The retention by protists of foreign plastids and other organelles obtained from algal prey is an ecologically important example of mixotrophy and also represents a potential pathway for the symbiogenetic evolution of novel permanent plastids. A gymnodinoid dinoflagellate isolated from the Ross Sea, Antarctica (RSD) retains plastids from its haptophyte prey Phaeocystis antarctica. It is a member of the Kareniaceae, a dinoflagellate family whose other members all contain permanent tertiary plastids of haptophyte origin. A subset of its cells also contain foreign nuclei. The following chapters describe experiments that indicate the RSD's selectivity for P. antarctica in feeding and plastid uptake, when compared to other potential prey; and observations that demonstrate survival of plastid-retaining RSD for over two years in the absence of its prey. Further experiments assess the resilience of P. antarctica and the RSD in response to the prolonged darkness of the austral winter. / Biology