Occurrence of 16:2(n-4) and 18:2(n-4) fatty acids in the lipids of the hydrothermal vent shrimps Rimicaris exoculata and Alvinocaris markensis: nutritional and trophic implications

Adults of 2 species of vent shrimp, Rimicaris exoculata and Alvinocaris markensis, were sampled from the Snake Pit and TAG hydrothermal vent sites on the mid-Atlantic ridge. Fatty acid analyses indicated high abundances of 16:2(n-4) and 18:2(n-4) in the Lipids of R, exoculata, with the highest proportions of these fatty acids detected in the digestive gland [14.5% 16:2(n-4) and 23.8% 18:2(n-4)]. Lipid extracted from abdominal muscle of R. exoculata also contained these fatty acids, although in lower proportions [2.0% 16:2(n-4) and 14.5% 18:2(n-4)]. By contrast, lipid extracted from the same tissues in A. markensis contained relatively low proportions of (n-4) fatty acids (1.9 to 3.0%), but was substantially enriched in the phototrophic, microplanktonic biomarkers 20:5(n-3) and 22:6(n-3). GC-IRMS (gas chromatography with isotope ratio mass spectrometry) analysis of the (n-4) dienoic fatty acids established delta(13)C values of -11.0 to -11.4 parts per thousand which is consistent with the fatty acids being derived from chemosynthetically fixed carbon. By contrast, those fatty acids which are characteristic of phototrophic microplankton were isotopically lighter, i.e. -17.1, -17.3 and -15.8 parts per thousand for 20:4(n-6), 20:5(n-3) and 22:6(n-3) respectively. The non-methylene interrupted dienes (NMIDs) 20:2 Delta 5,13 and 22:2 Delta 7,15 were detected in all samples although in small amounts with R. exoculata containing the highest amounts. The (n-4) dienes and NMIDs are consistent with a substantial dietary input of bacterially derived 16:1(n-7) and 18:1(n-7) fatty acids for this species. Thus, these results are consistent with R. exoculata adopting a bacterivorous mode of nutrition whilst A. markensis is essentially a scavenger (necrophagous) and suggest that the 2 species are not in direct competition for their food supply.