Prey size spectra and food preferences in thecate heterotrophic dinoflagellates

Laboratory experiments were conducted to determine prey size spectrum and food preference in the thecate heterotrophic dinoflagellates Protoperidinium pallidum, P. steinii, and Zygabikodinium Tenticulatum. Algal cultures of different taxonomic groups and different size were offered as prey. In addition to observations of feeding, growth rate was used as a measure of the quality of the different prey for population maintenance. The two Protoperidinium species show some degree of food niche differentiation, with preference for different prey types. Protoperidinium pallidum showed a preference for diatoms over dinoflagellates, and only diatom prey was able to support population growth. The smaller P. steinii had positive population growth on both dinoflagellates and diatoms, bur the highest growth rate was supported by dinoflagellates, and P. steinii showed a preference for this prey type. A common feature of the Protoperidinium species was the failure of small flagellates to support growth. The third dinoflagellate, Z. lenticulatum, showed a preference for diatoms over dinoflagellates, although dinoflagellates and one small flagellate, in addition to diatoms, were able to support growth. Species from the Diplopsalis group can be regarded as generalists in feeding compared to Protoperidinium species, which show more specialization. The predator:prey size ratio yielding best growth was, in terms of equivalent sphere diameter, about 1:1 for P. pallidum and Z. lenticulatum and 2.3:1 for P. steinii.