Crustose

Crustose is a growth form used in lichenology to describe lichens that develop a crust-like thallus tightly adherent to the substrate. The thallus forms a flat, crusty layer that is often immovable and difficult to remove, with little to no raised margins; it may be inconspicuous or brilliantly colored. Crustose lichens occur on rocks (saxicolous), on tree bark (corticolous), and on soil, in a wide range of climates from Arctic to tropical.

In contrast to foliose and fruticose forms, crustose lichens lack a distinct, leafy or shrubby appearance. They consist of a fungal partner with photosynthetic partners (green algae and/or cyanobacteria) embedded in a continuous, tightly bound thallus. Reproduction is primarily through sexual structures such as apothecia or perithecia that protrude above the thallus; many also propagate via dispersal units that bud off from the crust.

Taxonomically, crustose lichens are diverse and include genera such as Caloplaca, Acarospora, Verrucaria, and Graphis. Identification typically requires microscopic examination and chemical spot tests to detect lichen substances.

Ecologically, crustose lichens contribute to rock weathering, soil formation, and nutrient cycling; due to slow growth, they can serve as long-term environmental indicators for air quality and climate change.