This group of unusual fungi are mainly associated in Scotland with the native pinewoods of the Caledonian Forest.
Tooth-fungi are widely distributed, particularly in the forests of the world’s northern temperate regions. Many species, including the earpick fungus (Auriscalpium vulgare), the drab tooth fungus (Bankera fuligineoalba) and those in the genus Phellodon, occur both in Europe and North America. Others occur in parts of Asia – the wood hedgehog (Hydnum repandum) and the scaly tooth fungus (Sarcodon imbricatus) have been recorded from Japan and Korea, in addition to Europe and North America. The mealy tooth fungus (Hydnellum ferrugineum) is known from North Africa, and the bitter tooth fungus (Sarcodon scabrosus) has been recorded in India. A larger number of species of both Hydnellum and Sarcodon occur in continental Europe than in the UK.
Distribution in Scotland
The wood hedgehog is widespread in Scotland, occurring in deciduous woods as well as with pine and spruce trees, and is the tooth fungus which people most often encounter. The earpick fungus is also fairly common, while the other species are mainly found in pine plantations and woodlands, particularly the remnants of the Caledonian Forest. The orange tooth fungus (Hydnellum aurantiacum) and the blue tooth fungus (Hydnellum caeruleum) are restricted to the native pinewoods, and the greenfoot tooth fungus (Sarcodon glaucopus) is only known from Abernethy, Glen Affric and Strathfarrar. Several other tooth fungi species found in Scotland are not featured here, as they do not occur in native pinewoods.
In recent decades substantial declines in tooth fungi have been documented in the Czech Republic, Denmark and Holland, where eight species have reportedly become extinct. The main reason for this is thought to be eutrophication – increased nitrogen in the soil – due to fertiliser use and possibly airborne pollution. Tooth fungi are on the Red Lists of threatened species in a number of European countries, including the UK. Here, Hydnellum aurantiacum is classified as Critically Endangered, Bankera fuligineoalba, Hydnellum caeruleum, Hydnellum ferrugineum and Sarcodon scabrosus are all classified as Endangered, and a further seven pinewood tooth fungi species are rated as Vulnerable. However, these designations may not accurately reflect the species’ status, as insufficient fieldwork has been carried out to locate them.
A total of 14 tooth fungi species (not all of them pinewood species) are covered by a UK Biodiversity Action Plan (BAP), under the government’s response to the Convention on Biological Diversity agreed at the 1992 Earth Summit. As the implementation of this BAP has proceeded, surveys have revealed that at least some pinewood tooth fungi species are more widespread in Scotland than was previously thought.
As with all fungi, the above-ground mushrooms of the tooth fungi are just the fruiting bodies – the main part of each fungus is an underground network of filaments or hyphae known as a mycelium, which persists throughout the year. The hyphae are cylindrical, thin-walled branching tubes containing the cytoplasm of the fungus, and are the fungal equivalent of cells in plants and animals. Hyphae differ from plant cells in some key ways though – they are made of chitin instead of cellulose, and they allow the free flow of cytoplasm through them, whereas cells have impermeable walls which prevent the movement of fluid from one cell to another.
Fungi grow by means of the hyphae absorbing nutrients and using these to extend the hyphal structure. Because they lack chlorophyll, fungi cannot utilise the sun’s energy to produce their own food, in the way that green plants can. Most tooth fungi are mycorrhizal, and obtain their energy from the host tree they grow with.
Fruiting bodies appear, usually in late summer and autumn, when aggregates of fungal hyphae combine to form the familiar structures commonly known as mushrooms. Some species of tooth fungi, such as Hydnum repandum, Bankera fuligineoalba and those in the genus Sarcodon, have relatively soft fruiting bodies which grow rapidly, and these typically survive for only a few days before they rot or are eaten by maggots (fly larvae). The Hydnellum and Phellodon tooth fungi produce their fruiting bodies more slowly, with the growth taking place at the perimeter of their caps. This outward growth can result in them engulfing twigs, grass stalks etc, and the fruiting bodies often have pine needles or other forest floor debris embedded in their caps. They can also fuse with neighbouring fruiting bodies to form caps with multiple stems. Their mushrooms are tougher or leathery in consistency, more irregular in shape, and can persist for several weeks.
The fruiting bodies of most tooth fungi are between 4 – 10 cm. in diameter, although those of the drab tooth fungus can reach 20 cm. in size. Because of its growth habit on fallen pine cones, the earpick fungus has a smaller fruiting body, with a cap less than 2 cm. wide. Many tooth fungi fruiting bodies are bright shades of beige, cream or even pink when they are young, but darken with age to a more nondescript brown. The Hydnellum and Phellodon species often exhibit concentric ring patterns in their colouration, because growth occurs at their cap edges. The blue and orange tooth fungi are distinctive for their bright and unusual colours.
The main distinguishing feature of tooth fungi are the teeth or spines on the undersides of their caps. These take the place of the gills in the more familiar types of mushrooms, and serve the same function, to produce and release large quantities of spores, which the fungus uses for reproduction. The teeth form dense masses, and in many species they look more like bristles – a characteristic which has given rise to their alternative common name of hedgehog fungi. In most of these fungi the teeth or spines are about 3 mm. long, but in the scaly tooth fungus they can reach 12 mm. in length.
The wood hedgehog and the scaly tooth fungus are both edible, but most other tooth fungi are inedible, because of their tough consistency or bitter taste. None are poisonous. Commercial wild fungi collectors in Scotland have a voluntary ban on collecting the scaly tooth fungus, because of its rarity.
The spores are tiny unicellular structures which are produced as a result of the sexual fusion of hyphae from two fungal colonies. In the tooth fungi, the spores mostly take the shape of knobbly, irregularly-formed spheres, and are typically 3 – 8 microns in size. An individual fruiting body releases huge numbers of spores – tens of millions or more – which are distributed mainly by the wind. The very few spores which land in suitable sites germinate, when a hypha known as the germ tube emerges from the spore to begin the growth of a new fungal colony.
The most important ecological relationships involving tooth fungi are the mycorrhizal ones they have with trees. Most of the species covered by this profile, with the exception of the earpick fungus, have a mycorrhizal association with Scots pine (Pinus sylvestris). Some species, including the wood hedgehog, are also associated with broadleaved trees. In this symbiotic, mutually-beneficial relationship, the hyphae of the fungus wrap around the root hairs of a tree. In the exchange which takes place at the plant-fungus interface, the fungus obtains sugars which the tree produces by photosynthesis, while the tree gains nitrogen, phosphorus and potash which the fungus takes up from the soil.
As tooth fungi in Scotland have often been found growing on bare sites such as eroded river gravels, path edges and acid, sandy soils, it is thought that they play a role in nutrient cycling or recycling, and in assisting the colonisation of these areas by trees.
In contrast to many fungi, tooth fungi are largely unattractive to insects, while their tough consistency and bitter tastes render them unpalatable to animals. However, some of the soft-bodied species such as the wood hedgehog are eaten by slugs, snails, fly larvae and small mammals, including wood mice (Apodemus sylvaticus).
Unlike the other tooth fungi, the earpick fungus is saprotrophic, meaning that it grows on dead organic material. In Scotland it occurs on fallen pine cones (elsewhere it also occurs on spruce cones), which its spores probably come into contact with once the cones have fallen. By helping to break down the cones’ woody structure, this fungus plays a role in returning the nutrients in the cones back to the soil.