Long-chain 5, 9-dienoic fatty acids, commonly called demospongic acids, belong to a group of non-methylene-interrupted (NMI) fatty acids that are found in sponges. Interestingly, some of the 5, 9-dienoic fatty acids have been reported to be good inhibitors, particularly in vitro, of human DNA topoisomerase I, which is a target for drug design against cancer, and the enoyl-ACP reductase of Plasmodium falciparum, which causes the most severe form of malaria. On the other hand, it was demonstrated that sponges are not the only source of 5, 9-dienoic fatty acids with even- and odd- numbered chain lengths, but also patellogastropod limpets and nudibranchs, which belong to the largest class of mollusks, gastropods, and cnidarians such as sea anemones and gorgonians. In addition, the intertidal gastropods, Cellana grata and Collisella dorsuosa, and the marine bivalves, Megangulus zyonoensis and Perna canaliculus, are rich sources of a number of NMI fatty acids, including novel ones, and their structurally related positional isomers, which may have some bioactive effects. This review provides an update on the enormous diversity of structures and biological role of unusual and rare fatty acids with a 5, 9-double bond system (5, 9, x-trienoic, 5, 9, x, y-tetraenoic and 5, 9, x, y, z-pentaenoic fatty acids). Future directions in intertidal gastropods and marine bivalves research are also discussed.

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