Retention efficiency of 0.2 to 6 μm particles by the appendicularians Oikopleura dioica and Fritillaria borealis

Abstract

We used suspensions of 0.2, 0.5, 0.75, 1, 2, 3 and 6 μm fluorescent beads in combination with analytical flow cytometry to determine the efficiency of retention by small (165 μm trunk length), medium (347 μm) and large (689 and 734 μm) Oikopleura dioica, and by large (585 μm) Fritillaria borealis. Large O. dioica and F. borealis were the most efficient at retaining the 2 μm beads, and small and medium O. dioica were most efficient for 1 μm beads. Large O. dioica and F. borealis showed efficiencies of ca. 15 % for 0.2 μm, 33 % for 0.5 μm, 58 % for 0.75 μm beads, 66 % for 1 μm beads and 88 % for 2 μm beads. However, small O. dioica showed higher efficiencies, measuring ca. 10 % for 0.2 μm, 43 % for 0.5 μm, 72 % for 0.75 μm beads, 87 % for 1 μm beads and 93 % for 2 μm beads. The combination of our measured appendicularian particle-retention efficiency spectra with typical particle size-distribution spectra in the ocean indicates that large and small appendicularians obtain 80 % of their diet from particles smaller than 15 and 7 μm respectively, and that the smallest particles represent a significant part of their diet only when they strongly dominate the biomass size spectra. Comparison with data from the literature indicates that although the appendicularian:prey length ratio is extremely high, the appendicularian:prey body-carbon ratio (14538:1) is within the reported range for mesozooplankton (1:1 to ca. 3 x 106:1), and statistically undistinguishable from that of copepods (1603:1).