Seasonal and diel variation in the rhythmicity ofIdotea baltica(Pallas) andIdotea granulosaRathke

Abstract

The swimming activity of Idotea baltica and Idotea granulosa was investigated by means of photocells. The activity patterns of both species showed great seasonal variations. Both species showed no swimming activity in winter (Idotea baltica from November-March, and Idotea granulosa from September-March). The rest of the year both species had one activity peak between midnight and sunrise. Idotea baltica was nocturnal in early spring, in summer the activity was high all 24 hours, besides the nocturnal maximum an increased activity was evident in the afternoon till two hours after SS. At the end of September and the beginning of October I. baltica was nocturnal. The same variation in the activity pattern was found for I. granulosa. Under artificial light/dark conditions both species were nocturnal. When the light/dark change was right-angled this was the dominating Zeitgeber.