Effects of simulated light regimes on maturity and body composition of Antarctic krill, Euphausia superba

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.