Oral: Hydrodynamic study on the water-hopping behavior of mudskipper*

Mudskippers, amphibious fish capable of living in water and on land, exhibit an intriguing behavior of hopping on the water's surface without submerging most of their bodies. Although their speed and momentum have been analyzed, questions regarding body-fin movement during splashes and energy efficiency remain unanswered. This study aims to explore the hydrodynamics of water-hopping in mudskippers through experimental setups and theoretical analysis. Using a saltwater tank and high-speed imaging, we capture the fin and body movements during the fish's splash. Our observations reveal that mudskippers extend and maintain their pectoral fins during the impact and then flap their caudal fins to hop when the water splash reaches its maximum radius. The cost of transport (CoT) for water-hopping is found to be smaller and more efficient compared to swimming and crawling. To simulate this behavior, we set up an impact experiment in a water tank with a pneumatic actuator. Two acrylic rods, one with artificial pectoral fins and one without, are thrown into the water surface at the same impact angle and angle of attack as the mudskipper. Our findings demonstrate that the unique anatomical feature of the pectoral fin, which can be bent downward to maximize form drag during water impact, prevents mudskippers from full submersion and allows them to gain momentum from the rebound flow generated by their impact.