Fatty acid characteristics in two symbiotic gastropods from a deep hydrothermal vent of the west Pacific

Two deep-sea gastropods, Ifremeria nautilei and Alviniconcha hessleri, collected on a hydrothermal site of the North Fiji Basin (Southwestern Pacific) were analysed for polar and neutral lipids using gas-liquid chromatography/mass spectrometry. A high level of monounsaturated fatty acids (MUFAs) and a low level of omega 3 series polyunsaturated fatty acids (PUFAs) indicated that nutrition of both gastropods was related to a food web based mainly on bacterial supply. From differences in MUFA distribution between the 2 gastropods, it appeared that most of the energy requirements of A. hessleri were supplied by sulfur-oxidizing endobacteria whereas I. nautilei probably had a mixotrophic diet based on endogenous as well as exogenous bacteria. Given the relatively high level of linoleic acid, which represented from 2 to 8% of the phospholipid fatty acids, hydrothermal gastropods did not appear to be depleted in omega 6 PUFAs. It was hypothesized that they obtain linoleic acid from a pathway different to that in heterotrophic marine molluscs. In contrast to omega 6 PUFAs, both hydrothermal gastropods appeared to be depleted in omega 3 PUFAs, indicating the limited importance of photosynthesis-based food supplies. Some non-methylene-interrupted dienes, particularly 20:2 omega 9,15 which represented from 9 to 18% of the phospholipid fatty acids, may be synthesized by deep-sea symbiotic molluscs in order to restore the depleted omega 3 PUFAs considered as essential for animals. Gills of both gastropods had high levels of neutral lipids, mainly MUFAs that may have originated from degradation of endobacterial phospholipids.