Description: Thallus in tufts to 8 cms long. Erect polysiphonous axes terete with 9 - 11 pericentral cells, ecorticate. Branches alternate and complanate.

Habitat: Rare. Epilithic and epiphytic at extreme low littoral to 10 m in moderately sheltered shores.

Distribution: Recorded only from scattered sites in the south-west of Britain: Cornwall, Devon and Dorset in England; South Wales and in Ireland from Cos Cork, Clare and Galway. Europe: Mediterranean and France. Further afield: the Canary Islands.

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

iNaturalist: Pterosiphonia pennata at iNaturalist World Species Observations database.

WoRMS: Pterosiphonia pennata at World Register of Marine Species. Accepted name: Xiphosiphonia pennata (C.Agardh) Savoie & G.W.Saunders, 2016. AphiaID: 1027802.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Rhodymeniophycidae; Ceramiales; Rhodomelaceae; Pterosiphonieae; Pterosiphonia