Use of Lipid Profiles of Euphausia superba as a Parameter for Determining Dietary Preferences and the Abundance and Spatial Distribution of krill

From the abstracts of some of the referenced papers:During a period of short-term (19 days) starvation, total lipid in the digestive gland of Euphausia superba Dana decreased from 21 to 9% dry weight. Total lipid per digestive gland decreased significantly during starvation compared to Day 0 individuals, falling from 1960 (plus or minus 172) TO 385 (plus or minus 81) micrograms. Polar lipid was the major lipid class utilised during starvation, falling from 1510 (plus or minus 225) to 177 (plus or minus 46) micrograms per digestive gland (76 to 45%). Absolute levels of tricylglycerol fell from 300 (plus or minus 41) to 76 (plus or minus 5) micrograms; however, relative levels remained unchanged. The relative level of free fatty acid increased significantly with starvation (4 to 39%) with absolute levels ranging from 79 (plus or minus 1) to 156 (plus or minus 20) micrograms per digestive gland. Absolute levels of all fatty acids per digestive gland declined continually until the end of the starvation period. The long-chain polyunsaturated acids eicosapentaenoic and docosahexaenoic, decreased with starvation from 37 to 36% and 15 to 10%, respectively whereas the saturated fatty acid, palmitic acid (16:0), increased from 15% to 22%. Cholesterol, the major sterol in this organ, increased from 17 (plus or minus 20) to 44 (plus or minus 13) micrograms per digestive gland by Day 3, and by Day 19 had returned to levels found in the digestive gland of Day 0 individuals. Desmosterol followed a similar pattern to cholesterol, increasing from 3 (plus or minus 1) micrograms per digestive gland on Day 0 to 11 (plus or minus 4) micrograms on Day 3, and falling to 2 (plus or minus 1) micrograms on Day 19. Other sterols in the digestive gland, predominantly of algal origin, fell from the levels found in Day 0 individuals to near zero amounts by Day 6. The digestive gland of E. superba plays a dynamic role during short term starvation in terms of lipid content and composition. The relative levels of polar lipids, free fatty acids and cholesterol in the digestive gland may provide reliable indices of the nutritional condition of E. superba in the field. Sterols in the digestive gland are indicative of recent dietary composition of krill, and may also be used to quantify dietary input from individual phytoplankonic species.The fatty acid profiles of Euphausia superba, the Antarctic prymnesiophyte, Phaeocystis pouchetii, and a temperate diatom, Phaeodactylum tricornutum were analysed and compared. The lipid content, lipid class, fatty acid and sterol composition of E. superba fed on each cultured phytoplankton and a mixed diet of both species, were determined. No significant difference was found between total lipid levels of E. superba reared on each of these different diets. Phaeocystis pouchetii, although deficient in a number of the essential fatty acids, is apparently an adequate food source for E. superba. The proportion of polyenoic acids varied within lipid classes although there was no significant difference between levels of the long chain polyunsaturate 20:5 (n-3) in the total lipid of E. superba fed on these diets. This acid was found to be less than 1% of the total lipid in Phaeocystis pouchetii compared to 37% in Phaeodactylum tricornutum. This suggests that krill may possess the ability to convert exogenous shorter chain fatty acids to 20:5 (n-3) and 22:6 (n-3). Significant differences were detected in the isomeric ratio of 16:1 (n-7c)/16:0 between krill fed the diatom compared to the prymnesiophyte diet. Significant differences were also detected in several shorter chain fatty acids and between fatty acids within their lipid classes. Such differences may have the potential to be used as biochemical signatures to provide information on food sources and possible feeding grounds of E. superba. Phaeocystis pouchetii in a very late stationary phase, although not used in this feeding study, was found to contain 11% of 22:6 (n-3) for which there are few substantive sources in natural algal populations.Freshly caught male and female Euphausia superba from the same swarm exhibited different rates of mortality subsequent to capture. Mortality was significantly higher for reproductive males (100%, n - 68) than for females (3%, n = 186) within the first 3 days of capture. Total lipid and triacylglycerol levels in male, female and juvenile Euphausia superba were analysed and compared. All reproductive male krill analysed from this swarm had low lipid levels (1 to 3% dry weight) with negligible triacylglyecerol stores (0 to 2% of total lipid). Somatic lipid stores in female and juvenile krill ranged from 8 to 30%, of which up to 40% was triacylglycerol. The levels of algal sterols in the digestive gland of males, females and juveniles indicate that all krill had been feeding recently. An analysis of the sex ratio of krill catches derived from data collected over seven summers from the Prydz Bay region showed a decrease in the proportion of males with increasing size. There was a sharp decline in numbers of male krill once they attained a length of 51 to 55mm. Low lipid levels in reproductive male krill may be due to reproductive costs. The resulting low storage-lipid levels are accompanied by high mortality in male krill.Several aspects of copper toxicity to Euphausia superba were examined. Baseline copper concentrations in Euphausia superba caught and frozen in the Antarctic was found to be in the range 55.2-82.6 micrograms per gram, dry weight. Total body copper concentrations were measured in animals exposed to various ambient copper concentrations. Euphausia superba were able to regulate copper to a constant level beyond which copper became lethal at estimated ambient bioavailable copper ion concentrations of 140 million M at an LT50 of 3.25 days. Krill died when the total body concentration reached approximately 250-300 micrograms per gram dry weight. Survival rates were found to be directly related to free copper ion concentrations which were determined by altering the free cupric ion activity using the metal ion complexing agent, NTA.

引自部分参考文献的摘要：
在为期19天的短期饥饿胁迫下，南极磷虾（Euphausia superba Dana）消化腺中的总脂质含量以干重计从21%降至9%。与饥饿第0天的个体相比，饥饿过程中单只消化腺的总脂质含量显著下降，从1960±172微克降至385±81微克。
极性脂是饥饿过程中主要被利用的脂质类别，单只消化腺的极性脂含量从1510±225微克降至177±46微克（占总脂质的比例从76%降至45%）。三酰甘油（triacylglycerol）的绝对含量从300±41微克降至76±5微克，但其相对占比未发生变化。游离脂肪酸的相对占比随饥饿胁迫显著升高，从4%升至39%，其单只消化腺的绝对含量范围为79±1微克至156±20微克。单只消化腺内所有脂肪酸的绝对含量在整个饥饿周期中持续下降。
长链多不饱和脂肪酸二十碳五烯酸（eicosapentaenoic acid, EPA）与二十二碳六烯酸（docosahexaenoic acid, DHA）的相对占比随饥饿分别从37%降至36%、从15%降至10%；而饱和脂肪酸棕榈酸（16:0）的占比则从15%升至22%。
胆固醇作为该器官中的主要甾醇，在饥饿第3天时单只消化腺的胆固醇含量从17±20微克升至44±13微克，至第19天时恢复至饥饿第0天个体消化腺中的水平。脱胆固醇（desmosterol）的变化趋势与胆固醇相似：第0天时单只消化腺内的含量为3±1微克，第3天升至11±4微克，至第19天回落至2±1微克。消化腺内其余主要源自藻类的甾醇，从第0天个体的水平持续下降，至第6天时已接近零含量。
南极磷虾的消化腺在短期饥饿过程中，其脂质含量与组成均呈现动态变化。消化腺内极性脂、游离脂肪酸与胆固醇的相对占比，可作为野外南极磷虾营养状况的可靠评估指标。消化腺内的甾醇可反映磷虾近期的日粮组成，还可用于量化单个浮游植物物种的膳食贡献量。

本研究分析并比较了南极磷虾、南极定鞭金藻（Phaeocystis pouchetii）与温带硅藻三角褐指藻（Phaeodactylum tricornutum）的脂肪酸谱。研究还测定了分别投喂以上两种单种培养浮游植物、以及二者混合日粮的南极磷虾的脂质含量、脂质类别组成、脂肪酸组成与甾醇组成。投喂不同日粮的南极磷虾，其总脂质水平未出现显著差异。尽管棕囊藻（Phaeocystis pouchetii）缺乏多种必需脂肪酸，但它仍是南极磷虾的适宜食物来源。不同脂质类别内的多烯酸占比存在差异，但投喂以上日粮的南极磷虾总脂质中长链多不饱和脂肪酸20:5(n-3)的含量未出现显著差异。在棕囊藻中，该脂肪酸占总脂质的比例不足1%，而在三角褐指藻中该比例为37%。该结果表明，南极磷虾具备将外源性短链脂肪酸转化为20:5(n-3)与22:6(n-3)的能力。
投喂硅藻日粮与定鞭金藻日粮的南极磷虾，其16:1(n-7c)/16:0的异构体比值存在显著差异。多种短链脂肪酸以及不同脂质类别内的脂肪酸之间，也存在显著差异。此类差异有望作为生物化学标记物，用于解析南极磷虾的食物来源与潜在摄食区域。尽管未应用于本摄食研究，但处于极晚期静止期的棕囊藻中，22:6(n-3)的占比可达11%，而自然藻类种群中具备此类高占比的物种较为罕见。

从同一虾群中刚捕获的南极磷虾雌雄个体，在捕获后的死亡率存在显著差异。在捕获后的前3天内，性成熟雄性个体的死亡率（100%，n=68）显著高于雌性个体（3%，n=186）。研究对雄性、雌性以及幼体南极磷虾的总脂质与三酰甘油水平进行了分析与比较。本研究中所有取自该虾群的性成熟雄性磷虾，其脂质水平较低（以干重计为1%~3%），且三酰甘油储存量可忽略不计（占总脂质的0~2%）。雌性与幼体磷虾的体脂储存量占比为8%~30%，其中三酰甘油占比最高可达40%。雌雄个体与幼体消化腺内的藻类甾醇水平表明，所有受试磷虾近期均有摄食行为。
对普里兹湾海域7个夏季的磷虾捕捞数据进行性别比例分析后发现，雄性个体的占比随磷虾体长增加而降低。当雄性磷虾体长达到51~55mm时，其种群数量出现急剧下降。性成熟雄性磷虾的低脂质水平可能源于繁殖活动的能量消耗。由此产生的储存脂质水平低下，会导致雄性磷虾的死亡率升高。

本研究还探讨了铜对南极磷虾的毒性效应的多个方面。在南极捕获并冷冻保存的南极磷虾，其体内铜的基线浓度范围为55.2~82.6微克/克干重。研究测定了暴露于不同环境铜浓度下的南极磷虾的体铜总浓度。南极磷虾可将体内铜浓度调控至恒定阈值，当环境中生物可利用铜离子浓度达到140×10^6 M时，铜将达到致死剂量，此时的半致死时间（LT50）为3.25天。当南极磷虾体内总铜浓度达到约250~300微克/克干重时，个体将死亡。研究发现，磷虾的存活率与游离铜离子浓度直接相关；该游离铜离子浓度可通过金属离子络合剂NTA调控游离铜离子活性来确定。