Grazing in the heterotrophic dinoflagellate Oxyrrhis marina: Size selectivity and preference for calcified Emiliania huxleyi cells

Selective grazing on nanophythoplankton by the heterotrophic dinoflagellate Oxyrrhis marina was investigated in laboratory experiments. O. marina was offered 2 mixtures of differently sized algae: (1) Micromonas pusilla (2 mu m Equivalent Spherical Diameter), Emiliania huxIeyi (5 mu m ESD), and Tetraselmis suecica (7 mu m ESD); and (2) Nannochloris sp. (2 mu m ESD), Isochsysis galbana (5 mu m ESD), Rhodomonas sp. (8 mu m ESD), and Tetraselmis sp. (10 mu m ESD). O. marina grazed selectively on the larger algal species (ESD greater than or equal to 7 mu m), and also selected for the larger tells within each species. O. marina discriminated against E. huxleyi. To test this selection, O. marina was offered mixtures of equal concentrations of the similarly sized algae E. huxleyi and I. galbana. In these mixtures, O. marina preferred the calcified E. huxleyi cells over noncalcified E. huxleyi cells and discriminated against I. galbana. This result is also interpreted in terms of size-selective grazing.