Food and feeding in northern krill (Meganyctiphanes norvegica sars)
Early feeding studies on Meganyctiphanes norvegica described the morphology of the feeding appendages and the actual process of food uptake and digestion. Insights into diurnal, seasonal and ontogenetic pattern in feeding activity and diet were derived from field studies on the Clyde Sea population. Since then, technical advances have confirmed some of the early assumptions and rejected others. Submersible, remotely operated vehicles and echosounders, for instance, proved that M. norvegica stay often close to the seabed and feed on particles in the epibenthic layer and sediment-water interface. Scanning electron microscopy showed that mandibles of the so-called carnivorous M. norvegica have an elaborated grinding region, which allows efficient feeding on diatoms. Three-dimensional silhouette video imaging revealed mechanoreception, not vision, as the main sensory modality involved in proximity prey detection by M. norvegica. Fatty acid analysis and stomach content microscopy have now been conducted on M. norvegica across a range of environments including the Gulf of Maine, Greenland Sea, Barents Sea, Scandinavian fjords, the Kattegat and Mediterranean Sea. Regional and seasonal differences in the trophic environment are reflected in their daily ration and in the relative importance of copepods versus phytoplankton in their diet. Overall, phytoplankton is an important food source for M. norvegica during the spring bloom and part of the summer, but copepods are dominant in autumn and winter. Depending on their vertical co-occurrence, M. norvegica can feed on a range of copepods from early stages of Oithona spp. up to adult Calanus spp. There are clear ontogenetic differences in diet, with adults feeding more on copepods and benthic food items than early post-larvae. Future studies should link diet to simultaneously measured growth and reproduction and emphasise comparison across the spectrum of environments inhabited by this versatile species.