Description: Up to 20 cms long, tufted with erect ecorticate filaments, numerous and irregularly branched, interwoven and dense. Forms felt-like patches on species of Fucus and other species.

Habitat: Common. Epiphytic mainly on Ascophyllum and Fucus.

Distribution: Widely distributed around the British Isles. Europe: Portugal, Spain, France, Norway and Iceland. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut and Long Island, New Jersey, Delaware and Virginia. Further afield: the Canary Islands, Cape Verde Islands, West coast Africa and in the Pacific from Alaska to Chile.

Similar Species: Other species previously included in Ectocarpus such as Giffordia, Pylaiella and others.

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

iNaturalist: Spongonema tomentosum at iNaturalist World Species Observations database.

WoRMS: Spongonema tomentosum at World Register of Marine Species.