Perhaps the most notable, colorful, and ubiquitous group of lichens are the ones collected under the “Crustose” category. They’re also, because of their small size and rather cryptic features, among the most difficult to identify.

What distinguishes a crustose lichen from other types? These lichens are very closely attached to the substrate, which is often rock, but may be bark, wood, soil, or even stranger things. In some cases, they may even be partially embedded in the rock!

Crustose lichens lack a lower cortex. The cortex is a layer made up of closely packed fungal hyphae that may be considered somewhat analogous to an epidermis. As shown below in this diagram from the Watcher, most of the body of the lichen is made up of more loosely packed hyphae, the medulla. Sandwiched between the cortex and medulla is a layer of algal cells (or occasionally cyanobacteria) as the photobiont, that is, the part of the lichen responsible for photosynthesis.

Remember that the types intergrade. Some crustose lichens appear somewhat loose or “foliose” on the margins—that is the edges may look more like typical lichens. (A special type called “squamulose” can also be somewhat intermediate in appearance between a foliose lichen and a true crustose species). Most often, foliose lichens can be separated fairly easily from the substrate on which they grow, but crustose lichens are so closely attached that researchers have to collect a bit of rock or other substrate with their specimens!

How can you identify them? Lichenologists often use chemical tests to tell various species apart, but sometimes even that is not enough. For many crustose species, it is important to look at microscopic characters such as the number and type of fungal spores in the reproductive structures (apothecia). The black apothecia (spore-producing structures) in this lichen must be cut into thin slices and examined under a microscope to determine its identity. As each apothecium is less than a millimeter (or 1/25 of an inch) across, that can be a challenge.

Fortunately, some crustose species (like this Rhizocarpon) are more recognizable. The neon green and black thallus of Rhizocarpon geographicum is characteristic, and a common part of our high altitude lichen communities. This species has been used to date the rocks on which it grows, in a technique called lichenometry.

Where can you see them? Almost anywhere! Crustose lichens are especially abundant on rocks in our foothills and mountain habitats. Look closely and consider that each different color on a boulder along the trail is likely to be a distinct species of lichen!