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 carallioides.

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

iNaturalist: Phymatolithon calcareum at iNaturalist World Species Observations database.

WoRMS: Phymatolithon calcareum at World Register of Marine Species. Accepted name: Phymatolithon calcareum (Pallas) W.H.Adey & D.L.McKibbin ex Woelkering & L.M.Irvine, 1986. AphiaID: 145199.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Corallinophycidae; Hapalidiales; Hapalidiaceae; Phymatolithon