Description: Minute thallus, as hemispherical cushions and no more than 4 mm across. Of branched monostromatic filaments within the host and with erect filaments rising and loosely aggregrated.

Habitat: Rare. Epiphytic on Ralfsia verrucoas in the upper littoral.

Distribution: West coast of Ireland and the south coast of England. Europe: Germany (Helgoland), Baltic Sea, Scandinavia. Further afield: the Canary Islands.

Similar Species: There are other small algae appearing as spots or stain-like patches. All difficult to identify and require microscopic examination.

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

iNaturalist: Compsonema microspongium at iNaturalist World Species Observations database.

WoRMS: Compsonema microspongium at World Register of Marine Species.