Description: Encrusting on rock and stone. A thick flat thallus spreading over the surface with no upright parts except crests formed where two plants meet. Growing to 20 cms in diameter and to 4 mm thick. Flat or knobbly with strong orbital ridges.

Habitat: Very common. Epilithic on rock in the low littoral and to 45 m or more deep.

Distribution: Widespread around the British Isles including the Shetlands. Europe: Portugal, Spain, France, Baltic Sea, Faroes and Norway, Spitzbergen and Iceland. Further afield: Morocco, Arctic Russia.

Key Identification Features:

• Large thick crust, not branched but knobbly.

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

iNaturalist: Phymatolithon purpureum at iNaturalist World Species Observations database.

WoRMS: Phymatolithon purpureum at World Register of Marine Species.