Two of the dominant Southern Ocean copepods, Calanus simillimus and Calanus propinquus, are known for their lipid storage via triacylglycerols indicating year-round activity, as opposed to diapausing species, which accumulate wax esters. We studied the lipid and fatty acid compositions of C. simillimus (CV stages) with focus on its ability to produce unusually long-chain monounsaturated fatty acids. Besides the biosynthesis of high-energy fatty acids with 20 and 22 carbon atoms, the occurrence of fatty acids with 24 carbon atoms accounting for up to 15% of total fatty acids is intriguing. Their double bond positions were unequivocally determined as (n-9), (n-11) and (n-13) by DMOX derivatisation. The dominant isomer was 24:1(n-11) contributing up to 8% to the total fatty acids. The major fatty acids were the isomers 22:1(n-11) and (n-9) averaging 20% and 10%, respectively. A re-evaluation of fatty acid data of C. propinquus also revealed 24:1 fatty acids exhibiting on average 5% with (n-11) and (n-9) as main isomers. The principal fatty acids were also 22:1(n-11) and (n-9), but in contrast to C. simillimus both fatty acids occurred in equally high amounts of about 20%. The de novo biosynthesis of these long-chain monounsaturated fatty acids generally represents a very efficient energy storage mode. Chain elongation to 22 and even to 24 carbon atoms in C. propinquus and further optimized by C. simillimus yields high-energy compounds for these triacylglycerol-storing copepods. Biosynthetic pathways for the fatty acids are proposed and discussed in view of the well-adapted life cycle strategies of the two species, which have to cope with a pronounced seasonal food supply in the Southern Ocean.