Fatty acid desaturase that introduces a cis double bond at the 8-position in 20-carbon polyunsaturated fatty acids incorporated in a glycerolipid that contain a Delta(8) double bond to yield (20:4(8,11,14,17)). The cytochrome b5 heme-binding domain acts as the direct electron donor to the active site of the desaturase, and does not require an external cytochrome. Belongs to the fatty acid desaturase type 1 family.