Antarctic krill inhabit areas of the Southern Ocean that can exceed 4.0°C, yet they preferentially inhabit regions with temperatures of −1.5 to ≤1.5°C. Successful embryonic development and hatching are key to their life cycle, but despite the rapid climatic warming seen across their main spawning areas, the effects of elevated temperatures on embryogenesis, hatching success, and nauplii malformations are unknown. We incubated 24,483 krill embryos in two independent experiments to investigate the hypothesis that temperatures exceeding 1.5°C have a negative impact on hatching success and increase the numbers of malformed nauplii. Field experiments were on krill collected from near the northern, warm limit of their range and embryos incubated soon after capture, while laboratory experiments were on embryos from krill acclimated to laboratory conditions. The hatching success of embryo batches varied enormously, from 0 to 98% (mean 27%). Both field and laboratory experiments showed that hatching success decreased markedly above 3.0°C. Our field experiments also showed an approximate doubling of the percentage of malformed nauplii at elevated temperatures, reaching 50% at 5.0°C. At 3.0°C or below, however, temperature was not the main factor driving the large variation in embryo hatching success. Our observations of highly variable and often low success of hatching to healthy nauplii suggest that indices of reproductive potential of female krill relate poorly to the subsequent production of viable krill larvae and may help to explain spatial discrepancies between the distribution of the spawning stock and larval distribution.

The accurate observation of physiological changes on in vivo samples of important animal species such as Euphausia superba (Antarctic krill) is an important goal in helping to understand how environmental changes can affect animal development. Using a custom made krill trap', live un-anaesthetized krill were confined for seven hours, during which three hours of optical imaging were obtained and no subsequent ill effects observed. The trap enabled two imaging methods to be employed: optical coherence tomography (OCT) and microscopy. OCT enabled internal structure and tissues to be imaged to a depth of approximately 2mm and resolution of approximately 12m. Microscopy was used to observe heart rate. During our experiments, we imaged a range of internal structures in live animals including the heart and gastric areas. The trap design enables a new generation of mixed modality imaging of these animals in vivo. These techniques will enable detailed studies of the internal physiology of live krill to be undertaken under a wide range of environmental conditions and have the potential to highlight important variations in behaviour and animal development.

Six Australian Antarctic Division personnel have recently returned from a 60-day voyage to the Weddell Sea, where they joined the international Winter Ice Study on Key Species.

Metachronal swimming, in which adjacent appendages stroke in sequence, is widespread among crustaceans inhabiting the transitional flow realm in which both viscosity and inertia effects are important. However, the design and operation of this propulsion system in response to various hydrodynamic, energetic, and behavioral needs have not been well investigated. We examine free-swimming Antarctic krill (Euphausia superba) as a model species and identify three distinct behavioral swimming gaits. The pleopod kinematics of these gaits, hovering, fast-forward swimming, and upside-down swimming, are quantified via image analysis of high-speed video. Pleopod stroke amplitude and frequency were found to vary significantly among these swimming modes. In order to increase swimming speed, krill were found first to increase stroke amplitude and secondarily to increase beat frequency. The kinematics of these distinct swimming modes provide insight as we consider multi-appendage metachronal swimming from a design standpoint. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key parameter in metachrony, the value of which is constrained to a narrow range for a wide variety of species.

Nucleic acid contents of tissue were determined from field-caught Antarctic krill to determine whether they could be used as an alternative estimator of individual growth rates which can currently only be obtained by labour intensive on-board incubations. Krill from contrasting growth regimes from early and late summer exhibited differences in RNA-based indices. There was a significant correlation between the independently measured individual growth rates and the RNA : DNA ratio and also the RNA concentration of krill tissue, although the strength of the relationship was only modest. DNA concentration, on average, was relatively constant, irrespective of the growth rates. The moult stage did not appear to have a significant effect on the nucleic acid contents of tissue. Overall, the amount of both nucleic acids varied considerably between individuals. Nucleic acid-based indicators may provide information concerning the recent growth and nutritional status of krill and further experimentation under controlled conditions is warranted. They are, however, reasonably costly and time-consuming measurements.

Growth trends of Antarctic krill with sex, length, season, and region as independent variables were modelled with a linear mixed model (LMM) using 10 yr of accumulated instantaneous growth rate (IGR) measurements. A model of inter-moult period (IMP) as a function of temperature, required to convert IGR to daily growth rate, was fitted to data from published constant-tempera lure rearing studies; this model was used to predict seasonal IMP using a model of the average sea surface temperature seasonal trend for each region. Smaller krill exhibited higher growth rates and, in general, we observed a progressive decrease in IGR and daily growth with increasing size (from over 0.2 mm d(-1) down to 0.05 mm d(-1) for 20 and 50 rum krill, respectively, in December in the Indian Ocean sector of the Southern Ocean). IGR decreased from summer to autumn, with small to negative values predominating across all length classes by autumn. December was a period of rapid growth in the Indian Ocean sector of the Southern Ocean, whereas a similar peak in growth rates appeared to occur a few months earlier in the southwest Atlantic sector. Significantly lower growth rates were exhibited by females in January and February (e.g. 0.063 and 0.050 mm d(-1) for 40 mm females) compared to males (0.119 and 0.090 mm d(-1) for 40 mm males) in the Indian Ocean sector. Since these estimates were based on growth measurements of individual animals, they may reflect the occurrence of sex-differentiated growth patterns in the natural environment. Growth rate differences between sexes Suggested that growth rate estimates based on composite length frequency distributions without sex differentiation may be problematic.