Hydromechanical signaling between the ciliate Mesodinium pulex and motile protist prey

The raptorial ciliate Mesodinium pulex (Claparéde & Lachmann, 1858) was fed 3 different prey species: Rhodomonas salina, Heterocapsa rotundata and Gymnodinium simplex. Each prey displayed different motility patterns, including dramatic and effective escape responses. M. pulex only fed while motionless and responded only to motile prey by either an attack or an escape. Both the attack and escape responses are associated with a strong directional component in M. pulex. M. pulex responded to motile prey when the critical fluid velocity exceeded ~15 to 42 μm s–1, depending on prey species. These values are similar to hydromechanical signal thresholds found in marine metazoans. Prey-induced escape responses in M. pulex are associated with fast-swimming prey such as G. simplex. Deformation rates calculated in these specialized cases exceed the reported critical deformation rate for escape in ciliated protists, suggesting that fast-swimming prey mimic an approaching predator. Microscopic observations indicate that the band of equatorial cirri encircling M. pulex is the principal detector of motile prey. We propose that M. pulex exhibits a specialized fluid velocity-sensitive mechanoreceptor analogous to the fluid motion mechanoreceptors found in metazoans. M. pulex fails to ingest the cryptophytic prey R. salina. Our observations suggest that this is due to mismatch between the capturing organelles of the ciliate and surface properties of the prey. Dinoflagellate prey are more effectively ingested, but a high percentage escaped from M. pulex. Pre-capture escape responses may be triggered by ciliate-generated hydromechanical signals and facilitated by the strong swimming ability of the flagellates. Hydromechanical signaling can affect the outcome of predator–prey interactions and may select for distinctive morphological and behavioral features in planktonic protists.