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.