The production of ‘food boluses’ by Antarctic krill and implications for organic matter transport

Antarctic krill (Euphausia superba) are a key species in the marine Antarctic ecosystem. Food boluses, a byproduct of feeding where krill form a compact food mass within their feeding basket, were formed and rejected under laboratory conditions. We explored the mechanism used to form these boluses by examining krill feeding behaviour and observed two distinct scenarios that can lead to the formation of boluses: 1) when concentrations of food items are too high for krill to filter feed efficiently; 2) when either biological or foreign particles are caught in the feeding basket and unable to be processed by the krill. We measured the frequency of rejection of the boluses, along with their composition, sinking rates, and carbon and nitrogen contents. For cell concentrations over 108 cells L-1, the frequency of rejection ranged from 2.6 to 17 boluses per hour. Bolus sinking velocities averaged 367 m d-1, with carbon and nitrogen contents averaging 24.1 µg C mm-3 and 2.3 µg N mm-3, respectively. Our findings suggest krill may display this behaviour in situ, and, if so, this behaviour may also occur in other krill species with similar feeding behaviour.