Thelocarpon epibolum Nyl.
A Species of Thelocarpon Lichen
Basis for Listing
Thelocarpon epibolum (a species of Thelocarpon lichen) may not technically be a lichen at all, it may be what is referred to as a “lichenicolous fungus”. Lichenicolous fungi are specialized fungi that grow on the thalli of lichens. Sometimes this relationship is parasitic and other times it is saprophytic. Although these species are not technically “lichenized” (i.e. they do not have their own algal partner), they have traditionally been studied by lichenologists because of the specialized nature of their substrates, as well as the fact that lichenologists collect them consistently while they collect the host species. According to Alstrup and Hawksworth (1990), there is some debate among lichenologists as to whether this species is lichenized or not.
Thelocarpon epibolum appears to be uncommon across its North American range, where it tends to occur in boreal and alpine regions. Although there is an abundance of potential habitat in the northern portions of Minnesota, the species has only been collected four times in the state, twice in Koochiching County (Border Lakes and Littlefork-Vermilion Upland subsections), once in Lake County (North Shore Highlands Subsection), and once in Cook County (Border Lakes Subsection). Thelocarpon epibolum was designated a special concern species in 2013.
Thelocarpon epibolum is a nondescript small lichenicolous fungus that is found on the dead tissues of certain lichens, such as Peltigera aphthosa (felt lichen), Solorina saccata (chocolate chip lichen), and Baeomyces rufus (cap lichen). The species is only detectible when fruiting bodies are present. These show up as small (around 0.01mm [0.0004 in.] in diameter) bright yellow-green pimple-like structures called “perithecia” that produce spores for the fungus (Alstrup and Hawksworth 1990). Due to the rapid breakdown of dead lichens, T. epibolum is somewhat ephemeral in nature.
In Minnesota, T. epibolum is distinct from all other lichenicolous fungi and lichens. The combination of the small size of the perithecia, bright yellow-green color, and proper host species makes the species unique among our fungi. There are other species of Thelocarpon in North America that may eventually be discovered in Minnesota; however, differentiating T. epibolum from other Thelocarpon species requires a compound microscope and the use of technical genus specific keys.
In Minnesota, T. epibolum has been collected from the thalli of both P. aphthosa and S. saccata, though other substrates have been reported elsewhere. Peltigera aphthosa and S. saccata are both northern boreal species and are typically restricted to the northernmost tier of counties, with a few exceptions. It is likely that the limited range of these host species in our area restricts the range of T. epibolum.
In Minnesota, S. saccata is restricted to cool and moist shaded cliffs and rock outcrops. These habitats are fairly uncommon themselves but are most abundant along the North Shore of Lake Superior, where S. saccata can be observed growing with the rare Peltigera venosa (fan lichen). Peltigera aphthosa, on the other hand, can be found in a wide range of northern habitats including cliffs, rock outcrops, cedar swamps (forested peatlands), and mixed fire-dependent forests.
On the host thallus, T. epibolum is typically found near the base of the lichen, on portions of dead thallus. It is not clear why, but T. epibolum is typically not found on the oldest (darkest) pieces of thallus, rather it is often observed growing on portions of the thallus that have only recently died and retain a lighter brown color.
Biology / Life History
Thelocarpon epibolum reproduces sexually. Spores are spread by wind, water, or potentially various animals and must land on a suitable host species.
Conservation / Management
Thelocarpon epibolum has likely always been uncommon in Minnesota. Even though apparently suitable habitat occurs throughout much of Northern Minnesota, this species has only been recorded from a handful of locations. Any activity that could disrupt or alter the humidity of sites where its host species occurs could be detrimental to T. epibolum.
Best Time to Search
Thelocarpon epibolum can be observed year-round, whenever lichens are not covered by snow.