Circinaria calcitrapa

Circinaria calcitrapa is a species of rock-dwelling crustose lichen in the family Megasporaceae. Described in 2011 from the Pyrénées-Orientales in southern France, it grows on inclined to vertical faces of non-calcareous schist in warm, dry, sun-exposed habitats from low elevations into the lower montane zone, and is easily confused with Aspicilia viridescens unless examined microscopically, but differs in having non-pruinose fruiting bodies, eight-spored asci, and smaller ascospores.

Aspicilia calcitrapa was described as a new species in 2011 by Claude Roux and Anders Nordin, based on material collected during surveys in protected areas of the Pyrénées-Orientales (southern France). The type specimen was collected near Nohèdes (Pyrénées-Orientales) at about 1,030 m elevation, growing on small faces of non-calcareous schist. The epithet calcitrapa is from Latin (French chausse-trappe, 'a trap'), and the authors note that without microscopic work the species is easily confused with A. viridescens. In the authors' ITS-based phylogeny, A. calcitrapa is placed close to A. aquatica (a species with a very different, more water-associated ecology).^[2] In 2024, Roux transferred Aspicilia calcitrapa to the genus Circinaria, publishing the new combination Circinaria calcitrapa.^[3]

Circinaria calcitrapa forms a pale crust on rock, typically 1–8 cm across and 0.15–0.7 mm thick. The surface is divided into angular patches (areoles) about 0.2–1.0 mm wide, with a colour ranging from brownish-white to greenish or pale ochre. A grey-black marginal line (the hypothallus) is sometimes visible around the edge of the thallus, and may be bordered by a whitish line.^[2]

The fruiting bodies (apothecia) are small (0.15–0.7 mm wide) and black, usually sitting in the areoles. The discs are dull and very finely roughened to nearly smooth, and lack a powdery coating. Under the microscope, the asci contain eight hyaline, ellipsoid spores, typically about 13.5–21.5 × 8.5–12 μm, and the pycnidia produce short, rod-shaped conidia, typically about 8–11.5 × about 1 μm. Standard chemical spot tests on the thallus are negative, and thin-layer chromatography did not detect lichen substances. The green pigment (caesiocinerea-green) in the epithecium and upper cortex shows an emerald-green reaction with nitric acid (N+).^[2]

This is a rock-dwelling species that grows on inclined to near-vertical faces of non-calcareous schist. In the original study it is characterised as a lichen of open, exposed sites, favouring dry, sunny, warm microhabitats, and occurring mainly at low elevations, sometimes reaching the lower montane zone.^[2]

Circinaria calcitrapa was reported originally only from the Pyrénées-Orientales. The authors list multiple collections from local nature reserves in that department (including Jujols, Nohèdes, and Nyer), suggesting it is a local component of xerothermic (warm, dry, sun-exposed) crust communities on siliceous rock.^[2] It was later recorded from the southwestern Alps, where it often grows with Pertusaria chiodectonoides.^[4]

Circinaria calcitrapa is the specific host of the lichenicolous fungus species Placopyrenium breussii.^[2]