Description: Thallus uniseriate, erect to 11 cms long. Rhizoidal filaments grow down forming cortication.

Habitat: Epiphytic in the lower littoral to 15 m.

Distribution: Common and generally distributed around the British Isles from the Shetlands to the Channel Islands. Europe: the Azores, Portugal, Spain, France, the Baltic and Iceland. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New Jersey and Virginia. Further afield: Canary Islands.

Similar Species: All species of Callithamnion and Aglaothamnion are similar.

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

iNaturalist: Callithamnion tetragonum at iNaturalist World Species Observations database.

WoRMS: Callithamnion tetragonum at World Register of Marine Species. Accepted name: Callithamnion tetragonum (Stackhouse) S.F.Gray, 1821. AphiaID: 144529.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Rhodymeniophycidae; Ceramiales; Callithamniaceae; Callithamnioideae; Callithamnieae; Callithamnion