Locomotory activity patterns ofCrangon vulgaris(fabricius) (crustacea, natantia)

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Abstract
The diellocomotory rhythm of Crangon vulgaris from a non-tidal area was investigated by means of photocells. Crangon was found to be nocturnal with the light-dark change acting as a Zeitgeber. The activity level did not change significantly during the year, but the activity pattern changed considerably. In winter (nLD 8: 16) Crangon showed two or three activity peaks, in summer (nLD 17: 7) only one. It was possible to shift the period of activity to any time of the day by displacing the dark period. If the lighted period was prolonged to more than 22 hours Crangon showed a phase-shift to a diurnal activity. At the same time the level of activity decreased markedly. No free-running rhythm persists in constant darkness. The rhythm was shown to be of a fully external origin. Certain environmental conditions, for instance the absence of a correct substratum, completely break down the rhytm. Such conditions exert a stronger influence than the Zeitgeber.