Prey size spectra and food preferences in thecate heterotrophic dinoflagellates

Laboratory experiments were conducted to determine prey size spectrum and f ood preference in the thecate heterotrophic dinoflagellates Protoperidinium pallidum, P. steinii, and Zygabikodinium Tenticulatum. Algal cultures of d ifferent taxonomic groups and different size were offered as prey. In addit ion to observations of feeding, growth rate was used as a measure of the qu ality of the different prey for population maintenance. The two Protoperidi nium species show some degree of food niche differentiation, with preferenc e for different prey types. Protoperidinium pallidum showed a preference fo r diatoms over dinoflagellates, and only diatom prey was able to support po pulation growth. The smaller P. steinii had positive population growth on b oth 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 fl agellates to support growth. The third dinoflagellate, Z. lenticulatum, sho wed a preference for diatoms over dinoflagellates, although dinoflagellates and one small flagellate, in addition to diatoms, were able to support gro wth. Species from the Diplopsalis group can be regarded as generalists in f eeding compared to Protoperidinium species, which show more specialization. The predator:prey size ratio yielding best growth was, in terms of equival ent sphere diameter, about 1:1 for P. pallidum and Z. lenticulatum and 2.3: 1 for P. steinii.