DETECTION OF INFRASONIC WATER OSCILLATIONS BY COPEPODIDS OF LEPEOPHTHEIRUS-SALMONIS (COPEPODA, CALIGIDA)

The cues that trigger infection of fish by parasitic copepods are larg ely unknown. We show that copepodids of the parasitic copepod Lepeopht heirus salmonis respond to uniform, linear accelerations, which are si milar 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 persp ex chamber, which was suspended like a swing in four wires from a stee l frame. The chamber was moved by a vibrator which was fed amplified s ignals from a sine wave oscillator. On stimulation, copepodids respond ed by executing swimming bursts of 1-3 s duration. There was no appare nt preferred swimming direction. Sensitivity was highest at 3 Hz, with a threshold value of 5 x 10(-3) m s(-2) (rms). At 1 Hz the threshold was <6 dB higher, and sensitivity was markedly reduced at 10 Hz, where the threshold was 1.8 x 10(-1) m s(-2) rms. These results indicate th at the copepodids may react to the near-field accelerations produced w ithin centimetres of a swimming fish. Acceleration sensitivity may the refore be a cue that triggers high-speed swimming and subsequent infes tation of the host. If this ability is present in holoplanktonic copep ods, it may facilitate detection and escape from predatory fish.