A universal hydrodynamic transition in confined marine invertebrate larvae
The ocean is teeming with a myriad of mm-sized invertebrate planktonic larvae, which thrive in a viscous fluid environment. Many of them rely on ciliary beating to generate fluid flows for locomotion and feeding. Their larval forms, local morphologies, and ciliation patterns exhibit remarkable diversity, producing intricate and dynamic 3D flows that are notoriously difficult to characterize in laboratory settings. Traditional microscopic imaging techniques typically involve gently squeeze-confining the soft larvae between a glass slide and cover slip to study their flows in quasi-2D. However, a comprehensive hydrodynamic framework for the low-to-intermediate Reynolds number (<1) flows in quasi-2D confinement, particularly in light of their complex forms, has remained elusive. Here, we demonstrate that vortices around larvae proliferate with increasing confinement and illuminate the underlying physical mechanism. We experimentally quantify confinement-induced flows in larvae of sea stars and sea urchins. The flows exhibited strikingly universal patterns: under weak confinement, all larvae generated two vortices, whereas under strong confinement, the number of generated vortices significantly increased. The experimental observations were well captured by a low Reynolds number theoretical model based on the superposition of confined Stokeslets. Building on experiments and theory, we developed a comprehensive framework for confinement-induced flows, which suggests that vorticity dynamics are primarily determined by local morphological features, rather than solely the body plan. Our work provides fundamental insights into form-functional relationships between larval morphology and flow generation. Our findings are broadly applicable to understanding flows generated by a wide range of ciliated organisms with complex forms and morphologies, from micro- to milli-length-scales.