Dynamics of prey selection by an omnivorous flagellate

Abstract

The heterotrophic flagellate Paraphysomonas imperforata , a raptorial grazer, sustained maximum specific growth rates of ca 1.5 d super(-1) at 20 degree C when fed 3 phytoplankton species of different sizes and shapes (the relatively small diatom Phaeodactylum tricornutum and haptophyte Isochrysis galbana , and the larger chlorophyte Dunaliella tertiolecta either singularly or in combinations of 2 species. When prey combinations included D. tertiolecta , the chlorophyte was grazed only after a large fraction of the other species was first grazed. Diatom and haptophyte were grazed concurrently when offered in combination. Changing the relative proportions of starting biomass of the different species in combination had no effect on the order of grazing. In all cases the switch to the chlorophyte occurred rapidly and the maximum ingestion rate attained after the switch was proportional to the contribution of the chlorophyte to total starting biomass.