Description: Fronds dichotomously branched to 40 cms long and to 2 mm thick, hollow in the older parts. Axis consisting of central cells with a cortex of assimilatory filaments and hairs.

Habitat: Uncommon. In muddy and sandy regions in the sublittoral.

Distribution: Mainly western in the British Isles: Ireland and Scotland including the Shetlands. Europe: Mediterranean, France, Baltic Sea and Norway.

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

iNaturalist: Spermatochnus paradoxus at iNaturalist World Species Observations database.

WoRMS: Spermatochnus paradoxus at World Register of Marine Species.