Description: Crustose, thin and firmly attached by the whole undersurface to rock. Very dark red. Surface smooth but with sunken conceptacles just visible with hand-lens.

Habitat: Common. Epiphytic, from the upper littoral into the sublittoral.

Distribution: Widespread around the British Isles including the Shetlands, probably more common than indicated by the records. Europe: Mediterranean, Netherlands and Norway. Greenland. Atlantic coast of North America: Canada to Massachusetts. Further afield: Canary Islands, California also Chile and Japan.

Similar Species: Almost identical to Hildenbrandia rubra and the freshwater species Hildenbrandia rivularis

Key Identification Features:

• Very small conceptacles in the thin crustose thallus.

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

iNaturalist: Hildenbrandia crouanii at iNaturalist World Species Observations database.

WoRMS: Hildenbrandia crouanii at World Register of Marine Species.