Mixoplankton are planktonic protists engaging in photo-autotrophy plus osmo-heterotrophy plus phago-heterotrophy, contrasting with non-phagotrophic phytoplankton (e.g., diatoms) and non-phototrophic zooplankton (e.g., tintinnids). All mixoplankton are mixotrophs, but not all mixotrophs are mixoplanktonic. Mixoplankton are often considered as inferior in their capabilities compared to diatoms that surrendered phagotrophy, and those zooplankton that lost phototrophy. However, such views undersell the synergistic activities of mixoplankton. Thus, the phototrophic capacity of mixoplankton provides a predatory phagotroph with a ready source of carbon and energy supplementing phagotrophy and retention of the 30% of resources that would otherwise have to be released in specific dynamic action. Phagotrophy brings in nutrients to support phototrophy. Beyond these generalisations, we know little about the whole integrated physiology and ecology of mixoplankton. To fully appreciate the comparative fitness of two species, we need to consider all aspects of their life cycles. The emphasis for plankton is usually placed on resource acquisition and the maximum specific growth rate without considering the metabolic and mortality costs of being unable to support the growth rate, and predatory pressures. This suggests that trait trade-offs are less useful for conceptual and simulation modelling than approaches more securely founded in physiology and evolution.