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Recent studies indicated a metabolic temperature sensitivity in both the arcto-boreal krill species Thysanoessa inermis and T. raschii which may determine these species' abundance and population persistence at lower latitudes. T. inermis currently dominates the krill community in the Barents Sea and in the high Arctic Kongsfjord. We aimed to increase the knowledge on the upper thermal limit found in this latter species (at ~ 12°C) by linking metabolic rate measurements with molecular approaches. Optical oxygen sensors were used to measure respiration rates in steps of 2°C (from 0°C to 16°C). To follow temperature- mediated mechanisms of passive response, i.e. as a proxy for molecular stress, we monitored the gene expression of the molecular chaperone heat shock protein 70 (Hsp70) during a sudden temperature exposure to 10°C with subsequent recovery at 4°C. Our results confirmed the presence of three (out of five) inducible Hsp70 isoforms. The high level of gene expression during recovery (up to 80-fold) indicated that the specimens have experienced molecular damages during temperature exposure. These findings confirmed the temperature sensitivity of T. inermis and suggest the upper thermal limit even below 12°C. The study gives a baseline and a basis for further studies concerning the thermal limit of arcto-boreal Thysanoessa spp. and in comparison to other krill species under different climatic regimes. [ABSTRACT FROM AUTHOR]

Our study deals with the lipid biochemistry of the krill community in the ecosystem of the high Arctic Kongsfjord (Svalbard). During the last decades, Kongsfjord experienced a change in krill species composition due to recent increased advection of Atlantic water masses carrying characteristic boreal as well as subtropical-boreal euphausiids into the ecosystem. The lipid biochemistry and trophic relationships of the species recently inhabiting the Arctic water masses are scarcely known, although a change in a krill population may have a significant impact on the ecosystem. A comparison of nutrition and energy storage strategies, stable isotopes, lipid profiles and fatty acid compositions showed remarkable differences between the krill species. These reflected the diverse feeding behaviours and specific adaptations to the environments of their origin: the boreal Meganyctiphanes norvegica and subtropical Nematoscelis megalops appear more carnivorous, have significantly lower mean lipid contents (29 % and 10 %, respectively) and a different energy storage pattern (triacylglycerols and polar lipids, respectively) than the arcto-boreal Thysanoessa inermis, which consists of up to 54 % of lipids mainly stored as wax esters (> 40 %). These differences may have significant implications for the rapidly changing marine food-web of Kongsfjord - especially for higher trophic levels relying on the nutritional input of animal lipids. [ABSTRACT FROM AUTHOR]

Polar environments like the high Arctic Kongsfjord are determined by pronounced seasonality leading to strong variations in primary production. Particularly during winter, food sources are scarce. Herbivorous krill, like the arcto-boreal Thysanoessa inermis are key components in the ecosystem of Kongsfjord and strongly relies on phytoplankton as a food source. Therefore, during polar night such species must be adapted to survive long periods without significant nutritional input. We investigated physiological mechanisms and the allocation of energy resources to explain how T. inermis manages to survive the Arctic winter. Adult specimens were caught in late summer and kept under starvation conditions for 28 days. Changes in metabolic rates (respiration and excretion) and biochemical composition (protein, lipid and fatty acid analyses) were monitored. In contrast to the Antarctic krill, Euphausia superba, and the subtropical E. hanseni, the arcto-boreal species did not reduce metabolism but used lipid reserves for survival. Assessed from total lipid stores and energy demand, the potential survival period was estimated at 63 days without food uptake, which is not sufficient to survive the entire winter. Results were compared to specimens that overwintered in-situ and discussed in relation to other euphausiids. In conclusion, T. inermis is well adapted to survive the Arctic winter provided that alternative food sources are available, but has a different strategy to cope with starvation than krill species from other latitudes. [ABSTRACT FROM AUTHOR]

Adult Euphausia hanseni, keystone and most abundant krill species in the northern Benguela Upwelling area, were sampled during late austral summer in two water masses (northern Benguela Current vs. Angola-Benguela-Front waters) along the Atlantic coast off Namibia. This study investigates the species' physiological performance at different temperatures which it naturally experiences during diel vertical migration as well as its capacity to physiologically adapt to seven consecutive days of starvation. Moulting rates, metabolic rates, carbon demand, total lipid and protein contents, citrate synthase activity and kinetics, C:N ratios and stable isotope ratios served as parameters to estimate the species' physiological condition and adaptive capability within the nutritionally poly-pulsed Benguela upwelling system to cope with short periods of food absence. Moulting rates correlated negatively with temperature. Metabolic rates, adjusted following the Q10 rule, declined significantly over the starvation period. Decreasing trends in the other parameters similarly suggest an adaptation to remain metabolically efficient. Ammonium excretion rates, oxygen to nitrogen ratios and stable isotopes showed strong distinctions between regions. Considerable differences were found between regions regarding the nutritional condition of E. hanseni. The total lipid content and the physiological reaction to starvation are different from euphausiids from other latitudes and help define E. hanseni as a true upwelling organism. [ABSTRACT FROM AUTHOR]