GRAZING IN THE HETEROTROPHIC DINOFLAGELLATE OXYRRHIS-MARINA - SIZE-SELECTIVITY AND PREFERENCE FOR CALCIFIED EMILIANIA-HUXLEYI CELLS

Selective grazing on nanophythoplankton by the heterotrophic dinoflage llate Oxyrrhis marina was investigated in laboratory experiments. O. m arina was offered 2 mixtures of differently sized algae: (1) Micromona s 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 selecti vely 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. marin a discriminated against E. huxleyi. To test this selection, O. marina was offered mixtures of equal concentrations of the similarly sized al gae 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 t erms of size-selective grazing.