Description: Minute tufts of uniseriate filaments, mucilaginous, no more than 2 mm long, branched.

Habitat: Epiphytic on small filamentous algae and on Zostera, epizoic and epilithic. Low littoral and sublittoral.

Distribution: Widespread around the British Isles. Europe: Mediterranean, Azores, Portugal, Spain, France, Netherlands, Baltic Sea, Norway and Faroes. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut and Long Island, New Jersey and Virginia, North Carolina. Further afield: Canary Islands, Morocco, Bermuda, Jamaica, Brazil, Peru, Chile, South Africa, Australia and Japan.

Similar Species: Stylonema cornu-cervi Reinsch is distinguishable having two or more cell-rows.

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

iNaturalist: Stylonema alsidii at iNaturalist World Species Observations database.

WoRMS: Stylonema alsidii at World Register of Marine Species.