Prey size spectra in naked heterotrophic dinoflagellates

Laboratory experiments were conducted to determine the prey size spectrum of the naked heterotrophic dinoflagellates Gyrodinium dominans, G. fusiforme and Katodinium glaucum. Dinoflagellates, diatoms, prymnesiophytes, a cryptophyte 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 prey for population maintenance. All the taxonomic groups, except the chlorophyte, were able to support growth in G. dominans, but not all species within the groups were. The prymnesiophytes and the chlorophyte were inadequate food for G. fusiforme, whereas the other groups tested supported growth. Fewer species were tested for K. glaucum, and only dinoflagellates and the cryptophyte yielded growth. For all three predators, the best growth was achieved on dinoflagellate prey and Heterocapsa triquetra supported the highest growth rate. There was a tendency for predator cell volume to increase with increasing cell volume of the prey. The cell volume of G. fusiforme and K. glaucum tended to decrease during starvation. In terms of equivalent spherical diameter, the predator: prey size ratio that supported growth was between 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.