Effects of dusk and dawn on locomotor activity rhythms in the norway lobster Nephrops norvegicus

The endogenous locomotor activity rhythm of N.norvegicus shows peaks during darkness, around 3 h after dusk. At the peaks, activity occurs mainly within the burrow, although the prawns repeatedly vacate the burrows during the same periods. The phase of the rhythm shifts with a change of light-dark regime and in response to changed timing of 'dusk' when 'dawn' is fixed. Activity is suppessed immediately at dawn, but recommences before complete darkness during gradually reduced light at simulated dusk. This difference in response to light at dusk and dawn indicates phase responsiveness which is characteristic of endogenous rhythms entrained by light. The light- entrained endogenous rhythm is complementary to the rhythm of emergence which, reflected in commercial catches, appears to be controlled by exogenous factors.