Temperature related differences

Krill is considered to be stenothermal and is sensitive to slight changes in temperature (Wiedenmann et al. 2008). Temperature has been found to influence PL structure and fatty acid composition (Farkas 1979), as well as to cause gel/liquid phase transformations that result in changes to membrane fluidity (McElhaney 1984). However, clear temperature effects have only been observed with differences of at least 15°C for marine animals (Mayzaud et al. 2000). Below this range of temperatures, the expected changes in lipid structure and composition would be minimal (Mayzaud et al. 2000). This is particularly true for krill, which are probably thermally stressed above about 5°C, and thus through evolution, they have physiologically adapted to temperatures below 5°C (Clarke and Morris 1983).

Total lipid, TAG, PL and generally fatty acid content and composition of whole immature krill showed no clear temperature effect between -1°C and 3°C at various time points over a year (Chapter 5). During the summer period in this study, the effects of temperature between sex and among tissues fractions were again complex, even though krill were incubated under a constant supply of food. At -1°C in November, immature, male and female krill generally contained higher mean total lipid, TAG and PL content in the thoracic components compared to the warmer temperatures. The major fatty acid content at -1°C in the thoracic components and digestive gland were also higher than at 1°C and 3°C. Overall, the warmer 1°C and

digestive gland for total lipid, TAG and PL content for each sex. However, lipid stores in the digestive gland at 3°C were higher than at -1°C and 1°C. In terms of the relative levels of TAG and PL, the storage lipid TAG generally decreased with increasing temperature, while PL was dominant and increased with increasing temperature.

In February, immature krill contained higher mean lipid and fatty acid content in the thoracic components at 3°C. Lipid and fatty acid content in the female thoracic components and digestive gland was generally higher in the cooler temperatures (- 1°C and 1°C), while highest values were recorded at 1°C for males.

Overall, there is some evidence to suggest that krill in the cooler temperatures, particularly at -1°C, have elevated lipid stores in the summer months (November and February), and thus are in better condition compared to krill at 3°C. This is also supported by the fact that female krill only mated in the -1°C treatment in November, suggesting that krill may have been more stressed and their energy resources were reduced below a threshold that is required to undertake the reproductive processes in warmer temperatures. However, the variation of mean total lipid and fatty acid content and composition in krill was substantial under controlled light and feeding conditions, resulting in no significant relationships between the sexes and temperatures during summer.

There is growing interest in the role of inter-individual variability in structuring populations and providing resilience to environmental change. This is highlighted by Goodall-Copestake et al. (2010) who examined the relative levels of variability in the population genetic marker cox-1 of Antarctic krill. They concluded that krill has one of the most diverse genomes of any metazoan and this diversity is seen as much within swarms as between swarms. A possibility of such diversity is probably a result of the relatively unrestricted mixing of krill within its large Southern Ocean habitat and its large population size, which prevents the overall loss of alleles through drift (Goodall- Copestake et al. 2010). What the present work demonstrates is a phenotypic expression of this described diversity, in the range of capacities in the storage and utilisation of lipids.

This high individual variation highlights krill’s ability to adapt and survive under various environmental conditions, within a natural temperature range in the Southern Ocean. Understanding the effects of temperature on the physiology of krill

acidification and possible alterations to the Southern Ocean trophic structure are needed to assess climate change impacts on krill.

6.6. Conclusion

Overall, there were relatively few significant differences in lipid composition between temperatures and between maturity stages of krill. There was a general trend of decreasing TAG with increasing temperature, and higher lipid content in females compared to males, but generally this was not statically significant. Interestingly, mated females in the -1ºC treatment contained lower lipid and fatty acid content, compared to un-mated females, suggesting utilisation of lipids during mating. Even though mating was observed, ovaries of females were only in pre- vitellogenesis, which likely explains why there were no major differences in lipid and fatty acid content between sexes. A depletion of lipids was observed in krill sampled in February for all sexes and temperatures, particularly TAG in digestive gland, which implies krill were possibly utilising their energy stores for reproductive purposes. However, the incubation influences such as diet and confinement cannot be quantified. We show possible evidence of elevated lipid stores in krill at cooler temperatures. Variability in krill lipid levels, particularly with temperature, demonstrates a physiological plasticity necessary in an environment where, although temperature is reasonably stable, trophic resources are highly seasonal.

-- Chapter seven --

General Discussion

Antarctic krill (Euphausia superba) is an ecologically significant species in the Southern Ocean. Not only do krill play a major role in the food web (Mangel and Nicol 2000; Alonzo et al. 2003a), but they are also attracting increasing interest as a target for commercial fishing (Kawaguchi and Nicol 2007). Over-exploitation of krill could have major effects on the whole Antarctic ecosystem and it is therefore imperative to have sound management procedures in place before the krill fishery expands. A key to adequately managing this important and expanding fishery, and to protect the overall ecosystem, is to understand the life history traits of krill, particularly growth, maturation and physiology, which are the driving factors influencing the variability in krill biomass. Despite the recognised importance and extensive research on krill, current knowledge of the ecology and biology is far from complete and much still remains to be studied (Nicol 2003).

The central aim of this thesis was to test hypotheses brought about from key questions raised in field studies and to evaluate the effects of environmental parameters (light, food availability and temperature) on the growth, maturation and physiology in krill. In addressing the main aims of this study, krill were incubated under controlled conditions for a complete annual cycle. Information on growth, maturation and physiology under various light, diet and temperature regimes throughout a full year is limited, hampering understanding of the effects a changing environment may have on krill. It is crucial to understand how krill adapts and survives under extreme and changing environmental conditions.

7.1. Temperature effects on growth and maturity

A clear seasonal cycle of growth and maturity was evident for both male and female krill in all temperature treatments (-1°C, 1°C and 3°C), under a natural Antarctic light cycle and plentiful food concentrations (Chapter 2). An observed progression of maturity and rapid growth occurred during spring, and shrinkage

(IMP). In the wild, krill generally have a longer IMP in winter compared to summer, which is regarded as the effect of the seasonal temperature cycle (Atkinson et al. 2006; Kawaguchi et al. 2006; Tarling et al. 2006). Our results indicated that if the temperature and food conditions are constant, krill have a higher moulting frequency during winter compared to the field. This is probably because krill have minimal energy requirements (no reproduction and less growth) in winter. If supplied with the same amount of food all year, krill might divert excess energy to driving the moulting cycle. This underlines the importance of further studying the energy budget and physiology in order to understand the processes of krill growth. It also showcases the importance of regular moulting to the condition of krill.

Temperature is the major influence on the rate of moulting and possibly growth in krill, with a rise in temperature resulting in shorter IMP (Hartnoll 2001). From this study, krill incubated under the highest temperature (3°C) did not result in the greatest growth. Under the current experimental condition, 1°C was considered optimum for krill growth. IMP was significantly shorter at 1°C than -1°C, but the difference in growth increment between the two temperatures was not significantly different. Although not statistically demonstrated, this should result in higher daily growth rate (DGR) at 1°C. IMP and all growth variables were significantly lower for krill in the 3°C treatment. Our findings support those from Atkinson et al. (2006), who suggested the existence of an optimal temperature (~0.5°C) for krill growth in the south-west Atlantic sector of the Southern Ocean. Furthermore, for the first time, this study has confirmed that compensation mechanisms do exist between IMP and instantaneous growth rate (IGR) in individual krill (faster moult times corresponding to slower growth increment). Compensation mechanisms had previously been reported for crustaceans in general, however, they had not been examined in krill (Kawaguchi et al. 2006). Understanding compensation effects is important for modelling growth trajectories of wild krill using IGR to quantify the distribution of size (age) frequencies with respect to season. My results will facilitate more accurate and robust model predictions for temperature-dependent growth for krill.