Description: Fronds rising from a discoid holdfast to 10 cms long. Terete stipes, frond branches into a flat blade 2 cms wide and irregular in shape. Only the tetrasporangial phase is known.

Habitat: Very rare. No details as to the habitat in the British Isles is avaliable. In Portugal is grows in shady places in the sublittoral.

Distribution: Only a few records for the whole of the British Isles where it is recorded only in the drift from Isles of Scilly and the Channel Isles. Europe: Portugal, Atlantic coasts of Spain and France. Further afield: Morocco.

Similar Species: Cryptonemia lactuca from which it is distinguished by a slight thickening at the base of the blade. Similar to a number of other algae with flat blades.

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

iNaturalist: Cryptonemia seminervis at iNaturalist World Species Observations database.

WoRMS: Cryptonemia seminervis at World Register of Marine Species.