Biological observation and physical modelling of water-hopping mudskipper*

Mudskippers, which are amphibious fish that can live in both water and on land, display a unique water-skipping behavior without fully submerging their bodies. However, the specifics of their body-fin movement and the interfacial dynamics during the water splash have not been fully understood. This study investigates the hydrodynamics of mudskippers' water-hopping through biological observation and physical modeling. By using a saltwater tank and high-speed imaging, we captured the movements of their fins and bodies during water impact with cavity deformation. Our findings indicate that mudskippers extend and hold their pectoral fins during impact and then use their caudal fins to hop when the water splash reaches its peak radius. Water impact experiment of artificial mudskippers demonstrated various impact modes, depending on the body angle, impact velocity, and impact angle. Analyzing the cavity dynamics identified the optimal impact conditions that allow the fish to maximize their jump velocity. These insights could improve future naval propulsion technology by increasing the thrust performance and efficiency of heavy-duty underwater vehicles.