Session J02: Locomotion: Interfacial

Interfacial Swimming Mechanics of Cypris Larvae Amphibalanus amphitrite

While some animals are adapted to the air-water interface, others traverse it occasionally for survival. This study examines the interfacial swimming of cypris larval Amphibalanus amphitrite, commonly known as acorn barnacles. These crustaceans, typically submerged, possess hydrophobic bodies that allow attachment to the air-water interface despite being denser than water. To better understand cyprid surface dynamics, we analyzed the swimming mechanics of Amphibalanus amphitrite using DeepLabCut, which provided Cartesian coordinates of cyprid body positions for every frame. This data enabled us to track the trajectories of cyprids swimming on the air-water interface over various timescales, relating velocity distribution to various types of random walks. By integrating these insights with a fluid mechanics model of single-stroke trajectories, we estimated the power exerted by cyprids to swim at the interface. Our findings offer new perspectives on the adaptations and energy expenditure of these organisms in navigating the interfacial environment, shedding light on their unique survival strategies.    

Juvenile eels climb up wet rough walls

Eels, Anguilla marmorata, migrate from the sea to rivers at an early age. Vertical migration is challenging because the eel has to climb slippery surfaces against gravity. Previous research reported that only juvenile eels with a body length of less than 12 cm migrate vertically. However, the strategy of movement on vertically wet surfaces utilized by juvenile eels is rarely understood. In this study, we experimentally and theoretically investigate the climbing behavior of juvenile eels. We discovered that eels climb only on wet and rough surfaces. Rough and moist walls with small water flows simulate 60% eels with a body length of 6 cm to climb. Juvenile eels apply surface tension and friction to balance their body weight. Eels greater than 10 cm fail to climb the same surfaces. Comprehending the climbing behavior of eels carries significant implications for both ecological conservation and materials technology.   

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.   

Marangoni effect enhances an ultrafast escape and its wake impairs predator's locomotion in water striders unlike rove beetles

Marangoni effect is used by water striders (Veliidae) and rove beetles (Staphylinidae) to induce an ultrafast escape response against inter- or intra-specific predators. However, it is unclear if the surfactants secreted by these insects that help reduce surface tension of water can also affect predator's locomotion during the chasing. We discovered that the wake produced by Rhagovelia and Microvelia bugs during Marangoni propulsion forms a wide persistent layer at the water surface that significantly degrades the locomotion performance of a pursuer. In contrast, we found that the wake left by Rove beetles during Marangoni propulsion is very narrow and dissipates quickly, which is ineffective to affect the predator's locomotion. This double advantage of Marangoni effect in water striders (speeding up prey escape and slowing down a pursuer) can be an adaptation not only against interspecific predators, but also to counteract cannibalism. Our results can be used for bio-inspired interfacial micro vehicles with the capacity to escape quickly and deter pursuers using the Marangoni effect.   

Diving of whirligig beetles: a novel method for transitioning from near-surface to underwater locomotion

Primarily water surface-dwelling whirligig beetles (Gyrinidae) dive to a full submergence when threatened by aerial predators. Their morphology and thrust generation mechanism adapted for their neustonic life (living on the water surface) suggests that there are no means for vertical momentum generation. Despite this limitation, they display rapid diving behavior saving themselves from the threat from above. Here, we present the mechanism behind whirligig beetle’s fast diving. The result of our in vivo experiments on their diving suggests their rapid transition from surface to underwater is initiated by bending their abdomen downward. The new body conformation generates additional drag on the abdomen, which torques the body in a favorable orientation for diving.