Microfluidics for hydrodynamic investigations of sand dollar larvae

The life cycle of most marine invertebrates includes a planktonic larval stage before metamorphosis to bottom-dwelling adulthood. During the larval stage, ciliary-mediated activity enables feeding (capturing unicellular algae) and transporting materials (e.g., oxygen) required for the larva’s growth, development, and successful metamorphosis. Investigating the underlying hydrodynamics of the ciliary activities is valuable for addressing fundamental biological questions (e.g., phenotypic plasticity) and advancing engineering applications (e.g., biomimetic design). We combined microfluidics and fluorescence microscopy as a miniaturized particle image velocimetry approach to study ciliary-mediated hydrodynamics during suspension feeding in sand dollar larvae (Dendraster excentricus). First, feasibility was confirmed by examining the underlying hydrodynamics (ciliary-mediated vortex patterns) for low- and high-fed larvae. Next, ciliary hydrodynamics were tracked from 11 days post-fertilization (DPF) to 20 DPF for 21 low-fed larvae. Microfluidics enabled the examination of baseline activities (without external flow) and behaviors in the presence of environmental cues (external flow). A library of qualitative vortex patterns and quantitative hydrodynamics (velocity and vorticity profiles) was generated. Velocities were used to examine the role of ciliary activity in transporting materials. Given the laminar flow and the viscosity-dominated environments surrounding the larvae, overcoming the diffusive boundary layer is critical. Péclet number analysis for oxygen transport suggested that ciliary velocities help overcome the diffusion-dominated transport. The approach was used to examine the hydrodynamics of two additional marine larvae (P. helianthoides and S. purpuratus). Microfluidics provided a scalable and accessible approach for investigating the ciliary hydrodynamics of marine organisms.