The well-known fatty acids with a Δ5,9 unsaturation system were designated for a long period as demospongic acids, taking into account that they originally occurred in marine Demospongia sponges. However, such acids have also been observed in various marine sources with a large range of chain-lengths (C(16)-C(32)) and from some terrestrial plants with short acyl chains (C(18)-C(19)). Finally, t...