Cetraria

Cetraria is a genus of fruticose lichens that associate with green algae as photobionts. Most species are found at high latitudes, occurring on sand or heath, and are characterised by their "strap-like" form with spiny edges. The lobes can range from narrow and linear to broader and flattened, often forming loose or densely packed cushions. Their distinctive spiny margins serve both a defensive role and aid in vegetative reproduction through fragmentation. The genus was created by Erik Acharius in 1803 and belongs to the large family Parmeliaceae. While originally a species-rich genus, taxonomic revisions since the 1960s have split many species into new genera, though the exact circumscription remains debated among lichenologists.

Several Cetraria species have cultural and economic importance, particularly C. islandica (Iceland moss), which has been widely used in European traditional medicine for treating digestive and respiratory ailments. This species was also historically important as a famine food in Northern Europe and continues to find applications in modern cosmetics and pharmaceutical products. Cetraria species are also ecologically important, serving as indicators of air quality and climate change due to their sensitivity to environmental conditions and ability to accumulate various elements, including potentially toxic heavy metals and radioactive isotopes.