Sources of cyclopropanoid fatty acids in the mummichog Fundulus heteroclitus

The saturated fatty acids in lipids of the mummichog Fundulus heteroclitus (L.) contain the characteristic bacterial cis-9,10-methylenehexadecanoic and cis-9,10-methyleneoctadecanoic acids in addition to straight-chain, iso, anteiso and multi-branched fatty acids. To ascertain whether the cyclopropanoid fatty acids (CFA) might be used as a marker for bacterial components in the mummichog diet, the saturated fatty acids of detritus were compared with those of fish intestines and their contents, and with those of eviscerated fish. The relative amounts of individual fatty acids in each sample indicated that microorganisms or invertebrate animals that consume microorganisms constitute a part of the mummichog's diet. A similar analysis of anterior and posterior sections of the fish intestines showed a higher concentration of CFA in the posterior section, suggesting that at least some of the cyclic acids might originate from the resident bacterial gut population. When a group of mummichogs was maintained on a beef-liver diet devoid of cyclic acids, carcass lipids were not depleted of CFA. Instead, an increase in concentration was observed. It was concluded that the intestinal microflora have a role in supplying CFA to mummichog lipids. Since the overall results indicate that these acids are derived from commensal bacteria as well as from bacteria associated with the diet, it is not possible to use such compounds as food-chain markers.