A New Look at Medaka: Surprising Deep Diving Behavior Uncovered in the Lab

Medaka (Oryzias latipes and O. sakaizumii) are small freshwater fish native to East Asia, commonly found in shallow waters. However, their presence and behavior in deeper habitats remain poorly understood due to observational challenges. Here, we introduce a novel experimental system--a 9-meter-tall cylindrical tank made from vertically connected transparent PVC pipes--that enables direct observation of medaka diving behavior. Our results demonstrate that medaka voluntarily dive to depths of up to 9 meters and remain at the bottom for extended periods, a behavior contrasting with that of zebrafish (Danio rerio), which rarely reached such depths. Moreover, when fish were introduced from the bottom of the tank, zebrafish exhibited repeated vertical movements, ascending and descending but rarely reaching the water surface, whereas medaka consistently remained at the bottom. While primarily descriptive, these findings represent a significant and previously unrecognized aspect of medaka ecology, challenging the traditional view of them as purely surface-dwelling fish. This study provides a unique tool and foundational insight that opens new avenues for ecological and physiological investigations of fish depth preferences.