Prey selection and rejection by a microflagellate; Implications for the study and operation of microbial food webs

In a series of experiments, measurements were made of both numbers and biovolumes of the phototrophs Dunaliella primolecta (Butcher) (7.6 mu m diameter), Isochrysis galbana (Parke) (4.5 mu m), and Micromonas pusilla (Butcher) (1.5 mu m), together with the phagotrophic dinoflagellate Oxyrrhis marina (Dujardin) (typically 16-20 mu m). This enabled the calculation of the 'equivalent encounter distance' (l(eq)), which gives a measure of the distance an average sized predator would have to swim in order to encounter a biovolume of prey equal to its own cell volume. If predation of a given prey type continues when its l(eq) is greater than that of an alternative prey item, then the predator is deemed to be demonstrating a preference for the former item. When confronted with all three phototrophs, Oxyrrhis selected Dunaliella first but, despite the 25-fold difference in cell volume, showed no preference for Isochrysis over Micromonas. Oxyrrhis may also reject Isochrysis on occasion, an event which seems to be associated particularly with elevated C:N ratios in the phototroph. Oxyrrhis has been seen to exhibit cannibalism when in the presence of Isochrysis biovolumes (biomass concentrations) an order of magnitude above that of Oxyrrhis. Such plasticity in prey selection makes it very difficult to predict the outcome of predator-prey interactions, especially where (as between Dunaliella and Isochrysis) there are also growth interactions between the prey species. It also suggests that results obtained from short term studies of predation-kinetics, or in studies conducted under conditions such as in steady-state cultures and in continuous darkness, should not be generalised to more realistic environmental conditions.