Why does the jumping ciliate Mesodinium rubrum possess an equatorially located propulsive ciliary belt?

It has long been thought that jumping by the ciliate Mesodinium rubrum can enhance its nutrient uptake. However, jumping can be energetically costly and also dangerous by inducing hydrodynamic disturbances detectable by rheotactic predators. Here, a computational fluid dynamics (CFD) model, driven by published empirical data, is developed to simulate the jump-induced unsteady flow as well as chemical field around a self-propelled jumping ciliate. The associated phosphorus uptake, hydrodynamic signal strength, mechanical energy cost and Froude propulsion efficiency are also calculated. An equatorial ciliary belt (ECB), i.e. the morphology used by M. rubrum for propulsion, is considered. For the purpose of comparison, three other strategies (pulled or pushed by cilia, or towed) are also considered. Comparison of the CFD results among the four strategies considered suggests: (i) jumping enhances phosphorus uptake with simulated values consistent with available field data; (ii) the M. rubrum-like propulsion generates the weakest and spatially most limited hydrodynamic disturbance and therefore may effectively minimize the jump-induced predation risk; and (iii) the M. rubrum-like propulsion achieves a high Froude propulsion efficiency (∼0.78) and is least costly in mechanical energy expenditure among the three self-propelled strategies considered. Thus, using the ECB for propulsion can be essential in ensuring that M. rubrum is a successful ‘fast-jumping’ primary producer.