Physiology and metabolism of Northern krill (Meganyctiphanes norvegica Sars).
Advances in our understanding of the physiology and metabolism of Northern krill, Meganyctiphanes norvegica have been sporadic but significant. Despite problems with keeping M. norvegica in good condition in the laboratory, those who have tried, and succeeded, have contributed to a better knowledge of krill biology and challenged our understanding of some basic biological processes. Most recent work has been concentrated in the fields of digestive physiology, lipid biochemistry, respiration and anaerobiosis, metabolic properties, and pollutants. M. norvegica is capable of digesting an opportunistic, omnivorous diet, showing some digestive enzyme polymorphism and high levels of enzyme activity, the latter varying with season. It also seems capable of digesting cellulose and hemicelluloses, for example, laminarin. The biochemical composition of krill is relatively well known with some recent extensive work focusing on the previously little studied lipid and fatty acid composition, particularly with reference to reproduction, overwintering energy storage and as a nutrition marker. A high aerobic metabolism (but poor anaerobic capacity) is characteristic of M. norvegica, and how this is affected by temperature, low O(2), and season has attracted some attention, particularly in the context of diel vertical migration (DVM) across pronounced pycnoclines. Despite determining high metabolic turnover rates and a high physiological plasticity for this species, we know little of the regulative potential of metabolites, particularly their modulative effect on enzyme activity. Certainly a modest ability to maintain aerobic metabolism when encountering hypoxia, and little or no ability to osmoregulate in hyposaline conditions, does not prevent DVM in adults of this species. The ability to maintain aerobic metabolism develops early in ontogeny at about furcilia III (i.e. concurrent with first DVM behaviour). The respiratory pigment of M. norvegica, haemocyanin, has a low O(2) affinity and high temperature sensitivity (although temperature has the opposite effect on O(2) binding than found for nearly every other haemocyanin). Also surprising is the apparent use of haemocyanin as an energy source/store. While recent work has focused on physiological effects, the ecophysiological effects of transuric elements and trace metals, the effects of pollution generally are widely understudied.