author, to the variation in the species that may have been consumed on that particular occasion and to the habitat and the time of the year; oyster mushrooms fruit all year depending on the species and circumstance. They tend to be darker with a firmer texture when harvested in the colder months relative to the whiter and more delicate morphology of the warmer months. The inimitable “mycovore” Charles McIlvaine, author of the seminal One Thousand American Fungi, offers an ambrosial encomium: “Its very name implies excellence. The camel is gratefully called the ship of the desert; the oyster mushroom is the shellfish of the forest. When the tender parts are dipped in egg, rolled in bread crumbs, and fried as an oyster they are not excelled by any vegetable, and are worthy of place in the daintiest menu.” David Aurora notes in Mushrooms Demystified that oyster mushrooms are “a universal favorite – with a superb fishy texture and taste” which has probably contributed to the conceit that they smell fishy (like an oyster) as well. My personal experience is that an omelet made with wild oyster mushrooms is of singular succulence without a hint of fishy taste.

While oyster mushrooms globally proliferate in the forests and woodlands and are assiduously gathered and eaten by the mycophilic peoples of Europe and Asia, their real distinction is their fecundity. According to Chang and Miles in Mushrooms, 2nd Edition, oyster mushrooms are “by far the easiest and least expensive to grow of all industrially cultivated edible mushrooms.” The various species can selectively be grown year-round on a variety of substrates – almost any cellulosic thing will do. It takes about two weeks for the oyster mushroom mycelium to convert woody, dried plant materials into fresh, edible fruiting bodies at the ratio of 100 grams of substrate to 50 grams of mushrooms. While this is remarkable enough from the standpoint of crop yield, it is of equal note that the fungal crop is highly nutritive; the oyster mushroom, like most of the other edible fungi, is an exceptionally healthy food. One 15 gram oyster mushroom contains about 6 grams of protein, 4 grams of dietary fiber and all 8 of the essential amino acids. It is also an excellent source of B-vitamins, particularly Riboflavin, important for good vision, and Niacin, important for digestive and neurological function. According to the myco-evangelist Paul Stamets in Mycelium Running “Oyster mushroom cultivation can help alleviate poverty and hunger by recycling waste materials of little economic value and turning them into nutritious and medicinally beneficial products.” The implication is that the omnivorous Pleurotus mycelium has a potentially major role to play in ecological remediation, in this case mycoremediation – the use of fungi to remove toxic substances from the environment. Oyster mushrooms have an affinity for hydrocarbons, the polymeric backbone molecules of many industrial compounds, notably those derived from petroleum. Mushrooms in general and oyster mushrooms in particular also concentrate heavy metals, affording a means of agglomeration to facilitate extraction. Stamets continues with a eulogy: “If one mushroom can steer the world on the path of greater sustainability …. And helping communities integrate a complexity (of) waste streams, oysters stand out.” However, it is probably best not to conflate the mycoremediation and food production aspects of the oyster mushroom, as concentrated heavy metals and degraded petroleum would certainly have some deleterious health implications.

The changes in the taxonomy of the oyster mushroom over the last ten years have been profound. As a benchmark, the 2004 Second edition of Mushrooms by Chang and Miles provide that “more than 1,000 species of the oyster mushroom have been described throughout the world, in more than 25 related genera. However, only approximately 50 valid species are recognized in the genus Pleurotus.” These populations were defined according to the taxonomy of the Linnaean system wherein the generic relationships between living things are determined from their structure. The species to which an individual organism was assigned within the genus was defined by the ability to interbreed – that is to mate and produce fertile offspring. As it turns out, there are two problems with the Linnaean logic when applied to fungi. The first is that physical appearance is not necessarily an indication of relatedness; DNA similarities are a far more direct indicator – something not known ten years ago. DNA analysis of fungi has revealed that things that have the same structure are not related and things that are related have significantly different structures. The second problem is that, in the Kingdom Fungi, interbreeding is not a matter of male and female as it is in (almost) all other living things. According to Nicholas Money in Mr. Bloomfield’s Garden, “sexual behavior in mushroom-forming fungi spans monogamy and civility, to group sex and slaughter.”

So where does that leave the question of oyster mushroom identity? In a 1996 paper entitled “Recent Advances in Molecular Systematics of the Genus Pleurotus,” Duke Mycologist Rytas Vilgalys et al reported on both the phylogenetic (DNA) and mating compatibility of oyster mushrooms. DNA analysis tracks the evolutionary history and relationships of different individuals by evaluating the degree of matching between genomes. These studies revealed that oyster mushrooms have “considerable genetic diversity” and that the Pleurotus genus was genetically divided into “several major groups that correspond to – geographical provenance.” The P. ostreatus clade was determined to be a major taxonomic subdivision found mostly in the northern hemisphere that “appears to have evolved relatively recently, possibly since the Pleistocene.” The mating studies referenced in the review were much more involved. The only way to tell if two fungi are of the same species is to put a spore from each of two different fruiting bodies in a growth medium like agar and see if the hyphae that emerge join together to form a dikaryon (a cell with two different nuclei from which the Phylum name Dikaryomycota originates; karyon is the Greek word for nut and in the lexicon of biology, it is the nucleus of a cell). The analysis in this case was to demonstrate hyphae that did not interact and were thus different species, members of unique intersterility groups; there are a total of 15 intersterility groups in the genus Pleurotus worldwide (vice the 1,000 – or maybe 50 – cited above in 2002). In addition to P. ostreatus found mostly in eastern North America, and P. pulmonarius found mostly in western North America (both are also found in Europe and Asia), there are 6 other North American intersterility groups, or species. Vilgalys concludes with the recommendation that the mating studies be used for the classification in subsequent research and points out that “frequent name changes are inevitable as new information about species comes to light.” The answer to the question posed about oyster mushroom identity is that it is still a moot point.

One intriguing possibility as a means of establishing the defining characteristics of oyster mushrooms outside phylogenetic DNA analysis and mating studies has been suggested. According to Peterson et al on the University of Tennessee Mycology Lab web site, “many visible characters have been used to circumscribe the genus, but few have proven worthwhile.” These incomplete characterizations include the location of the stipe or stem (some are centrally located instead of on one side), the white color of the spores (some are violaceous) and whether the gills are decurrent, extending down the stem (some have a partial veil). The proffered singular defining characteristic of the oyster mushroom clade is the formation of “nematostatic microdroplets.” Nematodes are small, ubiquitous (about 80 billion per acre) roundworms that permeate the soil; a nematostatic compound paralyzes them. According to Bryce Kendrick in The Fifth Kingdom, Pleurotus fungi “secrete a substance that rapidly inactivates nematodes, allowing the fungus to colonize their inert bodies. Since Pleurotus species are often primary colonizers of dead wood, a substance notoriously deficient in nitrogen – the nematodes may be an important component of the fungal diet.” The toxin has been identified as trans-2-decenedioic acid. The oyster mushroom as nematode predator is not unique, as there are a number of carnivorous fungi, including Coprinus comatus, the Shaggy Mane, and Stropharia rugosoannulata, the Wine-Cap Stropharia. It should really come as no surprise that mushrooms eat worms. Fungi have had to struggle to survive against the difficult odds of being immobile like the plants but without their autotrophic ability. Fungi are heterotrophic – they must get their food from another source without being able to go look for it. This has resulted in high evolutionary pressures to forge new chemistries for survival – nematostatic acid is just one. Oyster mushrooms also synthesize lovastatin, a statin-type drug that has been used for cholesterol reduction therapy for cardiovascular disease since its approval by the FDA in 1987; it is marketed by Merck as Mevacor. It is not clear why oyster mushrooms generate statins, but it likely has something to do with survival – to eat and not be eaten. Fungi in general and oyster mushrooms in particular are masters of forging enzymes. They represent a nearly untapped resource of natural chemical experimentation that has endured for millennia.