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

Abstract

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 δ13C values of -11.0 to -11.4‰ 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% for 20:4(n-6), 20:5(n-3) and 22:6(n-3) respectively. The non-methylene interrupted dienes (NMIDs) 20:2Δ5,13 and 22;2Δ7,15 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.