[The control of tidal rhythm of emergence by an hourglass mechanism].

The arctic population of the intertidal midge Clunio marinus (location: Tromsö, Norway) shows a tidal rhythm of emergence (period: 12.4 hours) in midsummer. The emergence time exactly coincides with the initial exposure of the habitat during the ebb.When the animals were bred in a 24 hour light-dark cycle involving 16 hours of light, emergence occurred mainly 10-11 hours after light-on. When the animals were subsequently placed under constant light, no rhythm could be detected. When constant light was interrupted by a single period of 6 hours of darkness, only one peak of emergence was evoked, again 10-11 hours after light-on. Similar results were observed when in constant darkness a single period of 6 hours of lower temperature was offered.The mechanism of control differs from that of a southern population (location: St. Jean-de-Luz, France) which has a circadian clock mechanism and shows a free-running rhythm of emergence in constant light. It is postulated that the tidal rhythm of the Tromsö population is controlled by an hour-glass mechanism which starts its run of at least 10-11 hours during the preceding ebb.