Squeeze-confinement induced vorticity amplification in ciliated marine larvae*

Ciliated marine invertebrate larvae swim and feed in a viscous low Reynolds number (< 1) environment in the ocean. The larvae swim in three-dimensions (3D) using ciliary beating, resulting in flow fields that are often complex and 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. In non-spherical larvae, increasing confinement (smaller h) increases the number of vortices that form, and they come closer to the body surface. In both spherical and non-spherical larvae, decreasing confinement (larger h) gives rise to a pair of counter rotating vortices. Our results are broadly applicable for quantification of the fluid dynamical effects of 2D squeeze confinement for ciliated larvae with a variety of morphologies.