Effects of squeeze-confinement on flow fields around morphologically complex ciliated larvae*

Marine invertebrates undergo development through free-swimming larval stages that have a diverse range of shape morphologies and ciliation patterns. These larvae utilize ciliary beating for swimming and feeding at low Reynolds numbers (< 1). The standard microscopic technique used to study flow fields of these larvae involves confining the larvae between a glass slide and cover slip. This induces a quasi-two-dimensional (2D) confinement on them since the larvae swim in three-dimensions (3D) in their natural habitat. We study the effects of quasi-2D confinement of two marine invertebrate species’ larvae with different, complex shape morphologies – sea stars (Patiria miniata) and sea urchins (Lytechinus variegatus). This enables us to understand how morphology may affect the resulting flow fields under different levels of confinement. We measure 2D confinement effects by varying the confinement parameter, defined as the height (h) between glass slide and cover slip. In both species, the number of vortices increase when the confinement is increased (smaller h). Conversely, when the confinement is decreased (larger h), both species create two large counter-rotating vortices. Our results suggest that complex larval shape morphologies are important in determining flow patterns under increased confinement (smaller h), and larval shape plays minimal role in determining flow patterns as confinement decreases.