Rhodotus

Discovering Rhodotus: A Fungal Enigma

Rhodotus is a genus that has long puzzled mycologists with its shifting taxonomic identity and intriguing characteristics. Imagine a mystery wrapped in the enigmatic layers of a mushroom’s cap—this is what Rhodotus represents, especially when it comes to the species Rhodotus palmatus, commonly known as the netted rhodotus or rosy veincap.

A Turbulent Taxonomic Journey

First named Agaricus palmatus by Jean Bulliard in 1785, Rhodotus has undergone numerous reclassifications. It’s like a game of musical chairs where the genus keeps changing seats among different families—Amanitaceae, Entolomataceae, Tricholomataceae, and even its own proposed family, Rhodotaceae. But molecular phylogenetics analysis has finally settled the debate, confirming that Rhodotus is part of the core euagarics clade within the order Agaricales.

From Synonyms to Species

The specific epithet palmatus refers to the resemblance of the cap surface to the lines in the palm of a hand, which is quite fitting for this species. But its common names—netted rhodotus, rosy veincap, and wrinkled peach—are equally evocative. Described with various synonyms such as Pleurotus subpalmatus, Lentinula reticeps, and Agaricus alveolatus, Rhodotus palmatus has a rich history that mirrors the complex nature of its taxonomic journey.

A Vulnerable Species

Rhodotus palmatus is not just a fascinating mushroom; it’s also a species in decline. With populations in Europe on the wane, it appears on Red Lists of threatened species in over half of European fungal Red Lists. This makes us wonder: what factors are contributing to its decline? Is it pollution, habitat loss, or something else entirely?

The Fruit Body and Its Characteristics

When you encounter Rhodotus palmatus, the first thing that strikes you is its unique cap. Initially convex but later flattening somewhat with age, the cap can reach widths of 2–6 centimeters (0.8–2.4 in). The edges are rolled inward, and the surface has a network of lightly colored ridges or veins that outline deep and narrow grooves—a condition technically termed sulcate or reticulate. Depending on lighting conditions, the color can range from salmon-orange to pink to red.

The gills are thick, packed close together, with veins similar in color but paler than the cap. Some of these gills do not extend fully and form two to four groups of roughly equal length—these are called lamellulae. The stem is 1.5–3.0 cm (0.6–1.2 in) tall and 0.4–0.6 cm (0.16–0.24 in) thick, often attached to the underside of the cap in a central or lateral manner.

Unique Phenomena

Rhodotus palmatus is known for its ‘bleeding’ phenomenon—red- or orange-colored liquid can be seen when it’s found in nature. This is also observed in laboratory cultures, where the orange drops appear on the mat formed by fungal mycelia before fruit bodies emerge. When exposed to a 10% aqueous solution of iron(II) sulfate (FeSO4), mature fruit bodies turn green—a common mushroom identification test known as iron salts.

Spore and Taste Characteristics

The spores are spherical with wart-like projections, while the basidia are club-shaped and 4-spored. Cheilocystidia are present on the gill edge, and clamp connections are found in hyphae. Chlamydospores, thick-walled cells developed from single hyphal compartments, add another layer of complexity to this species.

Rhodotus palmatus has no distinguishable odor but a ‘bitter’ taste, typically listed as unknown or inedible. It shows moderate antibacterial activity against Bacillus subtilis and weak antifungal activity against Saccharomyces cerevisiae and Aspergillus fumigatus.

Natural Habitat and Distribution

Rhodotus palmatus grows scattered on rotting hardwoods, preferring low-lying logs in shaded areas with periodic flooding. It’s a pioneer species in fungal colonization of dead wood, often found growing on dark-stained wood, especially the dried-out upper parts of trunks that have lost their bark.

With a circumboreal distribution, R. palmatus has been reported from various countries including Canada, Iran, Hungary, Italy, Poland, Slovakia, Denmark, Sweden, Norway, Germany, the area formerly known as the USSR, Korea, Japan, and New Zealand. In the United States, it’s found in Indiana and elsewhere in eastern North America.

Interestingly, light at the red end of the visible spectrum is required for the development of R. palmatus fruit bodies, contrary to the typical requirement for blue light seen with many other mushroom species. Fruiting occurs in the presence of green, yellow or red light with wavelengths above 500 nm, but only when blue light (under 500 nm) is absent.

Conservation Status

In Europe, Rhodotus palmatus has been a candidate species in over half of European fungal Red Lists. It’s listed as critically endangered, endangered, or near threatened (or the equivalent) in 12 countries. In Hungary, it gained legal protection in 2005, making it a fineable offense to pick them.

As we delve deeper into the world of Rhodotus palmatus, one can’t help but wonder about its future and what steps need to be taken to ensure its survival. The journey from Agaricus palmatus to Rhodotus palmatus is not just a tale of taxonomy; it’s a story of resilience in the face of environmental challenges.