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#79 Forked tails of the cercariae of blood flukes 血吸蟲尾蚴的尾巴為何分岔

Today we are going to have a short, fun story.
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Flukes are a group of deadly parasites with complex life cycles. They could be roughly classified into 4 groups: liver flukes, lung flukes, intestinal flukes and blood flukes. Before an embryo of flukes could become sexually mature, it must go through several developmental stages. These different stages of larvae are named as miracidia, sporocysts, rediae, cercariae, and metacercariae, according to their morphologies. Among these, cercariae are motile larvae with long tails.

The mission of cercariae is clear. They would leave their first intermediate hosts, various kinds of snails, actually, and try to invade and encyst on a second intermediate host or a vector plant. The cercariae that reach their goal would lose their tails and become metacercariae. If their final hosts, usually humans or mammals, eat them raw, they would be infected by these flukes. As an illustration, the life cycle of Clonorchis sinensis, a liver fluke, is shown below.
Life cycles of Clonorchis sinensis, a liver fluke.(ref: https://www.cdc.gov/parasites/clonorchis/biology.html)

However, this is not the case for blood flukes. The cercariae of blood flukes do not encyst on second intermediate hosts or vector plants. Instead, they invade the skin of their final host directly! Besides, there are only 12 hours for cercariae to complete their great mission since they do not feed during this developmental stage. Interestingly, among these 4 groups of flukes, only the cercariae of blood flukes possess forked tails. Is there any connection between? How does a forked tail help the cercariae of blood flukes swim efficiently to complete their mission?

Life cycles of Schistosoma spp., or blood flukes. Noted that their cercariae directly invade the skin of their final hosts. 
(ref: https://www.cdc.gov/parasites/schistosomiasis/biology.html)

Before we start any model in fluid mechanics, we should specify the Reynold number, which directly determines the properties of flows. Calculated from the organism's typical body lengths, speeds, and viscosity, the Reynold number is about 0.3. To swim in this domain, where inertial forces are negligible, all swimmer must change their shape in a way that breaks time-reversal symmetry. The authors of suggested reading observed that there are 3 distinct swimming modes, termed tail-first mode, free-sinking mode, and body-first mode. Their forked tails are open during tail-first mode and free-sinking mode while they are closed during body-first mode, which is only found during skin penetration. They spend most of their time in free-sinking mode and use tail-first mode to swim back near to the water surface. How does tail-first mode operate so efficiently to allow them to spend most of their time sinking?
Three swimming modes. Ref: http://www.nature.com/nphys/journal/vaop/ncurrent/extref/nphys3924-s2.mov
The authors modeled the cercariae as a swimmer with 2 joints. One between their bodies and tails, the other between forks and their tails. The forks are not actively movable but act as torsional springs which change their relative direction with respect to their tails. By wagging their tails in frequency comparable to the relaxation time of the torsional spring, their motions break time-reversal symmetry and enable rapid swimming. The authors also built a scaled-up robot model with 2 joints. As they adjusted the spring constant of torsional springs, they found that robots with neither a free tail-fork joint nor a fixed tail-fork joint could swim. This proved that an elastic, forked tails are essential for efficient swimming of cercariae of blood flukes. If there is a new way that could break such elastohydrodynamic effect, it could then be a new, eco-friendly environmental control for this deadly disease.


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Suggested Reading: Krishnamurthy, D., Katsikis, G., Bhargava, A., Prakash, M. (2016). Schistosoma mansoni cercariae swim efficiently by exploiting an elastohydrodynamic coupling. Nature Physics doi:10.1038/nphys3924.

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