This study introduces biodegradable particles as a sustainable, low-cost, and effective alternative to synthetic tracers for flow visualization of biological and underwater systems using particle image velocimetry (PIV). Biodegradable alternatives address critical challenges associated with conventional synthetic tracers (e.g., silver-coated glass beads), such as potential environmental impact, toxicity, and high costs. We characterized various biodegradable particles to determine their density, size, and light-scattering properties. Corn and arrowroot starch emerged as the most promising candidates, with densities closely matching deionized and artificial seawater (35 ppt), minimizing buoyancy effects. Both particles exhibited uniform size distributions, with mean diameters of 10.9 (corn) and $8.7\mu \mathrm{m}$ (arrowroot). PIV experiments using a translating airfoil, live brine shrimp, and live jellyfish confirmed that starch particles accurately visualized velocity and vorticity fields, effectively capturing flow structures in brine shrimp and jellyfish swimming. The nontoxic, digestible nature of starch makes them ideal for biological and ecological studies, minimizing risks to live organisms and the environment. Thus, we highlight the potential of natural starch as an eco-friendly replacement for synthetic tracers in PIV experiments, promoting sustainable and frugal practices in flow visualization without compromising performance.

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• Biological fluid dynamics
• Swimming
• Particle image velocimetry