Feeding by Antarctic Krill, Euphausia superba: Does Size Matter?

Summary

Antarctic Krill, Euphausia superba (25–30 mm total length), were fed uni-algal cultures of a flagellate, Isochrysis galbana (= 4 μm sphere), a pennate diatom, Phaeodactylum tricornutum (equivalent spherical diameter (ESD) = 4 μm), and 2 centric diatoms, Ditylum brightwellii (ESD = 30 μm) and Thalassiosira eccentrica (ESD = 40 μm). Experiments were conducted at −0.5°C for 3–4 d, after an initial acclimation period of 4 d. The feeding response varied with the size of the phytoplankton cell. The maximum clearance rates for any individual feeding on the 4 phytoplankton species were: 103 ml animal⁻¹ h⁻¹ for I. galbana, 167 ml animal⁻¹ h⁻¹ for P. tricornutum,281 ml animal⁻¹ h⁻¹ for D. brightwellii, and 357 ml animal⁻¹ h⁻¹ for T. eccentrica. The maximum clearance rates of all individuals were directly proportional to the calculated spherical radius squared (CSR)² of the cells, described by the equation y = 61.75 + 0.233 CSR² where y is the clearance rate (N = 20, r = 0.694). No critical concentration was established for I. galbana at concentrations as great as 25000 cells ml⁻¹, indicating that E. superba does not feed very effectively on small round cells. Because the variation in the maximum clearance rate is closely linked to the size, but not the species, of phytoplankton being ingested, the physical dimensions, not the chemical composition, of the cell are probably the most important parameters affecting the maximum clearance rate. These feeding responses are important both to the energetics of E. superba, and to the modification of the phytoplankton community in which Krill feed.