Description: Thallus up to 25 cms long, membranous and one celled thick. Margin lobed and undulate.

Habitat: Epilithic and epiphytic. In the littoral.

Distribution: The only British record is from the Orkney Islands collected about 1837. Europe: Helgoland, Norway, Faroes and Iceland. Greenland. Atlantic coast of North America: Canada. Further afield: Japan.

Similar Species: Ulva has a thallus two cells thick; Monostroma is one cell thick. Enteromorpha is in general tubular and bounded by a membrane one cell thick which may split or become compressed and appear to be two cells thick.

Key Identification Features:

• Membraneous, one cell thick.

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

iNaturalist: Monostroma undulatum at iNaturalist World Species Observations database.

WoRMS: Monostroma undulatum at World Register of Marine Species. Accepted name: Protomonostroma undulatum (Wittrock) K.L.Vinogradova, 1969. AphiaID: 145946.

Classification: Biota; Plantae; Viridiplantae; Chlorophyta; Chlorophytina; Ulvophyceae; Ulotrichales; Monostromataceae; Monostroma