LIPID AND LIPID CARBON STABLE-ISOTOPE COMPOSITION OF THE HYDROTHERMALVENT SHRIMP MIROCARIS-FORTUNATA - EVIDENCE FOR NUTRITIONAL DEPENDENCEON PHOTOSYNTHETICALLY FIXED CARBON

Mirocaris fortunata were sampled from the Lucky Strike hydrothermal ve nt area (Eiffel Tower site) on the mid-Atlantic ridge during the Frenc h DIVA 2 cruise (June 1994). Small adults (17 to 22 mm total length), although morphologically identical, could be divided into 2 categories on the basis of pigmentation, lipid composition and C-13/C-12 stable isotope ratios of fatty acids. Highly pigmented small adults (8.6 to 9 .2 mu g carotenoid shrimp(-1)) contained higher levels of total lipid than similar-sized individuals containing lower levels of pigment (0.9 to 2.9 mu g carotenoid shrimp(-1)). Lipid class analysis indicated th at wax esters comprised 62.5% of total lipid in the former group. Thes e pigmented shrimp also contained high proportions of polyunsaturated fatty acids (PUFA), particularly the phototrophic microplanktonic mark ers 20:5(n-3) and 22:6(n-3) (14.0 and 33.5% respectively). By contrast small adults (22 mm) and adult shrimp (25 to 26 mm) with low levels o f carotenoid pigmentation contained lower amounts of total lipid, litt le or no wax ester and low levels of 20:5(n-3) and 22:6(n-3), but did contain 16:2(n-4) and 18:2(n-4) and the non-methylene interrupted dien es 20:2 Delta 5,13 and 22:2 Delta 7,15. GC-IRMS analysis of all fatty acids and fatty alcohols in the pigmented small adults indicated delta (13)C values of -18.2 to -27.7 parts per thousand, which is consistent with a photosynthetic carbon source for these compounds. The C-13/C-1 2 isotope composition of fatty acids from low-pigmented small adults a nd adults was more variable (-12.5 to -33.1 parts per thousand) and su ggests a bimodal distribution which may be attributable to differing n utritional sources or the physiological/reproductive status of these s hrimp. Samples of eggs, which are carried by the female on the pleopod s, represented approximately 57% of total somatic lipid which indicate s a substantial reproductive investment by this species. The egg lipid s comprised high proportions of triacylglycerols (64.4 to 78.0% of tot al lipid) whilst the fatty acid composition was dominated by the monou nsaturated fatty acids 16:1(n-7), 18:1(n-7) and 18:1(n-9), which accou nted for 65.7 to 33.5% of total fatty acids. By contrast, PUFA were re latively minor components of egg lipids, particularly 20:5(n-3) and 22 :6(n-3), which accounted for only 1.1 and 2.9% of total egg fatty acid s respectively. This indicates that the reproductive investment by thi s species is supported mainly by material derived from bacterial chemo synthesis. The potential for M. fortunata hedge betting by producing l arvae which either metamorphose at the vent site or adopt a bathypelag ic lifestyle and delay metamorphosis to facilitate more widespread dis persal is discussed.