Die Flügelschnecke Clione limacina: Außergewöhnliche Lipide als Anpassungsstrategie an den Lebensraum Arktis

Abstract

The main topic of this thesis was to investigate of the ecological role of lipids in the life-strategy of the pteropod Clione limacina and the adaptations to the Arctic environment. Since the sea butterfly Limacina helicina is the only food of C. limacina and the development of both species occurs simultaneously, lipid biochemical investigations of L. helicina are as well important for the assessment of the life-strategy of C. limacina. The basic questions of this thesis were studied by combining field work and experiments. The results about the seasonal lipid dynamics of C. limacina and L. helicina clearly demonstrate that lipids play a key role in both species during the ontogenetic development and reproduction. Limacina helicina is a true omnivore feeding on available particles. The fatty acid composition reflects its diet which ranged from mostly phytoplankton in spring and summer to degraded organic material in late autumn and winter. In autumn the large females of L. helicina die after spawning, and the lipid-rich veligers develop to juveniles which both are overwintering stages. For Clione limacina the seasonal field studies have shown that the fatty acids originating from the food correlate with tri-acylglycerols (TAG), whereas the odd-chain fatty acids and the monounsaturates 16:1(n-7) and 18:1(n-7) correlate with 1-O-alkyldiacylglycerols (DAGE) and hence were mostly synthesised de novo. Both lipid classes were depleted during the ontogenetic development, maturation and reproduction in summer which underlines their importance as energy source within these processes. Due to enhanced accumulation of DAGE during autumn and a more slowly catabolism than TAG during long-term starvation this lipid class is most likely of major importance during food scarcity and over-wintering. Until now only little is known about the life-cycle of the Arctic Clione limacina. The length-frequency distribution of C. limacina revealed no unequivocal population structure why it was assessed by principal component analysis (PCA). From the seasonal lipid accumulation, de novo lipid biosynthesis and the population structure defined by the fatty acid based PCA it was concluded that C. limacina has at least a two-year life cycle or even longer. In contrast, a life cycle of one year was proposed for Limacina helicina which based on the strong depletion of lipids in females, the relatively high presence of lipids in veligers, but also on the disappearance of larger specimens in late September.The feeding experiment showed that Clione limacina assimilated up to 80% of the ash-free dry mass of Limacina helicina. This high assimilation rate supports the very effective utilisation of food. After 5 respectively 7 days of digestion of one specimen of L. helicina, C. limacina showed highest lipid contents. Since the uptake of lipids by food ingestion accounted only for one third of the increase of lipids in C. limacina most of the lipids must be synthesised de novo from non-lipid constituents like proteins and carbohydrates. Because odd-chain fatty acids are absent in the only food of C. limacina they have to be synthesised de novo. The feeding experiment also demonstrates that C. limacina is also capable to synthesise de novo considerable amounts of 16:1(n-7), 18:1(n-7) and the alkyl moieties 16:0 and 15:0. Important knowledge about the life-strategy of Clione limacina was obtained by two different approaches of starvation experiments. The first experiment was carried out to investigate the role of lipids during long-term starvation, whereas the second was performed to examine the major sites of lipid storage, the significance of lipid droplets and the utilisation of non-lipid constituents during food-absence. The studies revealed that the lipid catabolism of C. limacina is depended on the ontogenetic stage and reproduction (egg-production). Lipid-rich animals which obviously were in gonad- or egg-development utilised lipids more quickly than post-reproductive or immature animals. These experiments also clearly demonstrated that the long-term starvation ability of C. limacina was determined by the lipid content and composition. In both experiments DAGE were utilised more slowly than TAG which shows that DAGE serve as long-term energy. The digestive gland exhibited much higher lipid levels than the lipid-droplets containing trunk. During long periods of starvation Clione limacina utilised body constituents obviously not essential for survival which are cells of the body-wall and gonads. To survive extremely long periods of starvation, respectively food scarcity, the combination of various physiological strategies seems indispensable. For C. limacina the most important strategies to withstand almost one year (exactly 356 days) without any food are very low metabolic rates in close connection with a low lipid catabolism and utilisation of body constituents not essential for survival.