Description: Thallus erect and filamentous throughout, attached by prostrate axes with rhizoids, grows erect to 1 cm long. Branches opposite, in one one plane and delicate. The final branchlets secund.

Habitat: Very rare. Epiphytic and epilithic in the sublittoral to 20 m.

Distribution: In the British Isles known only from the south-west coasts. Europe: Spain and France. Further afield: Canary Islands and the Pacific.

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

iNaturalist: Antithamnion densum at iNaturalist World Species Observations database.

WoRMS: Antithamnion densum at World Register of Marine Species.