Choreocolax polysiphoniae is a minute marine parasitic alga in the division Rhodophyta.

It grows as an irregular sphere on the fronds of the alga, reaching no more than 1 mm in extent.

[3][4][5] Parasitic on Polysiphonia lanosa, the filaments grow into the host.

[3] In Ireland it has been confidently recorded from counties Down, Antrim[6] and Waterford[7] and at scattered sites around the British Isles including the Shetland Islands.

[8] Cruciate tetrasporangia are produced all year round in the cortex.