Investigating the intraspecific effect of cell concentration in mediating Oxyrrhis marina swimming behaviors

Heterotrophic protists are known to respond to a multitude of abiotic and biotic stimuli which confers a strong selective advantage in marine environments that are frequently dilute and heterogeneously distributed. In this laboratory study, we investigated the role of intraspecific signals in mediating Oxyrrhis marina swimming behavior that could be utilized to enhance dispersive behaviors and reduce competition between intraspecific predators. Using video and image analysis, three-dimensional movement behaviors of O. marina (on scales of micrometers and seconds) were simultaneously quantified with population-scale vertical distributions (on scales of centimeters and hours) and used in dispersal and encounter rate estimates. Three different concentrations of O. marina were filmed in both the absence and presence of the prey alga species, Isochrysis galbana, in at least triplicate films every 30 minutes for three hours at five horizons in 1-L experimental tanks. We found that the cell-cell interactions in the absence of prey cells resulted in modified swim behaviors that increased model estimates of encounter rates by 9%; however, individual swim behaviors between treatments were not significantly different in the presence of prey cells. Also, the relative proportion of the population near the top of the tank significantly decreased by 22% and 16% in both the absence and presence of prey cells, respectively, from low to high O. marina concentrations. These results suggest that O. marina can respond to the intraspecific cell concentration in the absence of competing signals which can ultimately result in significant changes to distributions, growth and grazing rates