Cetrelia is a genus of leafy lichens in the large family Parmeliaceae. They are commonly known as sea-storm lichens, alluding to the wavy appearance of their lobes. The name of the genus, circumscribed in 1968 by the husband and wife lichenologists William and Chicita Culberson, alludes to the former placement of these species in the genera Cetraria and Parmelia.

The main characteristics of the genus are the broad, rounded lobes of the greyish-green lichen body, and the presence of tiny pores in the outer surface (the cortex) that enable gas exchange. The lower surface of the lichen is brown to black with few root-like rhizines to act as holdfasts, resulting in a fairly loose attachment to the surface the lichen is on. Sexual reproductive structures are usually not present, so the physical characteristics used to distinguish between Cetrelia species include the presence or absence of asexual reproductive structures such as isidia, soredia, and lobules. Cetrelia lichens are chemically diverse and produce several secondary chemicals: atranorin is the main compound produced in the cortex, while substances known as depsides or depsidones are compounds in the medulla (the tissue layer under the cortex) that can be used to help distinguish between species.

The number of species in Cetrelia depends on how the chemical variation in the genus is categorized: there are five distinct forms (morphotypes), and additional morphologically identical but chemically unique species (chemotypes). Although 19 Cetrelia species have been formally described, some authorities prefer to consider the morphologically similar taxa as chemotypes of the same species. Molecular phylogenetic analyses suggest that the chemical races are best treated as distinct species. Known predominantly from the Northern Hemisphere, most Cetrelia species are found in eastern or southern Asia. They prefer somewhat moist, cool habitats, and are most commonly found on tree trunks, but sometimes on rocks or on bryophytes over shaded boulders. The type species, Cetrelia cetrarioides, is widely distributed, having been recorded in Asia, Europe, and North America. Most Cetrelia species that occur in Europe are considered rare or threatened, and appear on the Regional Red Lists of several countries. In Asia, some Cetrelia lichens are used in the preparation of natural dyes, for the production of material with antibiotic-like properties used in traditional medicine, or in the preparation of litmus reagent.

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