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 Alvinocari s markensis, were sampled from the Snake Pit and TAG hydrothermal vent sites on the mid-Atlantic ridge. Fatty acid analyses indicated high a bundances of 16:2(n-4) and 18:2(n-4) in the Lipids of R, exoculata, wi th the highest proportions of these fatty acids detected in the digest ive 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, alt hough in lower proportions [2.0% 16:2(n-4) and 14.5% 18:2(n-4)]. By co ntrast, lipid extracted from the same tissues in A. markensis containe d relatively low proportions of (n-4) fatty acids (1.9 to 3.0%), but w as substantially enriched in the phototrophic, microplanktonic biomark ers 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 est ablished delta(13)C values of -11.0 to -11.4 parts per thousand which is consistent with the fatty acids being derived from chemosynthetical ly fixed carbon. By contrast, those fatty acids which are characterist ic 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 i n 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 s pecies. 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 di rect competition for their food supply.