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-' at 20°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. However, 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. From a hydrodynamic standpoint, specific clearance rate C' increased as the ratio of predator radius to prey radius R: r decreased and C' increased as R decreased for a given value of R: r. We suggest that the preference for the 2 smaller species is governed by the ability of the flagellate to adjust its own size downward to accomodate the smaller prey in order to maintain R : r at ca 2 : 1. When sized to graze these smaller species, the flagellate simply is too small to graze the chlorophyte. Thus, although there is clear evidence that the flagellate is a non-passive grazer and will not graze certain species at all, mechanoreception clearly plays a major role in the dynamics of grazing desired species. Raptorial grazers such as P. imperforata, by having the ability to graze prey almost as big as themselves, may be effective competitors with larger protozoa for nanoplankton-size food particles and also contribute to making the food chain (web) withln the microbial loop long and complicated with high losses of energy and materials.