Lipids in Euphausia superba

Aspects of the life history and overwintering survival strategies of Antarctic krill (Euphausia superba) were interpreted through analyses of lipid class, fatty acid and sterol content, and composition. Both laboratory and field studies were undertaken to provide information on the biochemical and physiological consequences of starvation, on dietary sources and on reproduction. Starvation in krill, both short term (19 days) and long term (130 days), was investigated. Lipids are a source of short term energy in krill. The relative levels of polar lipids, free fatty acids and cholesterol in the digestive gland provide indices of the nutritional condition of Euphausia superba in the field in the short term. However, throughout long term starvation levels of both triacylglycerol and polar lipid in krill did not change significantly. Therefore, lipid metabolism is not considered to be the major mechanism for energy production during long term starvation as is usually the case in most other polar zooplankton. Lipid profiles of krill fed on diets of Phaeocystis pouchetii, and of diets of diatoms were compared. Krill fed these two diets showed no significant differences in lipid levels. Phaeocystis pouchetii, although being deficient in a number of what are normally referred to as the essential fatty acids, was found to be nutritionally equivalent to diatoms as a food source for Euphausia superba. Krill may possess the ability to convert exogenous shorter chain fatty acids to the long chain essential polyunsaturated fatty acids, eicosapentaenoic (EPA, 20:5ω3) and docosahexaenoic (DHA, 22:6ω3); this hypothesis has also recently been proposed for penaeid prawns. The results from the present investigation of Euphausia superba suggest the ability to convert dietary derived fatty acids to EPA and DHA may be more widely spread than previously believed. The use by krill of bacteria as a nutrient source was also investigated. Bacterial cultures isolated from the stomach and the digestive gland of krill contained strains able to produce polyunsaturated fatty acids including eicosapentaenoic acid (20:5ω3). These findings may partially explain the high levels (approximately 50% of total fatty acids) of essential fatty acids found in the digestive gland of krill. Lipid biomarkers in krill were investigated. It was found that sterols in the digestive gland indicate composition of the recent diet of krill. Sterols can be also be used to quantify dietary input from individual phytoplanktonic species. Fatty acids, like sterols, can be used as biomarkers to provide information on food sources of Euphausia superba. The role of lipids in reproduction was investigated. Reproductive stores were determined in both male and female krill. High lipid levels were found in reproductive female krill and very low levels of lipid were found in male krill following reproduction. Despite male krill feeding actively during reproduction, the level of storage lipid (triacylglycerol) was depleted. Female krill regressed sexually after spawning and survived through to the following season. Male krill, however, died shortly after reproduction. Fewer males in the Antarctic krill population would reduce competition during the food-limited winter allowing resources to be preferentially invested in females. The lipid profile of Euphausia superba was compared to that of a temperate euphausiid, Nyctiphanes australis. Seasonal comparisons between the polar and temperate species were made. The potential of the euphausiid, Nyctiphanes australis as an aquaculture feed in terms of lipid, pigment and fluoride content was investigated. Like Euphausia superba, Nyctiphanes australis contained high levels of the essential long chain polyunsaturated fatty acids (PUFA); EPA and DHA. Astaxanthin was the main carotenoid in Nyctiphanes australis. Both high PUF A and astaxanthin levels suggest this species could serve as a suitable feed for the aquaculture industry