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 by-product of feeding where Antarctic krill form a compact food mass within their feeding basket, were formed and rejected under laboratory conditions. We explored the conditions leading to bolus formation by examining feeding behaviour of Antarctic krill in response to different phytoplankton types and concentrations. Two scenarios were observed that increased the likelihood of bolus formation: (i) when food concentrations exceed the krill's filtering capacity and (ii) when particles are caught in the feeding basket. We measured the frequency of rejection of boluses, along with their composition, carbon and nitrogen contents, and sinking rates. For cell concentrations approximately 10⁸ cells l^-1, the frequency of rejection ranged from 2.6 to 17.0 boluses per hour. The carbon and nitrogen contents averaged 24.1 µg C mm^-3 and 2.3 µg N mm^-3, and sinking rates averaged 367 m d^-1. Our findings suggest this behaviour may also occur in situ and could contribute to organic carbon export, with bolus sinking rates comparable to or exceeding those of Antarctic krill faecal pellets. If confirmed in the field, this behaviour may also occur in other krill species with similar feeding behaviours.