Hop-and-sink locomotion and escape in benthic copepods under localized suction flow

Abstract

Benthic copepods inhabit complex boundary layers where flow varies rapidly, yet their sensory-motor responses to localized hydrodynamic signals remain poorly understood. We quantified the behavior of the harpacticoid Tigriopus californicus under no-flow and localized suction flow using 3D particle tracking velocimetry of both copepod trajectories and the surrounding flow. Copepods frequently entered the water column and maintained a hop-and-sink gait, spending ~40% of time jumping with peak speeds up to 68 mm s−1. In the far field (>2 mm from the source), kinematic statistics were indistinguishable from no-flow controls and orientations were consistent with random swimming. In the near field, individuals performed directed escape jumps (mean 80.6 ± 17.9 mm s−1; max. 105.7 mm s−1). Escape onset occurred at relative speeds (≈ 6 mm s−1) and strain rates (≈ 5 s−1) characteristic of localized deformation, at distances from the source on the order of the organism size. These results reveal a context-dependent modulation of locomotion that preserves routine hop-and-sink gait under weak flow while enabling rapid evasion near localized disturbances. The findings are consistent with gradient-based mechanosensation, although direct contact with the suction inlet may have contributed to some escapes. Broader generalization will require replication across taxa, life stages and flow regimes.