Behavioral and mechanistic characteristics of the predator-prey interaction between the dinoflagellate Dinophysis acuminata and the ciliate Mesodinium rubrum.

Predator-prey interactions of planktonic protists are fundamental to plankton dynamics and include prey selection, detection, and capture as well as predator detection and avoidance. Propulsive, morphology-specific behaviors modulate these interactions and therefore bloom dynamics. Here, interactions between the mixotrophic, harmful algal bloom (HAB) dinoflagellate Dinophysis acuminata and its ciliate prey Mesodinium rubrum were investigated through quantitative microvideography using a high-speed microscale imaging system (HSMIS). The dinoflagellate D. acuminata is shown to detect its M. rubrum prey via chemoreception while M. rubrum is alerted to D. acuminata via mechanoreception at much shorter distances (89 ± 39 μm versus 41 ± 32 μm). On detection, D. acuminata approaches M. rubrum with reduced speed. The ciliate M. rubrum responds through escape jumps that are long enough to detach its chemical trail from its surface, thereby disorienting the predator. To prevail, D. acuminata uses capture filaments and/or releases mucus to slow and eventually immobilize M. rubrum cells for easier capture. Mechanistically, results support the notion that the desmokont flagellar arrangement of D. acuminata lends itself to phagotrophy. In particular, the longitudinal flagellum plays a dominant role in generating thrust for the cell to swim forward, while at other times, it beats to supply a tethering or anchoring force to aid the generation of a posteriorly-directed, cone-shaped scanning current by the transverse flagellum. The latter is strategically positioned to generate flow for enhanced chemoreception and hydrodynamic camouflage, such that D. acuminata can detect and stealthily approach resting M. rubrum cells in the water column.