Delta(9)-fatty acid desaturase that acts preferentially on stearoyl-CoA (octadecanoyl-CoA) producing the monounsaturated oleoyl-CoA ((9Z)-octadecenoyl-CoA), one of the most abundant monounsaturated fatty acid in Caenorhabditis elegans phospholipids and triacylglycerols. Also acts on palmitoyl-CoA (hexadecanoyl-CoA), heptadecanoyl-CoA and (11E)-octadecenoyl-CoA (trans-vaccenoyl-CoA), the monounsaturated fatty acids (MUFAs) produced are further used as substrates to synthesize polyunsaturated fatty acids (PUFAs) by several other desaturases and elongases (PubMed:16839188, PubMed:10872837, PubMed:29237573). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (By similarity). 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 + octadecanoyl-CoA = (9Z)-octadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + hexadecanoyl-CoA + O2 = (9Z)-hexadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + heptadecanoyl-CoA + O2 = (9Z)-heptadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (11E)-octadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,11E)-octadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; monounsaturated fatty acid biosynthesis. Lipid metabolism; fatty acid metabolism. Expressed in the intestine in adult worms and in all four larval stages. Additional expression in the hypodermis in all life stages. Expression is regulated by nhr-80 and nhr-49. The histidine box domains may contain the active site and/or be involved in metal ion binding. Cytochrome b5 CYTB-5.1 is specifically required for the desaturase activity, its knockdown or mutation alters the enzyme activity. Belongs to the fatty acid desaturase type 1 family.