Diel vertical migration and the haemocyanin of krill Meganyctiphanes norvegica

Abstract

Changes in environmental factors (PO2, temperature, salinity and food availability) influence the concentration of the respiratory pigment haemocyanin ((Hc)) present in many crus- taceans. We investigated the possibility that (Hc) and HcO2 affinity altered during the diel vertical migration (DVM) of Nordic krill Meganyctiphanes norvegica in the Gullmarsfjord, Sweden. The (Hc) decreased with increasing depth, for individuals both trawled and caged at different depths. Labora- tory experiments indicated that this pattern could not be explained by differences in PO2, tempera- ture or salinity. Alternatively, starvation had a significant effect on (Hc) over the course of a few h (≤10), i.e. a much shorter time scale than found for other crustacean species. Starved individuals showed a dramatic decrease in (Hc) compared with fed individuals. This decrease was exacerbated by an increase in temperature. We suggest that when M. norvegica migrates into deep water during the day, for whatever reason (predator avoidance or reduced prey abundance, or reduced ability to locate and capture prey), they cannot secure enough energy to meet routine metabolic demands; they therefore break down Hc and use it as an energy source. We conclude that there is likely to be a trade-off between the respiratory function of Hc and its importance in nutrition when krill migrate into deeper, nutritionally poorer waters during their DVM.