A nematocyst is a specialized organelle within cells of jellyfish and other Cnidarians that sting. Nematocysts are also present in some single-celled protists. They contain a barbed venomous thread that accelerates faster than almost anything else in the animal kingdom. Here we simulate the fluid structure interaction of the barbed thread accelerating through water to puncture its prey using the 2D immersed boundary method. For simplicity, our model describes the discharge of a single barb harpooning a single celled organism, as in the case of dinoflagellates. One aspect of this project that is particularly interesting is that the micron-sized barbed thread reaches Reynolds numbers above one, where inertial effects become important. At this scale, even small changes in speed and shape can have dramatic effects on the local flow field. This suggests that the large variety of sizes and shapes of nematocysts may have important fluid dynamic consequences. We find that reaching the inertial regime is critical for hitting prey over short distances since the large boundary layers surrounding the barb characteristic of vicious dominated flows effectively push the prey out of the way.

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Strychalski, Wanda, Sarah Bryant, Baasansuren Jadamba, Eirini Kilikian, Xiulan Lai, Leili Shahriyari, Rebecca Segal, Ning Wei, and Laura A. Miller. Fluid Dynamics of Nematocyst Prey Capture. In Understanding Complex Biological Systems with Mathematics, edited by Ami Radunskaya, Rebecca Segal, and Blerta Shtylla, 123-144. Cham: Springer International Publishing: Imprint: Springer, 2018.

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