Description: Thallus filamentous in small tufts erect to 4 cms. Prostrate axes spreads over substrate from which erect fronds grow as a turf. Latteral branches irregular, secund, alternate or paired. Branches whorled at the apicies.

Habitat: Epiphytic, epilithic or on dead shells. Low littoral rock pools to 25 m.

Distribution: Widely distributed in the British Isles including Shetlands. Europe: Mediterranean, Azores, Portugal, Atlantic coasts of Spain and France, Baltic Sea and Norway. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New Jersey, Delaware and Florida. Further afield: the Canary Islands.

Similar Species: Ptilothamnion pluma can be distinguished by the rhizoids and erect axes which arise from near the middle of the prostrate axial cells.

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

iNaturalist: Spermothamnion repens at iNaturalist World Species Observations database.

WoRMS: Spermothamnion repens at World Register of Marine Species.