Prey size spectra in naked heterotrophic dinoflagellates

Laboratory experiments were conducted to determine the prey size spectrum o f the naked heterotrophic dinoflagellates Gyrodinium dominans, G. fusiforme and Katodinium glaucum. Dinoflagellates, diatoms, prymnesiophytes, a crypt ophyte and a chlorophyte were offered as food. In addition to observations of feeding, growth rate was used as a measure of the quality of different p rey for population maintenance. All the taxonomic groups, except the chloro phyte, were able to support growth in G. dominans, but not all species with in the groups were. The prymnesiophytes and the chlorophyte were inadequate food for G. fusiforme, whereas the other groups tested supported growth. F ewer species were tested for K. glaucum, and only dinoflagellates and the c ryptophyte yielded growth. For all three predators, the best growth was ach ieved on dinoflagellate prey and Heterocapsa triquetta supported the highes t growth rate. There was a tendency for predator cell volume to increase wi th increasing cell volume of the prey. The cell volume of G. fusiforme and K. glaucum tended to decrease during starvation. In terms of equivalent sph erical diameter, the predator:prey size ratio that supported growth was bet ween 0.7:1 and 3.5: 1. The size ratios yielding best growth were 2.4:1 for G. dominans, 1.7:1 for G. fusiforme, and 2.3:1 for K. glaucum.