Description: Small filamentous alga with irregular branching. Branches long, less than 2 mm wide. Hollow in older parts with a cortex of 2 - 3 cell layers, the inner ones colourless. The surface is loosely clothed with short assimilatory filaments.

Habitat: Rare. Epilithic in the lower littoral into the sublittoral to 12 m.

Distribution: Scattered records with a western distribution in the British Isles from the Shetlands to south coast of Ireland and Cornwall. Europe: the Baltic and Norway. Atlantic coast of North America: Canada, Massachusetts, Connecticut and Long Island. Further afield: Japan.

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

iNaturalist: Acrothrix gracilis at iNaturalist World Species Observations database.

WoRMS: Acrothrix gracilis at World Register of Marine Species.