  The motility of many parasites is critical for the infection process of their
host, as exemplified by the transmission cycle of the blood fluke Schistosoma
mansoni. In their human infectious stage, immature, submillimetre-scale forms
of the parasite known as cercariae swim in freshwater and infect humans by
penetrating through the skin. This infection causes Schistosomiasis, a
parasitic disease that is comparable to malaria in its global socio-economic
impact. Given that cercariae do not feed and hence have a finite lifetime of
around 12 hours, efficient motility is crucial for the parasite's survival and
transmission of Schistosomiasis. However, a first-principles understanding of
how cercariae swim is lacking. Via a combined experimental, theoretical and
robotics based approach, we demonstrate that cercariae propel themselves
against gravity by exploiting a unique elastohydrodynamic coupling. We show
that cercariae beat their tail in a periodic fashion while maintaining a fixed
flexibility near their posterior and anterior ends. The flexibility in these
regions allows an interaction between the fluid drag and bending resistance: an
elastohydrodynamic coupling, to naturally break time-reversal symmetry and
enable locomotion at small length-scales. We present a theoretical model, a
'T-swimmer', which captures the key swimming phenotype of cercariae. We further
validate our results experimentally through a macro-scale robotic realization
of the 'T-swimmer', explaining the unique forked-tail geometry of cercariae.
Finally, we find that cercariae maintain the flexibility at their posterior and
anterior ends at an optimal regime for efficient swimming, as predicted by our
theoretical model. We anticipate that our work sets the ground for linking the
swimming of S. mansoni cercariae to disease transmission and enables
explorations of novel strategies for Schistosomiasis prevention.
