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 (PubMed:16839188, PubMed:10872837, PubMed:29237573). 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 by several other desaturases and elongases as substrates to synthesize polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes) (PubMed:16839188, PubMed:10872837, PubMed:29237573). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (By similarity). Partially inhibits expression of genes involved in beta-oxidation, such as ech-1 and acs-2, perhaps signaling via the actions of one of its fatty acid products (PubMed:15719061). May form part of a negative feedback loop with the transcription factor nhr-49 to limit beta-oxidation, in which nhr-49 stimulates expression of fat-7 and acs-2, and in turn fat-7 indirectly inhibits acs-2 and other genes also involved in beta-oxidation (PubMed:15719061). 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. 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. RNAi-mediated knockdown causes a significant increase in the abundance of C18:0 fatty acid, as well as a decrease in the levels of C18:1n9 and C18:2n6 fatty acids (PubMed:15719061). Slight reduction in polyunsaturated fatty acids (PUFAs) (PubMed:15719061). Widespread vacuole formation, germ line necrosis and shortened life-span (PubMed:15719061). 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.