Dynamics of lipid and fatty acid composition of the hyperiid amphipod Themisto: a bipolar comparison with special emphasis on seasonality
Résumé
Themisto in polar regions represents a direct link between the zooplankton community and higher predators. Lipid characteristics were reviewed for both polar populations: T. libellula and T. abyssorum in the Arctic, and T. gaudichaudii in the Antarctic. Lipids composition showed similarities for all species with lipid class structures dominated by neutral lipids and similar composition of fatty acid membrane lipids. Differences reflect specificities of food web structure. In the Arctic, feeding on Calanus resulted in high accumulation of wax esters, and high content in long-chain monoenes in triglycerides and wax esters. In the Antarctic, feeding on different calanoid copepods resulted in a dominance of triglycerides and high 18:1n-9 content in both neutral lipid classes. Strong similarities in total fatty acid composition of summer-collected Themisto were observed in both polar systems. In addition to summer published data, a seasonal survey of adult lipid composition was carried out in Kerguelen Bay over 18 months. Lipid class composition showed maximum values of neutral lipids in fall-winter to cope with low food abundance and late winter reproduction. The results concerning fatty acid structure underline the influence of the summer-winter and winter-spring transitions: triglycerides and wax ester fatty acid changes showed in winter maximum percentages of 18:1n-9 and minimum content of polyunsaturated fatty acids. It emphasized the overwintering strategy with continuous feeding, relative stability of lipid content but decreasing percentages of polyunsaturated acids in wax esters (mostly 18:4n-3), and to a lesser extent triglycerides (mostly 20:5n-3 and 22:6n-3) to maintain stability in phospholipids fatty acid composition.