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The article focuses on the methods undertaken to study the sea ice conditions that are needed for the successful larval development of krill at winter time. Topics discussed included krill sampling for the determination of their condition, ice core sampling for the identification of potential sources of food, and the time frame for the presentation of expected results.

Fin whales (Balaenoptera physalus quoyi) of the Southern Hemisphere were brought to near extinction by twentieth century industrial whaling. For decades, they had all but disappeared from previously highly frequented feeding grounds in Antarctic waters. Our dedicated surveys now confirm their return to ancestral feeding grounds, gathering at the Antarctic Peninsula in large aggregations to feed. We report on the results of an abundance survey and present the first scientific documentation of large fin whale feeding aggregations at Elephant Island, Antarctica, including the first ever video documentation. We interpret high densities, re-establishment of historical behaviours and the return to ancestral feeding grounds as signs for a recovering population. Recovery of a large whale population has the potential to augment primary productivity at their feeding grounds through the effects of nutrient recycling, known as 'the whale pump'. The recovery of fin whales in that area could thus restore ecosystem functions crucial for atmospheric carbon regulation in the world's most important ocean region for the uptake of anthropogenic CO2. [ABSTRACT FROM AUTHOR]

An appendix is presented of the activities of the PICES Working Group on Comparative Ecology of Krill in Coastal and Oceanic Waters around the Pacific Rim (WG) 23.

Antarctic krill (Euphausia superba) is a key species of the Southern Ocean, impacted by climate change and human exploitation. Understanding how these changes affect the distribution and abundance of krill is crucial for generating projections of change for Southern Ocean ecosystems. Krill growth is an important indicator of habitat suitability and a series of models have been developed and used to examine krill growth potential at different spatial and temporal scales. The available models have been developed using a range of empirical and mechanistic approaches, providing alternative perspectives and comparative analyses of the key processes influencing krill growth. Here we undertake an intercomparison of a suite of the available models to understand their sensitivities to major driving variables. This illustrates that the results are strongly determined by the model structure and technical characteristics, and the data on which they were developed and validated. Our results emphasize the importance of assessing the constraints and requirements of individual krill growth models to ensure their appropriate application. The study also demonstrates the value of the development of alternative modelling approaches to identify key processes affecting the dynamics of krill. Of critical importance for modelling the growth of krill is appropriately assessing and accounting for differences in estimates of food availability resulting from alternative methods of observation. We suggest that an intercomparison approach is particularly valuable in the development and application of models for the assessment of krill growth potential at circumpolar scales and for future projections. As another result of the intercomparison, the implementations of the models used in this study are now publicly available for future use and analyses. [ABSTRACT FROM AUTHOR]

Background: The Antarctic krill Euphausia superba is a keystone species in the Southern Ocean ecosystem. This crustacean has an ancestral clock whose main components have been identified and characterized in the past few years. However, the second feedback loop, modulating clock gene expression through two transcription factors, VRI and PDP1, has yet to be described. The presence of this second regulatory mechanism is suggested by the identification of its negative component, vrille, at the transcriptional level. Results: Here, we describe the second feedback loop of krill by identifying the positive component, pdp1, and functionally characterizing both pdp1 and vrille. Starting from the online transcriptome database KrillDB2, we identified and cloned three putative pdp1 sequences which were subsequently analyzed for tissue expression and functional activity using luciferase assays, individually and in combination with two vrille isoforms. Among the pdp1 isoforms, Espdp1_3 displayed higher expression levels in relevant circadian districts than the other two. Furthermore, EsPDP1_3 and EsVRI_2 exhibited the expected positive and negative regulation of the V/P-box in our in vitro system. Finally, Espdp1_3 and Esvrille also showed rhythmic expression in light–dark cycles, supporting their involvement in the regulation of the main circadian clock of the Antarctic krill. Conclusions: This study expands our knowledge about the molecular architecture of the Antarctic krill circadian clock by defining the components that take part in the modulation of clock expression, establishing a second feedback loop. [ABSTRACT FROM AUTHOR]

The Antarctic krill, Euphausia superba , has evolved seasonal rhythms of physiology and behaviour to survive under the extreme photoperiodic conditions in the Southern Ocean. However, the molecular mechanisms generating these rhythms remain far from understood. The aim of this study was to investigate seasonal differences in gene expression in three different latitudinal regions (South Georgia, South Orkneys/Bransfield Strait, Lazarev Sea) and to identify genes with potential regulatory roles in the seasonal life cycle of Antarctic krill. The RNA-seq data were analysed (a) for seasonal differences between summer and winter krill sampled from each region, and (b) for regional differences within each season. A large majority of genes showed an up-regulation in summer krill in all regions with respect to winter krill. However, seasonal differences in gene expression were less pronounced in Antarctic krill from South Georgia, most likely due to the milder seasonal conditions of the lower latitudes of this region, with a less extreme light regime and food availability between summer and winter. Our results suggest that in the South Orkneys/Bransfield Strait and Lazarev Sea region, Antarctic krill entered a state of metabolic depression and regressed development (winter quiescence) in winter. Moreover, seasonal gene expression signatures seem to be driven by a photoperiodic timing system that may adapt the flexible behaviour and physiology of Antarctic krill to the highly seasonal environment according to the latitudinal region. However, at the lower latitude South Georgia region, food availability might represent the main environmental cue influencing seasonal physiology. [ABSTRACT FROM AUTHOR]

• We advance an existing individual-based model simulating growth, reproduction and survival (life history) of Antarctic krill • In the new model, krill metabolism is light- and temperature dependent which leads to down-regulation during winter and up-regulation during summer • The light- and temperature dependent regulation of the metabolism enables individuals to store more energy throughout the year, grow to larger sizes and increase reproductive output, especially when winter food conditions are poor • When winter food is abundant, the down-regulation of metabolism hinders individuals to fully exploit the available energy • The new model adds physiological detail while remaining relatively simple which makes it suitable for spatial modelling studies simulating great numbers of individuals Antarctic krill up- and down-regulate their metabolism as a strategy to cope with the strong seasonal environmental fluctuations in the Southern Ocean. In this study, we investigate the impact of this light- and temperature dependent metabolic regulation on growth, reproduction and winter survival of krill. Therefore, we advance a bioenergetic growth model of krill by adding a data-derived scaling function of krill activity. With SERBIK (SEasonally Regulated BIoenergetic Krill growth model), we conduct a numerical experiment which tests the impact of such scaling on krill life history under two different winter food conditions: In the first scenario, we simulate life history of krill when winter food availability is low; in the second scenario, winter food availability is increased within realistic ranges. The results demonstrate that the scaling of metabolism is especially important during low food winters. Reducing metabolism during winter permits individuals to grow to larger body length, reproduce successfully and release a greater number of eggs. It further significantly reduces within-year size fluctuations caused by starvation during months with low food availability. Finally, SERBIK can be used in future spatial modelling studies which include movement of krill along latitudinal gradients and thus spatio-temporal gradients in light- and temperature. [ABSTRACT FROM AUTHOR]

The zooplankter Calanus finmarchicus is a member of the so-called "Calanus Complex", a group of copepods that constitutes a key element of the Arctic polar marine ecosystem, providing a crucial link between primary production and higher trophic levels. Climate change induces the shift of C. finmarchicus to higher latitudes with currently unknown impacts on its endogenous timing. Here we generated a daily transcriptome of C. finmarchicus at two high Arctic stations, during the more extreme time of Midnight Sun, the summer solstice. While the southern station (74.5 °N) was sea ice-free, the northern one (82.5 °N) was sea ice-covered. The mRNAs of the 42 samples have been sequenced with an average of 126 ± 5 million reads (mean ± SE) per sample, and aligned to the reference transcriptome. We detail the quality assessment of the datasets and the complete annotation procedure, providing the possibility to investigate daily gene expression of this ecologically important species at high Arctic latitudes, and to compare gene expression according to latitude and sea ice-coverage. Measurement(s) transcriptome • sequence feature annotation Technology Type(s) RNA sequencing • sequence annotation Factor Type(s) location • time Sample Characteristic - Organism Calanus finmarchicus Sample Characteristic - Environment polar Sample Characteristic - Location Arctic Ocean Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.13008035 [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba) are high latitude pelagic organisms which play a key ecological role in the ecosystem of the Southern Ocean. To synchronize their daily and seasonal life-traits with their highly rhythmic environment, krill rely on the implementation of rhythmic strategies which might be regulated by a circadian clock. A recent analysis of krill circadian transcriptome revealed that their clock might be characterized by an endogenous free-running period of about 12–15 h. Using krill exposed to simulated light/dark cycles (LD) and constant darkness (DD), we investigated the circadian regulation of krill diel vertical migration (DVM) and oxygen consumption, together with daily patterns of clock gene expression in brain and eyestalk tissue. In LD, we found clear 24 h rhythms of DVM and oxygen consumption, suggesting a synchronization with photoperiod. In DD, the DVM rhythm shifted to a 12 h period, while the peak of oxygen consumption displayed a temporal advance during the subjective light phase. This suggested that in free-running conditions the periodicity of these clock-regulated output functions might reflect the shortening of the endogenous period observed at the transcriptional level. Moreover, differences in the expression patterns of clock gene in brain and eyestalk, in LD and DD, suggested the presence in krill of a multiple oscillator system. Evidence of short periodicities in krill behavior and physiology further supports the hypothesis that a short endogenous period might represent a circadian adaption to cope with extreme seasonal photoperiodic variability at high latitude. [ABSTRACT FROM AUTHOR]

Recently, there has been focus on lowering emission levels of wood-based boards. However, the accuracy and correlation between EN 717-1 and ASTM D 6007 chamber methods at emission levels below 0.05 ppm are not well investigated, and information about their correlation to the EN 16516 method is limited. In this paper, the low emission level of interest was determined by measuring emissions from particles, fibers and pressed boards without glue. The effect of analytical methods and edge-sealing on chamber emissions was determined, and accuracies and correlations of the EN 717-1 and ASTM D 6007 chambers were defined at low emission levels (<0.05 ppm). In addition, some emission values were compared to those obtained with EN 16516. The EN 717-1 and ASTM D 6007 methods had high accuracy. The acetyl acetone and 2.4-dinitrophenylhydrazine analytical methods showed low standard deviations (<5%), except at emission levels below 0.02 ppm. This could be counteracted by using a direct reagent absorber solution. Opening 5% of the edge of boards affected emissions and was dependent on board type. ASTM D 6007 and EN 717-1 methods were highly correlated for both particleboards (r2 = 0.9167) and fiberboards (r2 = 0.9443) at emission levels below 0.05 ppm. EN 16516 emissions were 2.6 times greater than those of EN 717-1 at emission range <0.05 ppm, exceeding the conversion factor of two given in the German legislation. The EN 717-1 to EN 16516 correlation needs to be further evaluated for different board types and emission ranges. [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba) is a high latitude pelagic organism which plays a central role in the Southern Ocean ecosystem. E. superba shows daily and seasonal rhythms in physiology and behaviour, which are synchronized with the environmental cycles of its habitat. Recently, the main components of the krill circadian machinery have been identified and characterized. However, the exact mechanisms through which the endogenous timing system operates the control and regulation of the overt rhythms remains only partially understood. Here we investigate the involvement of the circadian clock in the temporal orchestration of gene expression by using a newly developed version of a krill microarray platform. The analysis of transcriptome data from krill exposed to both light-dark cycles (LD 18:6) and constant darkness (DD), has led to the identification of 1,564 putative clock-controlled genes. A remarkably large proportion of such genes, including several clock components (clock, period, cry2, vrille, and slimb), show oscillatory expression patterns in DD, with a periodicity shorter than 24 hours. Energy-storage pathways appear to be regulated by the endogenous clock in accordance with their ecological relevance in daily energy managing and overwintering. Our results provide the first representation of the krill circadian transcriptome under laboratory, free-running conditions. [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba, hereafter krill) is a pelagic living crustacean and a key species in the Southern Ocean ecosystem. Krill builds up a huge biomass and its synchronized behavioral patterns, such as diel vertical migration (DVM), substantially impact ecosystem structure and carbon sequestration. However, the mechanistic basis of krill DVM is unknown and previous studies of krill behavior in the laboratory were challenged by complex behavior and large variability. Using a new experimental set-up, we recorded the swimming activity of individual wild-caught krill under light–dark cycles. Krill individuals exhibited differential phototactic responses to the light regime provided. However, using a new activity metric, we showed for the first time a consistent nocturnal increase in krill swimming activity in a controlled environment. Krill swimming activity in the new set-up was strongly synchronized with the light–dark cycle, similar to the diel vertical migration pattern of krill in the field when the krill were sampled for the experiment, demonstrated by hydroacoustic recordings. The new set-up presents a promising tool for investigating the mechanisms underlying krill behavioral patterns, which will increase our understanding of ecological interactions, the spatial distribution of populations, and their effects on biogeochemical cycles in the future. [ABSTRACT FROM AUTHOR]

Annual rhythms are observed in living organisms with numerous ecological implications. In the zooplanktonic copepod Calanus finmarchicus, such rhythms are crucial regarding its phenology, body lipid accumulation, and global carbon storage. Climate change drives annual biological rhythms out of phase with the prevailing environmental conditions with yet unknown but potentially catastrophic consequences. However, the molecular dynamics underlying phenology are still poorly described. In a rhythmic analysis of C. finmarchicus annual gene expression, results reveal that more than 90% of the transcriptome shows significant annual rhythms, with abrupt and dramatic upheaval between the active and diapause life cycle states. This work explores the implication of the circadian clock in the annual timing, which may control epigenetic mechanisms to profoundly modulate gene expression in response to calendar time. Results also suggest an increased light sensitivity during diapause that would ensure the photoperiodic entrainment of the endogenous annual clock. [ABSTRACT FROM AUTHOR]

New convective cells occur at increased likelihood near cold pool gust fronts – as was shown by observations and high-resolution simulations alike. However, pinning down whether this is due to single gust fronts or collisions of multiple ones is much less settled. Conceptual models show that this very distinction, single vs. multi-CP collisions, may be key to the character of the resulting spatial organization of thunderstorms, and related extreme precipitation events.We assume that the likelihood of triggering convection scales with the height and strength of the updrafts, generated at the gust front of the CP. The faster the CP, the more vigorous updrafts are generated. However, the relation between the 'first generation' rain event, the strength of the resulting CP and the intensity of the triggered 'second generation' rain event seems not as straight forward. To understand this feedback, we consider a conceptual model that decomposes CPs into three players: (1) a cold air downburst that determines the initial potential energy, (2) a peripheral rotating torus – often referred to as a vortex ring – and (3) the updrafts generated in the ambient air in front of the CP vortex ring. The vortex ring could be caricatured as a snow plow shoveling air into the vertical due to its key role in the dynamic interaction of and between CPs. The introduction of a vortex ring allows us to relate the understanding of cold pool collisions back to the process of vortex interaction - a well-seasoned branch of classical fluid dynamics. This theoretical vortex ring model is compared to idealized large-eddy simulations. Preliminary results suggest that not only the initial potential energy from evaporation, but also the geometry of the rain cell plays a role for the dynamics and lifetime of the CP. This most likely is a consequence of the division into linear and rotational kinetic energy during the transfer of potential energy to kinetic energy. The linear kinetic energy is dominated by the spreading velocity of the cold pool gust front, while the rotational energy is stored in the peripheral vortex ring. The higher the initial downbursts from the rain event, the higher fraction of potential energy seems to be turned into rotational energy of the vortex ring. The hypothesized vortex ring further seems to reduce mixing at the CP front, letting the CPs spread as coherent structures over times that exceed the lifetimes predicted by conventional models based on the assumption of cylindrically shaped CPs. Regarding collisions, the simulations show that the maximum vertical velocities resulting from two colliding CPs are up to a factor three higher than at an undisturbed CP front and extend higher into the atmosphere. This supports the formation of deep convective events in contrast to shallow convection clouds that are often observed near single cold pool gust fronts. [ABSTRACT FROM AUTHOR]

The biochemical composition of Antarctic krill, Euphausia superba, is largely determined by their feeding behaviour. As they supply energy for animals of a higher trophic level and are also commercialized for human consumption, the interest in research on the species is high. Lipids, especially phospholipids, make up a high proportion of dry weight in krill. Seasonal changes are well documented in the fingerprint of free fatty acids analysed after hydrolysis of phospholipids, but the underlying intact polar lipids are rarely considered. In this study, we evaluated the compositions of intact phospholipids (IPLs) in the stomach, digestive gland and hind gut of Antarctic krill caught in summer and autumn at the Antarctic Peninsula region. Using high-resolution mass spectrometry, the fatty acid composition of 179 intact phospholipids could be resolved. Most IPLs were phosphatidylcholines, followed by phosphatidylethanolamines. Several very long chain polyunsaturated fatty acids up to 38:8, which have not been reported in krill before, were identified. The composition shifted to higher molecular weight IPLs with a higher degree of unsaturation for summer samples, especially for samples of the digestive gland. The data supplied in this paper provides new insights into lipid dynamics between summer and autumn usually described by free fatty acid biomarkers. [ABSTRACT FROM AUTHOR]

The physiological condition of larval Antarctic krill was investigated during austral autumn 2004 and winter 2006 in the Lazarev Sea. The condition of larvae was quantified in both seasons by determining their body length (BL), dry weight (DW), elemental and biochemical composition, stomach content analysis, and rates of metabolism and growth. Overall the larvae in autumn were in better condition under the ice than in open water, and for those under the ice, condition decreased from autumn to winter. Thus, growth rates of furcilia larvae in open water in autumn were similar to winter values under the ice (mean, 0.008 mm d-1), whereas autumn underice values were higher (0.015 mm d-1). Equivalent larval stages in winter had up to 30% shorter BL and 70% lower DW than in autumn. Mean respiration rates of winter larvae were 43% lower than of autumn larvae. However, their ammonium excretion rates doubled in winter from 0.03 to 0.06 µg NH4 DW-1 h-1, resulting in mean O:N ratios of 46 in autumn and 15 in winter. Thus, differing metabolic substrates were used between autumn and winter, which supports a degree of flexibility for overwintering of larval krill. The larvae were eating small copepods (Oithona spp.) and protozoans, as well as autotrophic food under the ice. The interplay between under-ice topography, apparent current speed under sea ice, and the swimming ability of larval krill is probably critical to whether larval krill can maintain position and exploit suitable feeding areas under the ice. [ABSTRACT FROM AUTHOR]

The krill species Euphausia superba plays a critical role in the food chain of the Antarctic ecosystem. Significant changes in climate conditions observed in the Antarctic Peninsula region in the last decades have already altered the distribution of krill and its reproductive dynamics. A deeper understanding of the adaptation capabilities of this species is urgently needed. The availability of a large body of RNA-seq assays allowed us to extend the current knowledge of the krill transcriptome. Our study covered the entire developmental process providing information of central relevance for ecological studies. Here we identified a series of genes involved in different steps of the krill moulting cycle, in the reproductive process and in sexual maturation in accordance with what was already described in previous works. Furthermore, the new transcriptome highlighted the presence of differentially expressed genes previously unknown, playing important roles in cuticle development as well as in energy storage during the krill life cycle. The discovery of new opsin sequences, specifically rhabdomeric opsins, one onychopsin, and one non-visual arthropsin, expands our knowledge of the krill opsin repertoire. We have collected all these results into the KrillDB2 database, a resource combining the latest annotation of the krill transcriptome with a series of analyses targeting genes relevant to krill physiology. KrillDB2 provides in a single resource a comprehensive catalog of krill genes; an atlas of their expression profiles over all RNA-seq datasets publicly available; a study of differential expression across multiple conditions. Finally, it provides initial indications about the expression of microRNA precursors, whose contribution to krill physiology has never been reported before. [ABSTRACT FROM AUTHOR]

The effect of different light regimes on the development of sexual maturity and body composition (carbon, nitrogen, lipid and protein) of Antarctic krill, Euphausia superba, was studied over 12 weeks under laboratory conditions. Krill were exposed to light-cycle regimes of variable intensity to simulate Southern Ocean summer, autumn and winter conditions, respectively using: (1) continuous light (LL; 200 lux max), (2) 12-h light and 12-h darkness (LD 12:12; 50 lux max), and (3) continuous darkness (DD). The sexual maturity of female and male krill exposed to LL and LD 12:12 showed an accelerated succession of external maturity stages during the experimental period, while krill exposed to continuous darkness showed no changes in external maturity during the course of the study. Changes in the maturity development of krill between the different light regimes are reflected in changes in body composition. Krill exposed to LL and LD 12:12 showed an increase in lipid utilization, indicating that the development of external maturation may be fuelled preferentially by lipid reserves. In contrast, values of total lipid content of krill held under continuous darkness indicated an unchanged lipid catabolism during the course of the study. Thus, the maturity development of krill was affected either directly or indirectly by the different simulated light conditions. Based on these results, and observations on the effects of simulated light regimes on feeding and metabolic rates of krill available from a previous study, we suggest that the Antarctic light regime is an essential cue governing the seasonal cycle of krill physiology and maturity, and highlight the importance of this environmental factor in the life history of krill. [ABSTRACT FROM AUTHOR]

Abstract: Melatonin, the chief secretory product of the vertebrate pineal gland is suspected to be a ubiquitous molecule principally involved in the transduction of photoperiodic information. Besides vertebrates, melatonin has been detected throughout phylogeny in numerous non-vertebrate taxa. In the present study, the occurrence of melatonin in Antarctic krill Euphausia superba and its possible role in mediating seasonal metabolic changes was evaluated. Melatonin was quantified by enzyme linked immunosorbent assay (ELISA) in high performance liquid chromatography (HPLC) purified extracts of eyestalks and hemolymph of krill sampled in the Lazarev Sea during the Antarctic winter and summer. In addition, oxygen uptake rates and the activities of the metabolic enzyme malate dehydrogenase (MDH) were recorded to assess the metabolic status of krill. Validation of melatonin measurements was carried out on the basis of three different extraction methods with parallel determination of melatonin by ELISA in crude extracts and in HPLC purified extracts, and after derivatization of melatonin under alkaline conditions in the presence of hydrogen peroxide. A significantly higher respiration rate and MDH activity was found in summer krill than in winter krill indicating that krill was in a state of reduced metabolic activity during winter. However, neither during winter nor during summer there were detectable melatonin concentrations in the visual system or hemolymph of krill. Based on these results, we question a mediating role of melatonin in the control of seasonal metabolic changes in Antarctic krill. [Copyright &y& Elsevier]

The copepod Calanus finmarchicus (Crustacea, Copepoda) is a key zooplanktonic species with a crucial position in the North Atlantic food web and significant contributor to ocean carbon flux. Like many other high latitude animals, it has evolved a programmed arrested development called diapause to cope with long periods of limited food supply, while growth and reproduction are timed to take advantage of seasonal peaks in primary production. However, anthropogenic warming is inducing changes in the expected timing of phytoplankton blooms, suggesting phenological mismatches with negative consequences for the N. Atlantic ecosystem. While diapause mechanisms are mainly studied in terrestrial arthropods, specifically on laboratory model species, such as the fruit fly Drosophila, the molecular investigations of annual rhythms in wild marine species remain fragmentary. Here we performed a rigorous year‐long monthly sampling campaign of C. finmarchicus in a Scottish Loch (UK; 56.45°N, 5.18°W) to generate an annual transcriptome. The mRNA of 36 samples (monthly triplicate of 25 individuals) have been deeply sequenced with an average depth of 137 ± 4 million reads (mean ± SE) per sample, aligned to the reference transcriptome, and filtered. We detail the quality assessment of the datasets and provide a high‐quality resource for the investigation of wild annual transcriptomic rhythms (35,357 components) in a key diapausing zooplanktonic species. [ABSTRACT FROM AUTHOR]

Over the past decades, two key grazers in the Southern Ocean (SO), krill and salps, have experienced drastic changes in their distribution and abundance, leading to increasing overlap of their habitats. Both species occupy different ecological niches and long-term shifts in their distributions are expected to have cascading effects on the SO ecosystem. However, studies directly comparing krill and salps are lacking. Here, we provide a direct comparison of the diet and fecal pellet composition of krill and salps using 18S metabarcoding and fatty acid markers. Neither species' diet reflected the composition of the plankton community, suggesting that in contrast to the accepted paradigm, not only krill but also salps are selective feeders. Moreover, we found that krill and salps had broadly similar diets, potentially enhancing the competition between both species. This could be augmented by salps' ability to rapidly reproduce in favorable conditions, posing further risks to krill populations. In this manuscript, Pauli et al. directly compare the diets of two key Southern Ocean grazers, krill and salps. The 18S profiles of diet and pellets from both species do not match the planktonic profile, suggesting that both are selective grazers with a similar diet. [ABSTRACT FROM AUTHOR]

Over the last decades, it has been reported that the habitat of the Southern Ocean (SO) key species Antarctic krill (Euphausia superba) has contracted to high latitudes, putatively due to reduced winter sea ice coverage, while salps as Salpa thompsoni have extended their dispersal to the former krill habitats. To date, the potential implications of this population shift on the biogeochemical cycling of the limiting micronutrient iron (Fe) and its bioavailability to SO phytoplankton has never been tested. Based on uptake of fecal pellet (FP)-released Fe by SO phytoplankton, this study highlights how efficiently krill and salps recycle Fe. To test this, we collected FPs of natural populations of salps and krill, added them to the same SO phytoplankton community, and measured the community's Fe uptake rates. Our results reveal that both FP additions yielded similar dissolved iron concentrations in the seawater. Per FP carbon added to the seawater, 4.8 ± 1.5 times more Fe was taken up by the same phytoplankton community from salp FP than from krill FP, suggesting that salp FP increased the Fe bioavailability, possibly through the release of ligands. With respect to the ongoing shift from krill to salps, the potential for carbon fixation of the Fe-limited SO could be strengthened in the future, representing a negative feedback to climate change. • Salps recycle iron in a more bioavailable form than krill • Per mol fecal pellet carbon, salps release more iron than krill • Possibly, salps increase the carbon fixation potential of the Southern Ocean Böckmann et al. show that salp fecal pellets release more iron than krill fecal pellets. Additionally, the iron recycled from salp fecal pellets is more bioavailable to Southern Ocean phytoplankton than iron recycled from krill fecal pellets. Increasing salp populations might increase the carbon fixation potential of the Southern Ocean. [ABSTRACT FROM AUTHOR]

Light regime is an important zeitgeber for Antarctic krill (Euphausia superba Dana, 1850), which seems to entrain an endogenous timing system that synchronizes its life cycle to the extreme light conditions in the Southern Ocean. To understand the flexibility of Antarctic krill's seasonal cycle, we investigated its physiological and behavioural responses to different light regimes and if an endogenous timing system was involved in the regulation of these seasonal processes. We analysed growth, feeding, lipid content, and maturity in a 2-year laboratory experiment simulating the latitudinal light regimes at 52°S and 66°S and constant darkness under constant food level. Our results showed that light regime affected seasonal cycles of growth, feeding, lipid metabolism, and maturity in Antarctic krill. Seasonal patterns of growth, feeding, and maturity persisted under constant darkness, indicating the presence of an endogenous timing system. The maturity cycle showed differences in critical photoperiods according to the simulated latitudinal light regime. This suggests a flexible endogenous timing mechanism in Antarctic krill, which may determine its response to future environmental changes. [ABSTRACT FROM AUTHOR]

Summary Biological clocks are a ubiquitous ancient and adaptive mechanism enabling organisms to anticipate environmental cycles and to regulate behavioral and physiological processes accordingly [ 1 ]. Although terrestrial circadian clocks are well understood, knowledge of clocks in marine organisms is still very limited [ 2–5 ]. This is particularly true for abundant species displaying large-scale rhythms like diel vertical migration (DVM) that contribute significantly to shaping their respective ecosystems [ 6 ]. Here we describe exogenous cycles and endogenous rhythms associated with DVM of the ecologically important and highly abundant planktic copepod Calanus finmarchicus . In the laboratory, C. finmarchicus shows circadian rhythms of DVM, metabolism, and most core circadian clock genes ( clock , period1 , period2 , timeless , cryptochrome2 , and clockwork orange ). Most of these genes also cycle in animals assessed in the wild, though expression is less rhythmic at depth (50–140 m) relative to shallow-caught animals (0–50 m). Further, peak expressions of clock genes generally occurred at either sunset or sunrise, coinciding with peak migration times. Including one of the first field investigations of clock genes in a marine species [ 5, 7 ], this study couples clock gene measurements with laboratory and field data on DVM. While the mechanistic connection remains elusive, our results imply a high degree of causality between clock gene expression and one of the planet’s largest daily migrations of biomass. We thus suggest that circadian clocks increase zooplankton fitness by optimizing the temporal trade-off between feeding and predator avoidance, especially when environmental drivers are weak or absent [ 8 ]. [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba) is a key species in the Southern Ocean with an estimated biomass between 100 and 500 million tonnes. Changes in krill population viability would have catastrophic effect on the Antarctic ecosystem. One looming threat due to elevated levels of anthropogenic atmospheric carbon dioxide (CO2) is ocean acidification (lowering of sea water pH by CO2 dissolving into the oceans). The genetics of Antarctic krill has long been of scientific interest for both for the analysis of population structure and analysis of functional genetics. However, the genetic resources available for the species are relatively modest. We have developed the most advanced genetic database on Euphausia superba, KrillDB, which includes comprehensive data sets of former and present transcriptome projects. In particular, we have built a de novo transcriptome assembly using more than 360 million Illumina sequence reads generated from larval krill including individuals subjected to different CO2 levels. The database gives access to: 1) the full list of assembled genes and transcripts; 2) their level of similarity to transcripts and proteins from other species; 3) the predicted protein domains contained within each transcript; 4) their predicted GO terms; 5) the level of expression of each transcript in the different larval stages and CO2 treatments. All references to external entities (sequences, domains, GO terms) are equipped with a link to the appropriate source database. Moreover, the software implements a full-text search engine that makes it possible to submit free-form queries. KrillDB represents the first large-scale attempt at classifying and annotating the full krill transcriptome. For this reason, we believe it will constitute a cornerstone of future approaches devoted to physiological and molecular study of this key species in the Southern Ocean food web. [ABSTRACT FROM AUTHOR]

Different methods have been used for the determination of the amount of formaldehyde present in the air. The Hantzsch method uses derivatization with acetyl acetone, and the fluorescence signal of the resulting chromophore is highly selective for formaldehyde. The 2,4-dinitrophenylhydrazine procedure allows measuring of carbonyl compounds in combination with high-performance liquid chromatography. In contrast, the 3-methyl-2-benzothiazolonehydrazone method delivers the sum of aliphatic aldehydes, although it is reported to be selective for formaldehyde. To compare the performance of these methods, 13 samples of wooden furniture were measured. Further characterization was performed with a formaldehyde source and one oriented-strand board. As expected, the results of the 2,4-dinitrophenylhydrazine and Hantzsch methods provided excellent correlation. However, the formaldehyde concentrations obtained from the 3-methyl-2-benzothiazolonehydrazone method showed almost no correlation with the 2,4-dinitrophenylhydrazine and Hantzsch values. The results demonstrated that there was significant error in calculation with the use of the 3-methyl-2-benzothiazolonehydrazone method for determining the presence of a specific aliphatic aldehyde in a mixture of volatile organic compounds in the air. In addition, emission tests of an artificial formaldehyde source revealed excessively high concentrations of formaldehyde with the 3-methyl-2-benzothiazolonehydrazone method. Therefore, it was found that the 3-methyl-2-benzothiazolonehydrazone method was not suitable for the determination of the presence of formaldehyde in the air due to interferences of other organic compounds and systematic analytical artifacts. [ABSTRACT FROM AUTHOR]

Antarctic krill ( Euphausia superba; hereafter krill) are an incredibly abundant pelagic crustacean which has a wide, but patchy, distribution in the Southern Ocean. Several studies have examined the potential for population genetic structuring in krill, but DNA-based analyses have focused on a limited number of markers and have covered only part of their circum-Antarctic range. We used mitochondrial DNA and restriction site-associated DNA sequencing ( RAD-seq) to investigate genetic differences between krill from five sites, including two from East Antarctica. Our mt DNA results show no discernible genetic structuring between sites separated by thousands of kilometres, which is consistent with previous studies. Using standard RAD-seq methodology, we obtained over a billion sequences from >140 krill, and thousands of variable nucleotides were identified at hundreds of loci. However, downstream analysis found that markers with sufficient coverage were primarily from multicopy genomic regions. Careful examination of these data highlights the complexity of the RAD-seq approach in organisms with very large genomes. To characterize the multicopy markers, we recorded sequence counts from variable nucleotide sites rather than the derived genotypes; we also examined a small number of manually curated genotypes. Although these analyses effectively fingerprinted individuals, and uncovered a minor laboratory batch effect, no population structuring was observed. Overall, our results are consistent with panmixia of krill throughout their distribution. This result may indicate ongoing gene flow. However, krill's enormous population size creates substantial panmictic inertia, so genetic differentiation may not occur on an ecologically relevant timescale even if demographically separate populations exist. [ABSTRACT FROM AUTHOR]

The Southern Ocean ecosystem is characterized by extreme seasonal changes in environmental factors such as day length, sea ice extent and food availability. The key species Antarctic krill ( Euphausia superba ) has evolved metabolic and behavioural seasonal rhythms to cope with these seasonal changes. We investigate the switch between a physiological less active and active period for adult krill, a rhythm which seems to be controlled by internal biological clocks. These biological clocks can be synchronized by environmental triggers such as day length and food availability. They have evolved for particular environmental regimes to synchronize predictable seasonal environmental changes with important life cycle functions of the species. In a changing environment the time when krill is metabolically active and the time of peak food availability may not overlap if krill's seasonal activity is solely determined by photoperiod (day length). This is especially true for the Atlantic sector of the Southern Ocean where the spatio-temporal ice cover dynamics are changing substantially with rising average temperatures. We developed an individual-based model for krill to explore the impact of photoperiod and food availability on the growth and demographics of krill. We simulated dynamics of local krill populations (with no movement of krill assumed) along a south-north gradient for different triggers of metabolic activity and different levels of food availability below the ice. We also observed the fate of larval krill which cannot switch to low metabolism and therefore are likely to overwinter under ice. Krill could only occupy the southern end of the gradient, where algae bloom only lasts for a short time, when alternative food supply under the ice was high and metabolic activity was triggered by photoperiod. The northern distribution was limited by lack of overwintering habitat for krill larvae due to short duration of sea ice cover even for high food content under the ice. The variability of the krill's length-frequency distributions varied for different triggers of metabolic activity, but did not depend on the sea ice extent. Our findings suggest a southward shift of krill populations due to reduction in the spatial sea ice extent, which is consistent with field observations. Overall, our results highlight the importance of the explicit consideration of spatio-temporal sea ice dynamics especially for larval krill together with temporal synchronization through internal clocks, triggered by environmental factors (photoperiod and food) in adult krill for the population modelling of krill. [ABSTRACT FROM AUTHOR]

Euphausiids constitute a major biomass component in shelf ecosystems and play a fundamental role in the rapid vertical transport of carbon from the ocean surface to the deeper layers during their daily vertical migration (DVM). DVM depth and migration patterns depend on oceanographic conditions with respect to temperature, light and oxygen availability at depth, factors that are highly dependent on season in most marine regions. Here we introduce a global krill respiration ANN (artificial neural network) model including the effect of latitude ( LAT ), the day of the year ( DoY ), and the number of daylight hours ( DLh ), in addition to the basal variables that determine ectothermal oxygen consumption (temperature, body mass and depth). The newly implemented parameters link space and time in terms of season and photoperiod to krill respiration. The ANN model showed a better fit ( r 2 = 0.780) when DLh and LAT were included, indicating a decrease in respiration with increasing LAT and decreasing DLh . We therefore propose DLh as a potential variable to consider when building physiological models for both hemispheres. For single Euphausiid species investigated in a large range of DLh and DoY , we also tested the standard respiration rate for seasonality with Multiple Linear Regression (MLR) and General Additive model (GAM). GAM successfully integrated DLh ( r 2 = 0.563) and DoY ( r 2 = 0.572) effects on respiration rates of the Antarctic krill, Euphausia superba , yielding the minimum metabolic activity in mid-June and the maximum at the end of December. We could not detect DLh or DoY effects in the North Pacific krill Euphausia pacifica , and our findings for the North Atlantic krill Meganyctiphanes norvegica remained inconclusive because of insufficient seasonal data coverage. We strongly encourage comparative respiration measurements of worldwide Euphausiid key species at different seasons to improve accuracy in ecosystem modeling. [ABSTRACT FROM AUTHOR]

Abstract: A change in photoperiod has been implicated in triggering a transition from an active to a quiescent state in Antarctic krill. We examined this process at the molecular level, to identify processes that are affected when passing a photoperiodic threshold. Antarctic krill captured in the austral autumn were divided into two groups and immediately incubated either under a photoperiod of 12h light:12h darkness (LD), simulating the natural light cycle, or in continuous darkness (DD), simulating winter. All other conditions were kept identical between incubations. After 7days of adaptation, krill were sampled every 4h over a 24h period and frozen. Total RNA was extracted from the heads and pooled to construct a suppression subtractive hybridisation library. Differentially expressed sequences were identified and annotated into functional categories through database sequence matching. We found a difference in gene expression between LD and DD krill, with LD krill expressing more genes involved in functions such as metabolism, motor activity, protein binding and various other cellular activities. Eleven of these genes were examined further with quantitative polymerase chain reaction analyses, which revealed that expression levels were significantly higher in LD krill. The genes affected by simulated photoperiodic change are consistent with known features of quiescence, such as a slowing of moult rate, a lowering of activity levels and a reduction in metabolic rate. The expression of proteases involved in apolysis, where the old cuticle separates from the epidermis, showed particular sensitivity to photoperiod and point to the mechanism by which moult rate is adjusted seasonally. Our results show that key processes are already responding at the molecular level after just 7days of exposure to a changed photoperiodic cycle. We propose that krill switch rapidly between active and quiescent states and that the photoperiodic cycle plays a key role in this process. [Copyright &y& Elsevier]

Abstract: We investigated the effect of short-term starvation (18days) on the physiology of adult Euphausia superba from the Lazarev Sea at the onset of summer. Metabolic data, elemental and biochemical composition as well as morphological parameters revealed that, at the beginning of the experiment, krill was in transition from winter to summer physiology, with some features typical for late winter/spring (low lipid reserves, low C:N ratio, elevated O:N ratio) and others for summer (high respiration rates, high MDH activity, large green digestive gland, short intermoult period (IMP) and fast growth). Starvation reduced body reserves drastically by more than 1% C per day. In relative terms, lipids (40%) and glycogen (30%) were reduced most and proteins by 10% of the initial value. Absolute consumption, however, was approximately 4% DM for lipids and proteins each, whereas contribution of glycogen was negligible. Within lipids, triacylglycerols (TAG) and phospholipids (PL) fell most dramatically from already low levels by 84% and 39%, respectively. Phosphatylcholine (PC) constituted 57% of PL and declined by 46%. As a result, proportions of the lipid classes changed with sterols increasing relatively. Metabolite changes were similar in cephalothorax and abdomen, although TAG in the cephalothorax fell more drastically. High metabolic activity at the beginning of starvation was quickly reduced to reach 53% after 18days, accompanied by a reduction in the abdominal activity of malate dehydrogenase (MDH) by 25%. Our results may provide some explanation why recruitment of some year-classes of krill fails. Despite execution of the experiment in spring (i.e. transitional physiology state) and its short duration, some changes in the activity of metabolic enzymes in the abdomen, representing lipolytic, glycolytic and proteolytic pathways, respectively, were measured. Rising activities of 3-hydroxyacyl-CoA dehydrogenase (HOAD) and glyceralaldehyde-3-phosphate dehydrogenase (GAPDH) indicated increased lipolytic and glycolytic fluxes, respectively, whereas declining glutamate dehydrogenase (GluDH) activity suggests reduced proteolytic flux. Activities of other enzymes from protein catabolism, alanine aminotransferase (AlaAT) and aspartate aminotransferase (AspAT), however, remained unchanged. Ratios calculated from these trends indicated a declining importance of protein use during the course of starvation compared with consumption of lipids and glycogen. These results suggest that constant-proportion enzymes from different catabolic pathways, and calculated ratios thereof, may be useful in detecting shifts between the consumption of different body reserves. [Copyright &y& Elsevier]

• After long-term acclimation, krill O 2 consumption was higher at temperatures > 3.5 °C. • Krill body length was significantly lower at temperatures ≥ 3.5 °C. • Seasonal differences between male and female growth patterns could be detected. • Findings directly relate to future population dynamics and fisheries management. The ongoing environmental changes in the Southern Ocean may cause a dramatic decrease in habitat quality. Due to its central position in the food web, Antarctic krill (Euphausia superba) is a key species of the marine Antarctic ecosystem. It is therefore crucial to understand how increasing water temperatures affect important krill life-cycle processes. Here, a long-term (August – March) laboratory acclimation experiment at different temperature scenarios (0.5 °C, 1.5 °C, 2.5 °C, 3.5 °C, 5 °C, 7 °C) was performed and the effects of elevated temperatures on whole animal parameters (O 2 consumption, body length, length of the digestive gland) were analyzed. The response of krill oxygen consumption to different experimental temperatures differed between acute/short-term and long-term acclimation. After 8 months, krill oxygen consumption remained unchanged up to temperatures of 3.5 °C and was significantly higher at temperatures > 3.5 °C. Krill acclimated to temperatures ≥ 3.5 °C were significantly smaller at the end of the experiment. Limited food intake and/or conversion may have contributed to this effect, especially pronounced after the onset of the reproductive period. In addition, the seasonal growth pattern in males differed from that of females. Together, our findings indicate that warming Southern Ocean waters are likely to increase metabolic rate in krill, possibly altering the amount of energy available for other important life-cycle processes, a finding directly related to future population dynamics and fisheries management. [ABSTRACT FROM AUTHOR]

• An individual-based model of Salpa thompsoni has been developed. • The model reproduces observed patterns at the individual and population level. • In the model large salp blooms require a founding population in early spring. The Southern Ocean near the Western Antarctic Peninsula (WAP) is strongly affected by climate change resulting in warmer air temperature, accompanied with reduced sea ice coverage, increased sea water temperature and potential changes in the abundances of two key grazer species Salpa thompsoni (salp) and Euphausia superba (Antarctic krill). While salp abundance is hypothesized to increase, krill abundance is hypothesized to decline with dramatic consequences for the entire food web of the Southern Ocean. A better understanding of the biotic interaction between krill and salps and their population dynamics is thus crucial. However, the life cycle of salps is complicated and barely understood. Therefore, we have developed an individual-based model describing the whole life cycle to better understand the population dynamics of salps and the conditions for blooms. The model has been used to explore if and under what conditions the empirical pattern of large variability in observed salp abundances at the WAP, generated by the long-term data of the US Antarctic Marine Living Resources Program (AMLR) can emerge from a small seeding population. The model reproduced this empirical pattern if daily growth rates of oozoids were higher than previously reported for the WAP (mean growth rate for oozoids ~ 1 mm d−1) and if growth rates of blastozooids were lower (mean growth rate ~ 0.2 mm d−1). The model suggests that a prerequisite for local salp blooms requires a small founding population in early spring. With climate change it has been suggested that more frequent and earlier transport of salps into the WAP or winter survival will occur. Hence, the risk of salp blooms in the WAP is likely to substantially increase. These findings highlight the importance for an improved quantitative understanding of how primary production and the southward advection of salps will be impacted by climate change. [ABSTRACT FROM AUTHOR]

The Southern Ocean ecosystem has undergone extensive changes in the past two centuries driven by industrial sealing and whaling, climate change and commercial fishing. However, following the end of commercial whaling, some populations of whales in this region are recovering. Baleen whales are reliant on Antarctic krill, which is also the largest Southern Ocean fishery. Since 1993, krill catch has increased fourfold, buoyed by nutritional supplement and aquaculture industries. In this Perspective, we approximate baleen whale consumption of Antarctic krill before and after whaling to examine if the ecosystem can support both humans and whales as krill predators. Our back-of-the-envelope calculations suggest that current krill biomass cannot support both an expanding krill fishery and the recovery of whale populations to pre-whaling sizes, highlighting an emerging human-wildlife conflict. We then provide recommendations for enhancing sustainability in this region by reducing encounters with whales and bolstering the krill population., (© 2024. The Author(s).)

Krill are vital as food for many marine animals but also impacted by global warming. To learn how they and other zooplankton may adapt to a warmer world we studied local adaptation in the widespread Northern krill (Meganyctiphanes norvegica). We assemble and characterize its large genome and compare genome-scale variation among 74 specimens from the colder Atlantic Ocean and warmer Mediterranean Sea. The 19 Gb genome likely evolved through proliferation of retrotransposons, now targeted for inactivation by extensive DNA methylation, and contains many duplicated genes associated with molting and vision. Analysis of 760 million SNPs indicates extensive homogenizing gene-flow among populations. Nevertheless, we detect signatures of adaptive divergence across hundreds of genes, implicated in photoreception, circadian regulation, reproduction and thermal tolerance, indicating polygenic adaptation to light and temperature. The top gene candidate for ecological adaptation was nrf-6, a lipid transporter with a Mediterranean variant that may contribute to early spring reproduction. Such variation could become increasingly important for fitness in Atlantic stocks. Our study underscores the widespread but uneven distribution of adaptive variation, necessitating characterization of genetic variation among natural zooplankton populations to understand their adaptive potential, predict risks and support ocean conservation in the face of climate change., (© 2024. The Author(s).)