Description: Unattached branched and calcareous thalli to 6 mm diameter. Attached thalli unknown in the British Isles but have been recorded elsewhere.

Habitat: The unattached form maerl beds in the sublittoral in sheltered areas.

Distribution: British Isles: common and widely distributed on southern and western coasts. Europe: Mediterranean, Atlantic coast of Spain and France, Baltic Sea and Norway. Further afield: Arctic Sea and Canary Islands.

Similar Species: Lithothamnion corallioides.

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

iNaturalist: Phymatolithon calcareum at iNaturalist World Species Observations database.