In situ swimming behavior of the Mariana snailfish Pseudoliparis swirei.

The hadal zone, defined as ocean depths below 6000 m, is one of Earth’s most extreme environments. The Mariana snailfish Pseudoliparis swirei is among the deepest known fishes, yet its natural behavior remains largely unknown. Here we quantify, for the first time, the three-dimensional swimming behavior of this species in its native habitat by analyzing video collected by a baited deep-sea lander at nearly 7000 m depth. Using an automated computer vision workflow that detects, tracks and infers visual depth from single camera footage, we reconstructed full trajectories for individual fish using 868 manually annotated instances. The results show that P. swirei exhibits a slow routine swimming speed of 0.16-0.18 m s⁻1 (0.62-0.80 body lengths s⁻1, mean ≈ 0.71 BL s⁻1), with low variability across individuals and only short acceleration events reaching up to ~ 0.50 m s⁻1. By combining reconstructed movement paths with a simple representation of near bottom flow, we estimate that the fish operates within a modest spatial range around the bait. A mechanistic advection-diffusion framework further indicates that P. swirei first detects bait odor at a distance of approximately 350 m. These findings provide the first quantitative view of locomotion in a hadal vertebrate, revealing gentle, steady gaits shaped by environmental constraints.