Detection of infrasonic water oscillations by copepodids of Lepeophtheirus salmonis (Copepoda Caligida)

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Abstract
The cues that trigger infection of fish by parasitic copepods are largely unknown. We show that copepodids of the parasitic copepod Lepeophtheirus salmonus respond to uniform, linear accelerations, which are similar to those found in front of a swimming fish. Copepodid responses to vibrations at 1, 3, 5 and 10 Hz frequency were filmed and analysed. The animals were stimulated in a completely water-filled, clear perspex chamber, which was suspended like a swing in four wires from a steel frame. The chamber was moved by a vibrator which was fed amplified signals from a sine wave oscillator. On stimulation, copepodids responded by executing swimming bursts of 1–3 s duration. There was no apparent preferred swimming direction. Sensitivity was highest at 3 Hz, with a threshold value of 5 × 10−3 m s−2 (rms). At 1 Hz the threshold was −1 m s−2 rms. These results indicate that the copepodids may react to the near-field accelerations produced within centimetres of a swimming fish. Acceleration sensitivity may therefore be a cue that triggers high-speed swimming and subsequent infestation of the host. If this ability is present in holoplanktonic copepods, it may facilitate detection and escape from predatory fish.
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