Description: Thallus small, erect to 5 cms long with dense irregular branches. Axes polysiphonous with about 24 pericentral cells, ecorticate. Rhizoids cut off from pericentral cells.

Habitat: Very rare. No details available.

Distribution: British Isles: few records from south west of England, Wales, Ireland and Guernsey. Europe: Mediterranean, Atlantic coasts of Spain and France. Further afield: the Canary Islands, West Indies and Australia.

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

iNaturalist: Polysiphonia opaca at iNaturalist World Species Observations database.

WoRMS: Polysiphonia opaca at World Register of Marine Species. Accepted name: Polysiphonia opaca (C.Agardh) Moris & De Notaris, 1839. AphiaID: 144653.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Rhodymeniophycidae; Ceramiales; Rhodomelaceae; Polysiphonieae; Polysiphonia