Going with the flow: hydrodynamic cues trigger directed escapes from a stalking predator.

Area of Science:

• Marine biology
• Hydrodynamics
• Predator-prey dynamics

Background:

• Freely suspended planktonic organisms face unique challenges in predator threat assessment compared to substrate-dwelling ones.
• Small planktonic prey are vulnerable to fluid motion, which can mask sensory cues from approaching predators.

Purpose of the Study:

• To quantify the hydrodynamic cues used by calanoid copepods (Bestiolina similis) to detect and escape slow-approaching larval clownfish (Amphiprion ocellaris).
• To investigate the sensitivity thresholds and directional accuracy of copepod escape responses to predator-generated water deformation.

Main Methods:

• Utilized a hydrodynamic model simulating a rigid sphere to estimate water deformation around a predator.
• Parametrized the model with measurements of fish size, approach speed, and distance to the copepod.
• Quantified copepod escape responses to live larval clownfish, measuring sensitivity to deformation rates and escape direction.

Main Results:

• Copepods (developmental stages CII-CVI) detected predator-induced water flow at deformation rates as low as 0.04 s⁻¹, significantly lower than predicted by artificial-mimic studies.
• Copepods accurately localized the predator, with 87% of escapes directed away (≥90°) from the threat.
• Survival depends on detecting subtle nonlinear signals within a predominantly linear deformation environment.

Conclusions:

• Planktonic copepods possess highly sensitive mechanosensory systems capable of detecting faint hydrodynamic signals from predators.
• Accurate localization and directional escape responses are crucial for copepod survival against visually cryptic predators.
• This study highlights the importance of understanding nonlinear hydrodynamics in predator-prey interactions within planktonic ecosystems.