Morphological Study of Sieve Efficiencies and Mandibular Surfaces in the Antarctic Krill, Euphausia superba

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Abstract
The filtering basket of Euphausia superba is a complex comb filter with a wide range of mesh intervals. Intersetal distances range from approximately 15–95 μm between primary setae to 4–18 μm between secondary setae (setules). Microsetules with spacing of 1–2 μm thicken boundary layers surrounding secondary setae. Reynolds numbers for setules and microsetules, ranging from 0.006 to 0.95, limit the feeding modes available to E. superba for suspension feeding. Relative efficiency of particle capture, estimated from intersetule mesh size and grazing experiments with small phytoplankton (< 30 μm), show an asymptotic efficiency for particles 12–28 μm. Setule frequency distribution conservatively characterizes particle size selection for these small particles. The fine structure of the mandibular pars molaris varies between krill species and shows specialized surfaces that can split diatom chains, cut or fracture hard tests, and grind particles smaller than 10 μm.