Description: Very small, erect uniseriate filaments only 3 to 6 cell long rising from prostrate axes.

Habitat: Endozoic in Sertularia, sublittoral to 30 m.

Distribution: Widely distributed in the British Isles. Europe: Atlantic coast of Spain and France, Baltic Sea, Norway, Faroes, Spitzbergen and Iceland. Greenland. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island and Connecticut. Further afield: California.

Similar Species: Similar to species of Acrochaetium, Colaconema and other microscopic red algae and requires microscopic examination for identification.

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

iNaturalist: Rubrointrusa membranacea at iNaturalist World Species Observations database.

WoRMS: Rubrointrusa membranacea at World Register of Marine Species. Accepted name: Rubrointrusa membranacea (Magnus) S.L.Clayden & G.W.Saunders, 2010. AphiaID: 495808.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Nemaliophycidae; Palmariales; Meiodiscaceae; Rubrointrusa