Environmental effects on the growth, maturation and physiology in antarctic krill (euphausia superba) over an annual cycle : an experimental approach

Antarctic krill, Euphausia superba, is a keystone species in the Antarctic ecosystem, being a major food source for most predators and the target of a substantial fishery. Despite being a critical component in the Southern Ocean, limited information exists on krill growth, maturation and physiology under various light, diet and temperature regimes throughout a full year. Without a comprehensive understanding of these factors, forecasting adaptations in a changing environment is hampered. This study examines the effects of the key environmental parameters (light, food availability and temperature) on growth, maturation and physiology in krill. Krill were incubated for an annual cycle under natural light (emulating the field environment), and constant food supply and temperature. Krill showed a clear seasonal cycle of growth and maturity in all three temperature treatments (-1¬∞C, 1¬∞C, 3¬∞C). Sex significantly affected the relationship with growth over a year. Overall, females showed higher growth rates than males, and growth rapidly decreased after the peak growth period towards the end of spring. Males peaked in growth and matured earlier than females and decreased growth at a considerably slower rate. Negative growth occurred towards the end of January for both sexes, coinciding with the regression of external sexual characteristics. There was a significant decline in intermoult period (IMP) with increasing temperature and some evidence to suggest that 1¬∞C was optimum for krill growth. The IMP was significantly lower at 1¬∞C than at -1¬∞C, but the difference in growth increment (GI) between the two temperatures was not significantly different, with all growth variables significantly lower at 3¬∞C. For the first time, this study has confirmed that compensation mechanisms do exist between IMP and instantaneous growth rate (IGR) for krill, resulting in short IMP/small IGR to long IMP/large IGR. Based on external sexual characteristics (female ‚- thelycum; male - petasma), krill exposed to a natural Antarctic light cycle or a fixed light/dark regime, progress under a natural maturation cycle of regression and re-maturation. However, when krill were maintained in complete darkness during sexual regression, the rate of regression accelerated and re-maturation occurred three months earlier in the following season. This flexible maturation cycle in response to conditions of total darkness at the time of regression means that krill can flexibly adjust their seasonal physiological cycle. Overall, light (in this case darkness) appears to be one of the most important factors influencing the krill maturation cycle. There was a strong significant increasing trend of respiration rates in krill with month in all experimental conditions; natural light cycle versus complete darkness, fed versus starved and different temperature regimes (-1¬∞C, 1¬∞C and 3¬∞C). The interaction of treatment with month, as well as generally the main effect of each treatment, was non-significant. Overall, from this study, it appears that light, food availability and temperature may not be the dominant environmental variables influencing the observed seasonal changes in metabolic rates. There was no significant difference throughout the year (except February) in total lipid and fatty acid content and composition of immature krill, and also between mature males and females in summer. The lipid and fatty acid concentrations were near depletion in February for all krill, indicating these reserves were possibly used for reproductive purposes rather than as an overwintering source. Mated females were only observed at -1¬∞C in November. Lipid and fatty acid levels were lower in mated compared to un-mated females, indicating utilisation of lipids during the mating process. There was no clear temperature effect on lipid and fatty acid content and composition at the various time points sampled; however, krill at the lower temperature, -1¬∞C, generally contained higher lipid and fatty acid content. This study has provided a solid basis for understanding the life history of krill over an annual cycle, which will enable more robust modelling for accurate assessments and management for the krill fishery. This research has further helped elucidate the effects of key environmental parameters on the growth, maturation and physiology in krill. It is crucial to expand on this knowledge so as to comprehend seasonal adaptation and survival of krill in a changing habitat, in light of predicted climatic change.