The occurrence and distribution of octadecapentaenoic acid in a natural plankton population. A possible food chain index.

It is shown that marine dinoflagellates under natural conditions synthesize the unusual fatty acid octapentadecaenoic (18:5omega3). This acid is very likely a characteristic of certain groups of phytoplankters and is not an artifact from artificial culture conditions. Various species of herbivorous copepods as well as contemporary carnivorous chaetognaths living in the same environment present traces of this fatty acid. The decreasing quantity of 18:5omega3 on moving up the food chain, and its absence in certain species, makes it a possible ecological tracer.