Selective grazing on nanophythoplankton by the heterotrophic dinoflagellate Oxyrrhis manna was investigated in laboratory expenments. 0. marina was offered 2 mixtures of differently sized algae: (1) Micromonas pusiUa (2 pm Equivalent Spherical Diameter), Emihania huxleyi (5 pm ESD), and Tetraselrms sueclca (7 pm ESD); and (2) Nannochloris sp. (2 pm ESD), Isochrysis galbana (5 pm ESD), Rhodomonas sp. (8 pm ESD). and Tetraselmis sp. (10 pm ESD). 0. marina grazed selectively on the larger algal species (ESD 2 7 pm), and also selected for the larger cells within each species. 0. marina discriminated against E. huxleyi. To test this selection, 0. marina was offered mixtures of equal concentrations of the similarly sized algae E. huxleyj and I. galband. In these mixtures, 0. marina preferred the calcified E. huxleyl cells over noncalcified E. huxleyj cells and discriminated against I. galbana. This result is also interpreted in terms of size-selective grazing.

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