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, 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.
Systematics
Historical development (1800s–1950s)
The taxonomic history of lichens now known as "" begins with Carl Linnaeus, who in his 1753 work Species Plantarum included five species within his broad concept of the genus Lichen. These species, including L. islandicus, would later be recognised as distinct from other lichens based on their unique characteristics. The genus Cetraria was circumscribed by the Swedish lichenologist Erik Acharius in 1803. He noted that its apothecia were intermediate between the scutellate and peltate types found in related genera, which along with other morphological features meant that species in this genus could not be properly placed in either Parmelia or Peltidea. He assigned Cetraria islandica as the type species, and included an additional seven species in his original circumscription: C. cucullata, C. nivalis, C. lacunosa, C. fallax, C. glauca, C. sepincola, and C. juniperina. Of these eight, only the type and C. sepincola remain in the genus.File:Cetraria islandica - Köhler–s Medizinal-Pflanzen-032.jpg|thumb|right|upright=1|Historical illustration of Cetraria islandica from Köhler's Medicinal Plants, showing thallus morphology and anatomical details. This species serves as the type species of the genus Cetraria.
In 1860, William Nylander began the first major taxonomic reorganisation of Cetraria, retaining only five species in the genus while moving 25 species to his newly described genus Platisma and one species to Dactylina. Although new genera were subsequently established—including Nephromopsis by Johannes Müller Argoviensis and Tuckermannopsis by Vilmos Kőfaragó-Gyelnik —most lichenologists continued to favour a broader concept of Cetraria. This broader interpretation was reflected in mid-20th century treatments, with Kseniya Aleksandrovna Rassadina including 76 species in the genus, while Veli Räsänen recognising 62 species shortly after.
Modern revisions (1960s–2000s)
Cetraria remained a broad and species-rich genus until the 1960s, when taxonomic revisions began splitting it into new genera. The development of molecular phylogenetics techniques revealed previously unknown evolutionary relationships, leading to extensive reclassification. This process started with the description of Asahinea and Platismatia, and continued over subsequent decades with the recognition of additional genera including Masonhalea, Ahtiana, Allocetraria, Vulpicida, Cetrariella, Arctocetraria, and Flavocetraria, among others. Species delimitation within the genus has proven particularly challenging, as genetic analysis often reveals minimal genetic distances between morphologically distinct groups. In 1992–1993, Ingvar Kärnefelt further reorganised cetrarioid classification, erecting ten new genera. In 2013, Andres Saag and colleagues accepted 38 species of Cetraria in their world list of 149 cetrarioid lichens.The phylogenetic understanding of these cetrarioid relationships soon faced new challenges. A 2009 molecular study demonstrated that only about 90 species distributed across 14 genera formed a true monophyletic "cetrarioid core" within Parmeliaceae. By 2011, researchers found that approximately half of the accepted genera within this core group were not actually monophyletic, suggesting the existing classification required further refinement. Their analyses revealed that many cetrarioid genera had been more narrowly circumscribed than comparable groups within Parmeliaceae, leading to debates about whether some previously split taxa should be reunited.
Current classification debates
In 2017, Divakar and colleagues used a "temporal phylogenetic" approach to identify temporal bands for specific taxonomic ranks in the family Parmeliaceae, suggesting that groups of species that diverged within the time window of 29.45–32.55 million years ago represent genera. They proposed to synonymise Allocetraria with Cetraria, because the former group of species originated relatively recently and fell under the timeframe threshold for genus level. The net result of this proposal was to reduce 13 previously accepted genera in the cetrarioid clade down to two. This approach received mixed responses from the lichenological community. The synonymy was not accepted in a later critical analysis of this technique for lichen systematics. Arve Elvebakk and colleagues expressed a similar opinion, stating that they would "prefer a model of 13 imperfectly defined cetrarioid core genera in addition to 'orphaned' species, over an alternative of only two widely defined ones, as a starting point for further phylogenetic studies".Thell and Divakar later argued that the revised generic circumscription of Cetraria and Nephromopsis should be accepted, as the alternative would require division into further new genera. They noted this arrangement is supported by characters of conidia and ascospores. However, some researchers maintain that strict application of temporal phylogenetics methodology for genus delimitation in the Parmeliaceae could have important implications for conservation legislation. For pragmatic reasons, some authorities retain genera such as Cetrariella and Vulpicida, and Flavocetraria and Tuckermannopsis.
A pragmatic approach to this taxonomic complexity appears in Bruce McCune and Linda Geiser's 2023 field guide to Pacific Northwest macrolichens, where they adopt a broad concept of Cetraria based on available molecular and morphological data, while acknowledging that generic placement within cetrarioid lichens remains unsettled. Their treatment reflects the ongoing challenge of reconciling traditional generic concepts with phylogenetic findings.
Naming
The genus name Cetraria is derived from the Latin cetra, meaning combined with the suffix -aria, indicating connection or possession. English common names that have been applied to members of the genus include "Iceland lichens", "Icelandmoss", and "heath lichens".Description
Genus Cetraria includes lichens that typically form dorsiventral thalli, which are distinctly differentiated between an upper and lower surface. The thallus can be leaf-like and loosely attached to the, often with ascending margins that may form rosette-like patterns, or it can be shrubby and erect with lobes that are channelled, sometimes tubular, and occasionally flat. In some species, the thallus forms conspicuous spine-like projections, growing in tufted clusters. The genus is known to reproduce primarily through vegetative means, with species often propagating through fragmentation of the thallus, which contains both fungal and algal partners. The colour of the thallus varies, ranging from dark brown or olive green to grey-green or even yellowish in certain conditions.The upper of Cetraria lichens may have one or two layers, with thick-walled hyphal cells forming the external layer. Beneath this lies a more delicate layer of hyphae arranged parallel to the surface. The internal medulla, which forms the core of the thallus, is typically white or bright yellow. Pseudocyphellae are often present on the lower surface or at the margins of the lobes. The lower surface generally lacks rhizines or has them only sparsely. Soralia, which are specialised structures for vegetative reproduction, are rarely found.
The sexual reproductive structures of Cetraria are apothecia, typically positioned at the margins of the thallus and often attached at an angle. The margin of the apothecium can sometimes appear notched or, and the is often curved inward. The, or uppermost layer of the apothecium, is usually red-brown to dark brown. The hymenium stains blue when exposed to iodine, while the is colourless. The, composed of paraphyses, is typically straight, sparsely branched, with swollen tips.
The asci are eight-spored, narrowly club-shaped, and feature a moderately large with an apical ring and a conical ocular chamber that may have a narrow to broad beak. This configuration is characteristic of the Lecanora-type ascus. The are colourless, single-celled, and vary in shape from ellipsoidal to nearly spherical.
Asexual reproductive structures, the pycnidia, are also present in Cetraria. These are flask-shaped structures that release conidia through a blackened opening. The pycnidia may be located on the surface of the thallus or at the ends of marginal projections. The walls of the pycnidia are two-layered, with the outer layer being thin and sometimes darkened. The conidia themselves are colourless and may be cylindrical, bottle-shaped, crescent-shaped, or lemon-shaped.
The chemical composition of Cetraria species includes a range of secondary metabolites, such as β-orcinol depsidones, fatty acids, as well as usnic acid and derivatives of pulvinic acid in some species. These compounds contribute to the lichen's defence mechanisms and other ecological roles.