Lichen

Lichens are remarkable organisms; they are stable combinations of an alga and/ or a cyanobacteria with a fungus, living together in a symbiotic association (5). The fungus causes the alga to release sugars, which allow the fungus to grow, reproduce and generally survive. The fungus provides protection for the alga, and enables it to live in environments in which it could not survive without the fungal partner (5). A general rule is that the fungal component of a lichen is unable to live independently, but the alga may live without the fungus as a distinct species (3). Many lichens are known to be very sensitive to environmental pollution, and they have been used as 'indicators' of pollution (4). The taxonomy of this species is not certain, and work is needed to investigate the taxonomic relationships within the Opegrapha genus (2).

Opegrapha paraxanthodes is a priority species under the UK Biodiversity Action Plan. The Species Action Plan aims to maintain the current populations and to enhance them where possible.

This lichen grows in a crust-like fashion; the thallus is thin and pale greenish-yellow in colour with a cracked surface (2). The Latin name of the genus Opegrapha means 'hidden writing', this refers to the long fruits, called 'lirellae', which often have a dark outer margin (5), and may resemble Chinese writing or hieroglyphs (6). In this species, however, the fruiting bodies are 'boat shaped' (7).

Inhabits crevices in cliffs and rocky outcrops, and prefers base-rich rocks such as calcareous sandstone and limestone in shaded areas, such as river valleys (2).

This species is believed to be endemic to Britain and Ireland, and so it occurs nowhere else (2). It has a wide distribution in the UK, but where it does occur it tends to be rare (4).

Classified as Lower Risk (Near Threatened) in Great Britain and receives general protection under the Wildlife and Countryside Act 1981 (2).

This lichen has been lost from two sites in the UK, one in Gwent, the other in Cumbria, and from a site in County Galway in Ireland. The reasons for the decline of this species are not known, and the threats facing it have not yet been identified (2).