The thalli of this species form dense tufts, each tuft up to 7.5 cm generally attached by rhizoids to the fronds of Ascophyllum.
The erect cylindrical branches divide pseudodichotomously, that is forming two equal branches, one of the branches growing from a lateral branch.
A transverse section shows an axial cell surrounded by 12 to 24 peraxial cells.
The axial cell is large and occupies about one third of the diameter of each branch, best seen in transverse section.
It is deep brownish red in colour.

Choreocolax polysiphoniae is a small parasitic alga which grows in a cushion-like manner to 1 mm in diameter on the branches of P. lanosa.
The thalli of this species form dense tufts, each tuft up to 7.5 cm generally attached by rhizoids to the fronds of Ascophyllum. The erect cylindrical branches divide pseudodichotomously, that is forming two equal branches, one of the branches growing from a lateral branch. A transverse section shows an axial cell surrounded by 12 to 24 peraxial cells. The axial cell is large and occupies about one third of the diameter of each branch, best seen in transverse section. It is deep brownish red in colour.
Choreocolax polysiphoniae is a small parasitic alga which grows in a cushion-like manner to 1 mm in diameter on the branches of P. lanosa.
Polysiphonia lanosa