Description: This common crustose lichen of siliceous rocks, especially, small, weathered fragments: scree rocks, pebbles and the like, on exposed slopes or along sheltered streams in upland country, will sooner or later come to the attention of the beginner. The thin, grey thallus (but sometimes reddish near water) is dominated by numerous black discs (<1.5mm diam.) with a slightly raised margin. It is common throughout Ireland, but irregularly recorded. A dab of K (Potassium hydroxide) will usually turn it yellow.

Key Identification Features:

• One of the smallest-fruited Porpidia lichens with numerous tiny, black discs
• It is most likely to be confused with Porpidia striata which is of a similar size but has a wrinkled margin. P. macrocarpa has much larger discs.

Original text submitted by Simon Davey

Distribution Map from NBN: Porpidia crustulata at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Porpidia crustulata at iNaturalist World Species Observations database.