Invited Talk: Squeeze confinement-induced changes in flow fields of ciliated marine larvae*

Ciliated marine invertebrate larvae swim and feed in a viscous low Reynolds number (< 1) environment in the ocean. Larvae swim in three-dimensions (3D) using ciliary beating, resulting in complex flow fields that are challenging to quantify in experimental studies. The conventional microscopic imaging configuration of trapping larvae in between a glass slide and cover slip induces a quasi-two-dimensional (2D) confinement. We systematically quantify the fluid dynamical effects of 2D squeeze-confinement on flows generated by ciliated larvae at low Reynolds numbers (< 1). We explore both spherical and non-spherical larval morphologies in our study. Spherical morphologies include coral larvae and non-spherical morphologies include sea star and sea urchin larvae. We vary the confinement parameter – the gap between the glass slide and cover slip (h) – and observe changes in the number of vortices, vortex size, and intensity. Across both morphologies, increasing confinement (smaller h) increases the number of vortices that form and decreasing confinement (larger h) gives rise to a pair of counter rotating vortices. We compare our experimental results with low Reynolds number theoretical predictions and find a very good agreement. Our results are broadly applicable for quantification of the fluid dynamical effects of 2D squeeze confinement for ciliated larvae with a variety of morphologies.