Description: A minute and inconspicuous alga probably parasitic formed of a group of short terete branches and no more than 5 mm in diameter.

Habitat: Very rare. Parasitic partly embedded in the frond of Coccotylus truncatus. Sublittoral to depths of 10 m.

Distribution: In the British Isles only known definitely from Ireland and the the Shetlands. Europe: Denmark, Baltic Sea, Norway, Iceland. Greenland. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts and Rhode Island. Further afield: Circumboreal in the Arctic Ocean and Black Sea.

Key Identification Features:

• Growing in Coccotylus truncatus (parasitic).

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

iNaturalist: Ceratocolax hartzii at iNaturalist World Species Observations database.

WoRMS: Ceratocolax hartzii at World Register of Marine Species.