Schistosoma mansoni cercariae swim effciently by exploiting an elastohydrodynamic coupling

The motility of many parasites is critical for infecting their host, as exemplified in the transmission cycle of the parasite Schistosoma mansoni(1). In its human infectious stage, submillimetre- scale forms of the parasite known as cercariae swim in freshwater and infect humans by penetrating the skin(1,2). This infection causes schistosomiasis, a disease comparable to malaria in global socio-economic impact(3,4). Given that cercariae do not feed and hence have a lifetime of around 12 hours(5,6), effcient motility is crucial for schistosomiasis transmission. Despite this, a first-principles understanding of how cercariae swim is lacking. Combining biological experiments, a novel theoretical model and its robotic realization, we show that cercariae use their forked tail to swim against gravity using a novel swimming gait, described here as a 'T-swimmer gait'. During this gait, cercariae beat their tail periodically while maintaining an increased flexibility near their posterior and anterior ends. This flexibility allows an interaction between fluid drag and bending resistance-an elastohydrodynamic coupling, to naturally break time-reversal symmetry and enable locomotion at small length scales(7). Finally, we find that cercariae maintain this flexibility at an optimal regime for effcient swimming. We anticipate that our work sets the ground for linking the swimming of cercariae to disease transmission, and could potentially enable explorations of novel strategies for schistosomiasis control and prevention.